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Review of Truck Characteristics as Factors in Roadway Design (2003)

Chapter: Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics

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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
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Suggested Citation:"Chapter 6 - Highway Geometric Design Criteria and Their Relationship to Truck Characteristics." National Academies of Sciences, Engineering, and Medicine. 2003. Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: The National Academies Press. doi: 10.17226/23379.
×
Page 113

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

61 CHAPTER 6 HIGHWAY GEOMETRIC DESIGN CRITERIA AND THEIR RELATIONSHIP TO TRUCK CHARACTERISTICS This chapter provides a review of the appropriateness of individual highway geometric design criteria to accommo- date trucks. The review includes the following highway geo- metric design criteria: • Stopping sight distance, • Passing sight distance and passing/no-passing zones on two-lane highways, • Decision sight distance, • Intersection sight distance, • Railroad-highway grade-crossing sight distance, • Intersection and channelization geometrics, • Critical length of grade, • Downgrades, • Acceleration lanes, • Deceleration lanes, • Lane width, • Horizontal curve radius and superelevation, • Pavement widening on horizontal curves, • Cross-slope breaks, and • Vertical clearance. Recommended changes in these geometric design criteria are presented in Appendix F. The review of each individual highway geometric design criterion includes a discussion of the criterion or policy cur- rently used by highway agencies, typically based on the Green Book, and a critique of that criterion based on available data or recent research concerning truck characteristics or con- cerning the traffic operational and safety effects of the crite- rion. These findings have been used to develop recommen- dations concerning the need to revise existing highway design policies to better accommodate trucks. The starting point for many of the geometric design reviews presented below is the FHWA Truck Characteris- tics report (2,3). This report reviewed all those design cri- teria in the 1984 Green Book and all those operational cri- teria in the 1988 MUTCD that were based on a passenger vehicle and assessed whether those criteria were adequate to accommodate trucks. In some cases, the analyses pre- sented in that report are still valid and are presented here. In other cases, changes in geometric design policy or in truck characteristics in the intervening years make the previous evaluation obsolete; a new review, based on up-to-date infor- mation, has been performed in these cases. Each highway geometric design criterion is discussed below. STOPPING SIGHT DISTANCE Current Geometric Design Criteria Sight distance is the length of roadway ahead that is visible to the driver. The minimum sight distance available on the roadway should be long enough to enable a vehicle traveling at the design speed to stop before reaching a stationary object in its path. This minimum sight distance, known as stopping sight distance, is the basis for design criteria for crest vertical curve length and minimum offsets to horizontal sight obstruc- tions. Not only is stopping sight distance needed at every point on the roadway, but stopping sight distance also forms the basis for a number of additional highway design criteria, including intersection sight distance and railroad-highway grade-crossing sight distance. Stopping Sight Distance Criteria Stopping sight distance is determined as the summation of two terms: brake reaction distance and braking distance. The brake reaction distance is the distance traveled by the vehi- cle from when the driver first sights an object necessitating a stop to the instant the brakes are applied. The braking dis- tance is the distance required to bring the vehicle to a stop once the brakes are applied. Stopping sight distance criteria in the Green Book have undergone a thorough recent review and have been revised in the 2001 edition based on research by Fambro et al. (37). Design values for stopping sight distance are based on the fol- lowing model, which is based on Green Book Equation 3-2: Metric US Customary (25) where where SSD = stopping sight distance, m SSD = stopping sight distance, ft t = brake reaction time, s; t = brake reaction time, s; V = design speed, km/h; V = design speed, mph; a = deceleration rate, m/s2 a = deceleration rate, ft/s2 SSD 1 Vt 1 V a 2 = +. .47 075SSD 0.278Vt 0.039 V a 2 = +

The first term in Equation 25 represents the brake reaction distance and the second term represents the braking distance. Equation 25 is not conceptually different from the stopping sight distance models used in previous editions of the Green Book, but the parameters of the model are now defined in ways that more realistically represent traffic situations encountered in emergency maneuvers. The design values for stopping sight distance are presented in Table 39, based on Green Book Exhibit 3-1. Correction of Stopping Sight Distance Criteria for Grades Stopping sight distance is also affected by roadway grade because longer braking distance is required on a downgrade and shorter braking distance is required on an upgrade. The Green Book criteria for grade effects on stopping sight dis- tance are derived with the following equation: 62 priate corrections for grade. This may explain why designers do not adjust stopping sight distance because of grade. Excep- tions are one-way roads or streets, as on divided highways with independent design profiles for the two roadways. For these separate roadways, adjustments for grade may be needed. Application of Stopping Sight Distance Criteria to Crest Vertical Curves Vertical crests limit the sight distance of the driver. Crest vertical curves designed in accordance with the AASHTO Green Book criteria should provide stopping sight distance at least equal to the design values in Table 39 at all points along the curve. The minimum length, L, of a crest vertical curve as a function of stopping sight distance is calculated based on Green Book Equations 3-43 and 3-44, as Metric US Customary (26)SSD V 3 2 a 32.2 G = ( ) ± 0SSD V 254 2 a 9.81 G = ( ) ±  In this equation, G is the percent of grade divided by 100, and the other terms are as previously stated. The stopping distances needed on upgrades are shorter than on level road- ways; those needed on downgrades are longer. On nearly all roads and streets, the grade is traversed by traffic in both directions of travel, but the sight distance at any point on the highway generally is different in each direction, particularly on straight roads in rolling terrain. As a general rule, the sight distance available on downgrades is larger than on upgrades, more or less automatically providing the appro- Metric US Customary When SSD is less than L, When SSD is less than L, (27) When SSD is greater than L, When SSD is greater than L, (28) where A = algebraic difference in grade L SSD 2158A= −2( )L SSD 658 A= −2( ) L A(SSD) 2 = 2158L A(SSD)2 = 658 Equations 27 and 28 are based on the mathematical prop- erties of a parabolic curve. The Green Book suggests that it is typical practice to use a minimum vertical curve length that is at least three times the value of the design speed (expressed in mph). For stopping sight distance, the driver eye height (h1) used by AASHTO is 1,080 mm [3.5 ft] and the object height (h2) used is 600 mm [2.0 ft]. Table 40 presents the minimum vertical curve lengths to attain the desirable stopping sight distance criteria in Table 39 as a function of design speed. TABLE 39 Design values for stopping sight distance (1) Metric US Customary Stopping sight distance Stopping sight distance Design speed (km/h) Brake reaction distance (m) Braking distance on level (m) Calculated (m) Design (m) Design speed (mph) Brake reaction distance (ft) Braking distance on level (ft) Calculated (ft) Design (ft) 20 13.9 4.6 18.5 20 15 55.1 21.6 76.7 80 30 20.9 10.3 31.2 35 20 73.5 38.4 111.9 115 40 27.8 18.4 46.2 50 25 91.9 60.0 151.9 155 50 34.8 28.7 63.5 65 30 110.3 86.4 196.7 200 60 41.7 41.3 83.0 85 35 128.6 117.6 246.2 250 70 48.7 56.2 104.9 105 40 147.0 153.6 300.6 305 80 55.6 73.4 129.0 130 45 165.4 194.4 359.8 360 90 62.6 92.9 155.5 160 50 183.8 240.0 423.8 425 100 69.5 114.7 184.2 185 55 202.1 290.3 492.4 495 110 76.5 138.8 215.3 220 60 220.5 345.5 566.0 570 120 83.4 165.2 248.6 250 65 238.9 405.5 644.4 645 70 257.3 470.3 727.6 730 75 275.6 539.9 815.5 820 80 294.0 614.3 908.3 910 130 90.4 193.8 284.2 285 NOTE: Brake reaction distance predicated on a time of 2.5 s; deceleration rate of 3.4 m/s2 [11.2 ft/s2] used to determine calculated sight distance.

Application of Stopping Sight Distance Criteria to Horizontal Curves Sight distance can also be limited by obstructions on the inside of horizontal curves, such as trees, buildings, retaining walls, and embankments. Horizontal curves designed in accor- dance with the Green Book should provide sight distance at least equal to the design values in Table 39 along the entire length of the curve. For a circular horizontal curve, the line of sight is a chord of that curve and the sight distance is measured along the centerline of the inside lane (see Figure 35). The minimum offset to a horizontal sight obstruction at the center of the curve (known as the middle ordinate of the curve) is computed in accordance with the following equation: 63 Specific Aspects of Stopping Sight Distance The critique that follows addresses the following aspects of stopping sight distance criteria: • Assumed speed for design, • Brake reaction time, • Deceleration rate (or coefficient of tire-pavement fric- tion) and braking distance, • Driver eye height, and • Object height. In addition, the critique addresses horizontal sight. Each of these factors is discussed below. Assumed Speed for Design. Prior to the 2001 Green Book, stopping sight distance was based on an assumed range of speeds, from the average running speed of traffic to the design speed, which resulted in a range of design values for stopping sight distance. The rationale for using this range of speeds was the assumption that drivers travel more slowly on wet pavements than on dry pavements; however, recent data have shown that drivers travel at about the same speeds on both wet and dry pavements. Therefore, the 2001 Green Book assumes that the initial speed of the vehicle prior to braking should be equal to the design speed of the roadway. This assumption appears to be as applicable to truck drivers as to passenger car drivers. Brake Reaction Time. The brake reaction time (t) is set equal to 2.5 s in the 2001 Green Book, as in past design poli- cies. This choice for brake reaction time has been confirmed Metric US Customary (29) where where R = Radius of curve, m; R = Radius of curve, ft; M = Middle ordinate, m M = Middle ordinate, ft M R R = −( )[ ]1 28 65cos . SSDM R R= −( )[ ]1 28 65cos . SSD TABLE 40 Design controls for stopping sight distance and for rate of vertical curvature (1) Metric US Customary Rate of vertical curvature, Ka Rate of vertical curvature, Ka Design speed (km/h) Stopping sight distance (m) Calculated Design Design speed (mph) Stopping sight distance (ft) Calculated Design 20 20 0.6 1 15 80 3.0 3 30 35 1.9 2 20 115 6.1 7 40 50 3.8 4 25 155 11.1 12 50 65 6.4 7 30 200 18.5 19 60 85 11.0 11 35 250 29.0 29 70 105 16.8 17 40 305 43.1 44 80 130 25.7 26 45 360 60.1 61 90 160 38.9 39 50 425 83.7 84 100 185 52.0 52 55 495 113.5 114 110 220 73.6 74 60 570 150.6 151 120 250 95.0 95 65 645 192.8 193 130 285 123.4 124 70 730 246.9 247 75 820 311.6 312 80 910 383.7 384 a Rate of vertical curvature, K, is the length of curve per percent algebraic difference in intersecting grades (A). K = L/A Critique of Geometric Design Criteria This section reviews the recent literature relevant to stop- ping sight distance criteria and their application to crest ver- tical curves and horizontal curves. The criteria are based on consideration of the passenger car as the design vehicle. The critique calls attention to differences between passen- ger cars and trucks that are relevant to stopping sight dis- tance design.

as appropriate for most drivers in several older studies (28, 58). Recent research by Fambro et al. (37) has confirmed that 2.5 s represents the 90th percentile of brake reaction time for all drivers. The brake reaction time is a driver characteristic and is assumed to be applicable to truck drivers as well as passen- ger car drivers. In fact, experienced professional truck driv- ers could reasonably be expected to have shorter brake reac- tion times than the driver population as a whole. On the other hand, the air brake systems historically used in tractor-trailer combination trucks have an inherent delay of approximately 0.5 s in brake application (18). It appears to be a reasonable assumption that the factors offset one another and that the 2.5-s brake reaction time is appropriate for both passenger car and truck drivers. Deceleration Rate and Braking Distance. The deceleration rate, a, in Equation 25 is set equal to 3.4 m/s2 [11.2 ft/s2] in the 2001 Green Book. This value was found by Fambro et al. (37) to represent the 10th percentile deceleration rate of passen- ger car drivers. This deceleration rate represents a comfort- able value for controlled braking by a passenger car and is within the driver’s capability to stay within his or her lane and maintain steering control during braking maneuvers on wet surfaces. Previous design policies were based on friction lev- els for locked-wheel braking, which carried with it a potential for loss of control of the vehicle. Thus, the 2001 Green Book 64 criteria are based on an assumed maneuver that is more appro- priate for trucks than previous editions of the Green Book. The review of braking distances in Chapter 5 of the report indicates that trucks equipped with antilock brakes can achieve deceleration rates in controlled braking nearly equal to the rate used for passenger car drivers in the Green Book. Thus, as antilock brakes come into widespread use, braking distances and deceleration rates for passenger cars and trucks should come closer together. NCHRP Synthesis 241 (19) noted that braking distances for passenger cars and trucks differ on dry pavement, but are nearly the same on wet pavements; wet pavements are, of course, the most critical situation for stop- ping sight distance. Appendix B discusses data collection activities undertaken to document the distribution of trailers with antilock brake sys- tems in the current vehicle fleet. Results of the field studies show that approximately 42 percent of trailers are equipped with antilock brake systems. As a comparison, Vehicle Inven- tory and Use Survey (VIUS) data from 1997 show that approx- imately 21 percent of trucks were equipped with antilock brake systems at that time. Thus, the percentage of trailers equipped with antilock brake systems has approximately doubled from 1997 to 2002. It is expected that, within 10 years, nearly all trailers will be equipped with antilock brake systems. Thus, there is good reason to expect that, within 10 years, most trucks will be able to stop on wet pavements in the same dis- tances as passenger cars. In addition, nearly all truck tractors are equipped with antilock brake systems. Figure 35. Diagram illustrating components for determining horizontal sight distance.

Driver Eye Height. The minimum crest vertical curve crite- ria for stopping sight distance in Table 40 are based on a driver eye height for passenger cars of 1,080 mm [3.5 ft]. The driver eye heights for trucks are much higher than for passen- ger cars, which may partially or completely offset their longer braking distances on crest vertical curves. However, the higher eye heights of truck drivers provide no comparable advantage at sight obstructions on horizontal curves unless the truck driver is able to see over the obstruction. The Green Book uses a value of 2,400 mm [8.0 ft] for truck driver eye height. Because this value is based on the results of recent research by Fambro et al. (37), it does not appear to be in need of updating. Object Height. The object height used in the 2001 Green Book to determine crest vertical curve lengths is 600 mm [2.0 ft], which was chosen as a conservative value to represent the taillight height of a passenger car. Previous editions of the Green Book used object heights of 100 and 150 mm [4 and 6 in.]. These lower object heights represented an arbitrary rationalization of possible hazardous objects that could be found in the roadway. Some researchers maintain that, histor- ically, these lower object heights represented a subjective tradeoff of the cost of providing sight distance to the pavement and did not represent any particular hazard (59). An accident study by Fambro et al. (37) found that virtually no accidents occur involving objects in the 100- to 150-mm [4- to 6-in.] range. Most collisions involve objects at least 600 mm [2 ft] high including, predominantly, other vehicles and, to a lesser extent, pedestrians, bicyclists, and animals. The choice of the 600-mm [2-ft] object, representing vehicle taillights, for use in the Green Book, was based on the work of Fambro et al. Horizontal Sight Obstructions Increased eye height provides truck drivers no advantage over passenger car drivers at a horizontal sight obstruction, unless the truck driver can see over the obstruction. How- 65 ever, Olson et al. indicate that the minimum offset to a hori- zontal sight obstruction, represented by the middle ordinate of the curve computed with Equation 29, is normally required only near the center of a horizontal curve (28). Figure 36 illustrates a sight distance envelope or clear sight zone within which horizontal sight obstructions should not be present. The figure illustrates that less offset to horizontal sight obstruc- tions is required within a distance to the ends of the curve equal to one-half the stopping sight distance. Another problem associated with stopping sight distance on horizontal curves is that the tire-pavement friction avail- able for braking is reduced by the portion of the available tire-pavement friction that is required for cornering (28, 60). Olson et al. expressed the available friction for braking on a horizontal curve as (28): (30) where f = coefficient of friction available for braking ft = total available coefficient of friction V = vehicle speed (mph) R = radius of curvature (ft) e = superelevation rate (ft/ft) Equation 30 implies that the required stopping sight dis- tances on horizontal curves should be longer than on tangents. Truck Considerations A sensitivity analysis conducted for the 1990 FHWA Truck Characteristics study (2, 3) concluded the following: • The 1984 Green Book stopping sight distance criteria were adequate for trucks with antilock brake systems. f f V15R e 2 t 2 2 2 = − −   Figure 36. Example sight obstruction envelope on horizontal curves for condition where the stopping sight distance is less than the length of the curve (28).

• The 1984 Green Book stopping sight distance criteria were adequate for trucks with conventional brake sys- tems and the best-performance driver at vertical sight restrictions and were nearly adequate at horizontal sight restrictions. • The 1984 Green Book stopping sight distance criteria were not adequate to accommodate trucks with con- ventional brake systems and poor-performance driv- ers, but changes in stopping sight distance criteria to accommodate poor-performance drivers would only be cost-effective for new construction or major recon- struction projects on rural two-lane highways that carry more than 800 trucks per day and rural freeways that carry more than 4,000 trucks per day. Given that antilock braking systems for trucks are coming into widespread use, these results suggest that changes to stopping sight distance policies to accommodate trucks should not be needed. The recommended stopping sight distances in the Green Book are based on passenger car operation and do not explic- itly consider design for truck operation. However, it does appear that the introduction of antilock brake systems on trucks minimizes any concern about stopping sight distance criteria for trucks in the long term. One situation in the Green Book indicates that every effort should be made to provide stopping sight distances greater than the design values in Table 39: Where horizontal sight restrictions occur on downgrades, particularly at the ends of long downgrades where truck speeds closely approach or exceed those of passenger cars, the greater height of eye of the truck driver is of little value, even when the horizontal sight obstruction is a cut slope. Although the average truck driver tends to be more experienced than the average pas- senger car driver and quicker to recognize potential risks, the Green Book states that it is desirable under such conditions to provide stopping sight distance that exceeds the values in Table 39 or the values derived from Equation 26. There is no indication in the literature whether the Green Book hypothesis that stopping sight distance for trucks is espe- cially critical at the end of long downgrades is correct. Further research on this issue would be desirable. Such research could be performed using a computer simulation model. The most critical situation for consideration in such research would appear to be the combination of a downgrade and a super- elevated horizontal curve. PASSING SIGHT DISTANCE AND PASSING/ NO-PASSING ZONES ON TWO-LANE HIGHWAYS Current Geometric Design and Marking Criteria Two major aspects of geometric design criteria for passing and no-passing zones on two-lane highways are addressed in 66 this section: passing sight distance and minimum passing zone length. This discussion addresses the Green Book criteria for passing sight distance, but also, for completeness, compares and contrasts these criteria with the criteria for passing sight distance and passing zone length in the FHWA Manual on Uniform Traffic Control Devices for Streets and Highways (MUTCD) (61). Passing Sight Distance Passing sight distance is needed where passing is permit- ted on two-lane, two-way highways to ensure that passing vehicles using the lane normally used by opposing traffic have a clear view ahead for a distance sufficient to minimize the possibility of collision with an opposing vehicle. Geometric Design Criteria. The current design criteria for passing sight distance on two-lane highways set forth in the 2001 Green Book are essentially unchanged from the criteria in the 1965 AASHTO policy and are based on the results of field studies conducted between 1938 and 1941 and validated by another study conducted in 1958 (62, 63, 64). Based on these studies, the Green Book policy defines the minimum passing sight distance as the sum of the following four distances: • dl = distance traveled during perception and reaction time and during initial acceleration to the point of encroachment on the left lane, • d2 = distance traveled while the passing vehicle occu- pies the left lane, • d3 = distance between passing vehicle and opposing vehi- cle at the end of the passing maneuver (i.e., clearance distance), and • d4 = distance traveled by an opposing vehicle for two- thirds of the time the passing vehicle occupies the left lane, or 2/3 of d2. Design values for the four distances described above were developed using the field data and the following assumptions stated in the Green Book: • The passed vehicle travels at uniform speed. • The passing vehicle reduces speed and trails the passed vehicle as it enters the passing section. (This is called a delayed pass.) • When the passing section is reached, the passing driver requires a short period of time to perceive the clear pass- ing section and to begin to accelerate. • Passing is accomplished under what may be termed a delayed start and a hurried return in the face of oppos- ing traffic. The passing vehicle accelerates during the maneuver, and its average speed during the occupancy

of the left lane is 16 km/h [10 mph] higher than that of the passed vehicle. • When the passing vehicle returns to its lane, there is a suitable clearance length between it and any oncoming vehicle in the other lane. The design values for the four components of passing sight distance are shown in Figure 37, based on Green Book Exhibit 3-4. Table 41, Figure 38, and Table 42 illustrate the development of the design values for passing sight distance. The columns in Table 41 not headed by a value of design speed represent field data from the sources cited above for the four components of the passing maneuver identified above. 67 Figure 38 illustrates the distances for these four compo- nents of the passing maneuver graphically, as well as the total passing sight distance, which is the sum of d1 through d4. Table 42 presents the design values of passing sight distance for design speeds from 30 to 130 km/h [20 to 80 mph]. In Table 42, the speeds used to compute the design values for passing sight distance differ from the design speed of the high- way. The speed of the passed vehicle is assumed to represent the average running speed of traffic. The speed of the passed vehicle is up to 36 km/h [22 mph] less than the design speed of the highway. The speed of the passing vehicle is assumed to be 15 km/h [10 mph] higher than the speed of the passed vehicle. The distance traveled during the initial maneuver period (dl) is computed in the Green Book as follows: Figure 37. Elements of passing sight distance for two-lane highways (1). TABLE 41 Elements of safe passing sight distance for design of two-lane highways (1) Metric US Customary Component of passing maneuver Speed range (km/h) Speed range (mph) 50-65 66-80 81-95 96-110 30-40 40-50 50-60 60-70 Average passing speed (km/h) Average passing speed (mph) 56.2 70.0 84.5 99.8 34.9 43.8 52.6 62.0 Initial maneuver: a = average accelerationa 2.25 2.30 2.37 2.41 1.40 1.43 1.47 1.50 t1 = time (sec)a 3.6 4.0 4.3 4.5 3.6 4.0 4.3 4.5 d1 = distance traveled 45 66 89 113 145 216 289 366 Occupation of left lane: t2 = time (sec)a 9.3 10.0 10.7 11.3 9.3 10.0 10.7 11.3 d2 = distance traveled 145 195 251 314 477 643 827 1030 Clearance length: d3 = distance traveleda 30 55 75 90 100 180 250 300 Opposing vehicle: d4 = distance traveled 97 130 168 209 318 429 552 687 Total distance, d1 + d2 + d3 + d4 317 446 583 726 1040 1468 1918 2383 a For consistent speed relation, observed values adjusted slightly. NOTE: In the metric portion of the table, speed values are in km/h, acceleration rates in km/h/s, and distances are in meters. In the U.S. customary portion of the table, speed values are in mph, acceleration rates in mph/sec, and distances are in feet.

The Green Book policy estimates the time for the initial maneuver (t l) as within the 3.6 to 4.5 s range, based on field data. Similarly, the average acceleration rate during the initial maneuver ranges from 2.22 to 2.43 km/h/s [1.38 to 1.51 mph/s]. The distance traveled by the passing vehicle while occu- pying the left lane (d2) is estimated in the Green Book from the following equation: 68 Based on field data, the Green Book assumes that the time the passing vehicle occupies the left lane ranges from 9.3 to 11.3 s for speed ranges from 50 to 110 km/h [30 to 70 mph]. The clearance distance (d3) is estimated in the Green Book to range from 30 to 90 m [100 to 300 ft], depending on speed. The distance traveled by an opposing vehicle (d4) is esti- mated as two-thirds of the distance traveled by the passing vehicle in the left lane. Conservatively, the distances d2 and d4 should be equal, but the Green Book assumes that the passing vehicle could abort its pass and return to the right lane if an opposing vehicle should appear early in the passing maneuver. Metric US Customary d2 = 0.278vt2 d2 = 1.47vt2 (32) where where t2 = time passing vehicle t2 = time passing vehicle occupies the left lane, s; occupies the left lane, s; v = average speed of passing v = average speed of passing vehicle, km/h vehicle, mph Metric US Customary (31) where where ti = time of initial maneuver, s; ti = time of initial maneuver, s; a = average acceleration, a = average acceleration, km/h/s; mph/s; v = average speed of passing v = average speed of passing vehicle, km/h; vehicle, mph; m = difference in speed of m = difference in speed of passed vehicle and passed vehicle and passing vehicle, km/h passing vehicle, mph d 0.47t v m at 21 i i = − +( )d 0.278t v m at21 i i= − +( ) Figure 38. Total passing sight distance and its components—two- lane highway (1).

Table 41 illustrates the derivation of the Green Book pass- ing sight distance criteria, representing the sum of the dis- tances dl through d4 for specific speed ranges. Table 42 pre- sents the Green Book passing sight distance criteria for specific design speeds. These design values range from 200 to 815 m [710 to 2,680 ft] for design speeds from 30 to 130 km [20 to 80 mph]. The Green Book criteria are used in highway design to determine if a particular highway project has sufficient length with passing sight distance to ensure an adequate level of service on the completed highway. The acceptable level of service for a particular project is consid- ered to be a design decision and is not specified in the Green Book. The Green Book criteria for passing sight distance are not used in the marking of passing and no-passing zones. Marking Criteria. The criteria for marking passing and no- passing zones on two-lane highways are set by the MUTCD. Passing zones are not marked directly. Rather, the warrants for no-passing zones are established by the MUTCD, and passing zones merely happen where no-passing zones are not war- 69 ranted. Table 43 presents the MUTCD passing sight distance warrants for no-passing zones. These criteria are based on prevailing off-peak 85th-percentile speeds rather than design speeds. The MUTCD passing sight distance warrants are substan- tially less than the Green Book passing sight distance design criteria. For example, at a speed of 100 km/h [60 mph], the AASHTO and MUTCD passing sight distance criteria are 670 m [2,135 ft] and 320 m [1,000 ft], respectively. The rationale for the MUTCD passing sight distance crite- ria is not stated in the MUTCD. However, the MUTCD war- rants are identical to those presented in the 1940 AASHTO policy on marking no-passing zones (65). These earlier AASHTO warrants represent a subjective compromise between distances computed for flying passes and distances computed for delayed passes. As such, they do not represent any par- ticular passing situation. Table 44 presents the basic assump- tions and data used to derive the MUTCD passing sight dis- tance warrants. TABLE 42 Passing sight distance for design of two-lane highways (1) Metric US Customary Assumed speeds (km/h) Passing sight distance (m) Assumed speeds (mph) Passing sight distance (ft) Design speed (km/h) Passed vehicle Passing vehicle Calculated Rounded for design Design speed (mph) Passed vehicle Passing vehicle Calculated Rounded for design 30 29 44 200 200 20 18 28 706 710 40 36 51 266 270 25 22 32 897 900 50 44 59 341 345 30 26 36 1088 1090 60 51 66 407 410 35 30 40 1279 1280 70 59 74 482 485 40 34 44 1470 1470 80 65 80 538 540 45 37 47 1625 1625 90 73 88 613 615 50 41 51 1832 1835 100 79 94 670 670 55 44 54 1984 1985 110 85 100 727 730 60 47 57 2133 2135 120 90 105 774 775 65 50 60 2281 2285 130 94 109 812 815 70 54 64 2479 2480 75 56 66 2578 2580 80 58 68 2677 2680 TABLE 43 Minimum passing sight distance for marking passing and no-passing zones on two-lane highways (61) Metric U.S. Customary 85th percentile speed or posted or statutory speed limit (km/h) Minimum passing sight distance (m) 85th percentile speed or posted or statutory speed limit (mph) Minimum passing sight distance (ft) 40 140 25 450 50 160 30 500 60 180 35 550 70 210 40 600 80 245 45 700 90 280 50 800 100 320 55 900 110 355 60 1,000 120 395 65 1,100 70 1,200

Minimum Passing Zone Length Another consideration in the marking of passing and no- passing zones on two-lane highways is the minimum length of a passing zone. The Green Book does not address passing zone lengths at all. The MUTCD indirectly sets a minimum passing zone length of 120 m [400 ft] by stating that, when two no- passing zones come within 120 m [400 ft] of one another, the no-passing barrier stripe should be continued between them. Critique of Geometric Design and Marking Criteria Passing Sight Distance Clearly, the AASHTO and MUTCD passing sight distance criteria are incompatible. The design values for the individual component distances in the Green Book criteria are question- able because, at high speeds, they are based on vehicle speeds less than the design speed of the highway. On the other hand, the definition of passing sight distance as the sum of the four distance elements (dl through d4) is extremely conservative, because it assumes that very early in the passing maneuver, the passing driver is committed to complete the pass. In fact, observation of two-lane highway operations shows that pass- ing drivers frequently abort passing maneuvers. The MUTCD passing sight distance criteria are based on a questionable premise, given that they represent a compro- mise between delayed passes and flying passes. A delayed pass is a maneuver in which the passing vehicle slows to the speed of the passed vehicle before initiating the passing maneuver. A flying pass is a maneuver in which the passing vehicle comes up behind the passed vehicle at a speed higher than the passed vehicle and initiates the passing maneuver without slowing down to the speed of the passed vehicle. Furthermore, both the AASHTO and MUTCD criteria are based on field data collected nearly 50 years ago. These field studies considered only passenger cars, not passing maneuvers involving longer and less powerful vehicles 70 such as trucks. Neither the AASHTO nor MUTCD models for passing sight distance contain a vehicle length term that could be used to examine the differences between passing sight distance requirements for trucks and passenger cars. Over the last three decades, researchers have recognized the inconsistencies between the AASHTO and MUTCD policies and have investigated alternative formulations of passing sight distance criteria. A total of 13 studies published since 1970 have questioned the premises of the AASHTO and MUTCD models and/or suggested revisions to those models (66–78). In the early 1970s, two studies independently rec- ognized that a key stage of a passing maneuver occurs at the point where the passing driver can no longer safely abort the pass and is, therefore, committed to complete it. One study called this the point of no return and another called it the critical position (66, 67). A 1976 paper added the insight that the critical position is the point at which the sight dis- tances required to abort the pass and to complete the pass are equal (68). Until the critical position is reached, the passing vehicle can abort the pass and return to the right lane behind the passed vehicle. Beyond the critical position, the driver is committed to complete the pass, because the sight dis- tance required to abort the pass is greater than the sight dis- tance required to complete the pass. The critical position concept has also been incorporated in research on passing sight distance requirements published in 1982, 1984, 1988, and 1989 (69, 70, 75, 76). Several of the studies cited above formulated passing sight distance models based on the critical position concept. How- ever, each of these models contained one or more logical flaws that made the model invalid. In 1988, however, Glennon for- mulated a new passing sight distance model that accounts for the kinematic relationships between the passing, passed, and opposing vehicles (75). The location of the critical position is determined as follows: (33) ∆C P I P L m m L L V m V(2.93 m L L d(2V m) I P = + + + − − + + − ( )   1 47 2 93 1 47 2 4 . . . ( ) ) TABLE 44 Derivation of MUTCD passing sight distance warrants (based on 1940 AASHTO policy) (65) Speed of passing vehicle (mph) 30 40 50 60 70 Assumed speed differential between passing and passed vehicles (mph) 10 12 15 20 25 Assumed speed of opposing vehicle (mph) 25 32 40 46 55 Required sight distance for flying pass (ft) 440 550 660 660 660 Required sight distance for delayed pass (ft) 510 760 1,090 1,380 1,780 600 800 1,000 1,200Recommended minimum sight distance (ft) 500

where ∆C = critical separation (distance from front of passing vehi- cle to front of passed vehicle at critical position) (ft) V = speed of passing vehicle and opposing vehicle (mph) m = speed difference between passed vehicle and passing vehicle (mph) d = deceleration rate used in aborting a passing maneu- ver (ft/s2) LP = length of passing vehicle (ft) LI = length of passed vehicle (ft) When the location of the critical position is known, the critical passing sight distance can be computed as follows: (34) The assumptions of the Glennon model are as follows: • The maximum sight distance during a passing maneu- ver is required at the critical position at which the sight distances required to complete the pass or to abort the pass are equal. • The speeds of the passing vehicle and opposing vehicle are equal. • The passing vehicle has sufficient acceleration capabil- ity to attain the specified speed difference relative to the passed vehicle by the time it reaches the critical position. • If the passing vehicle completes its pass, it returns to its normal lane with a 1-s gap in front of the passed vehicle. • If the passing vehicle aborts its pass, it returns to its nor- mal lane with a 1-s gap behind the passed vehicle. • The minimum clearance time between the passing vehi- cle and an opposing vehicle is 1 s. The derivation of the Glennon model, as given in Equa- tions 33 and 34, is presented in the literature and will not be repeated here (75). The Glennon model combined with accepted enforcement practices provides a very safety-conservative approach for marking passing and no-passing zones on two-lane highways. PSD 2V 2.93 L1.47mC P C = + −    ∆ 71 If the passing sight distance determined from Equation 34 is available throughout a passing zone, then it is ensured that a passing driver in the critical position at any point within that zone (even at the very end) has sufficient sight distance to complete the passing maneuver safely. In most terrain, pass- ing sight distance substantially greater than the minimum will be available throughout most of the passing zone. It must always be recognized that some drivers will illegally start a passing maneuver before the beginning of a passing zone (jumping) or complete it beyond the end of the zone (clip- ping). However, given that the sight distance requirements of passing drivers are lower in the early and later stages of a passing maneuver than at the critical position, the model pro- vides assurance that jumping and clipping drivers are unlikely to be greatly at risk of collision with an opposing vehicle. Finally, it should be recognized that the assumptions for a critical passing situation given above (e.g., passing and oppos- ing vehicles traveling at the design speed of the highway, 1-s clearance time to an opposing vehicle, and so forth) represent an extremely rare combination of events that does not occur often on two-lane highways. An advantage of the Glennon model is that the lengths of the passing and passed vehicles appear explicitly so that the sensitivity of the required passing sight distance to vehicle length can be examined. Minimum Passing Zone Length The MUTCD minimum passing zone length of 120 m [400 ft] is clearly inadequate for high-speed passes. A 1970 study evaluated several very short passing zones (79). In two passing zones with lengths of 120 and 200 m [400 and 640 ft], it was found that very few passing opportunities were accepted in such short zones and, of those that were accepted, more than 70 percent resulted in a slightly forced or very forced return to the right lane in the face of opposing traffic. A 1971 study recommended that the minimum length of a passing zone should be the sum of the perception-reaction dis- tance (d l) and the distance traveled while occupying the left lane (d2) (67). Table 45 illustrates several alternative criteria TABLE 45 Alternative criteria for minimum length of passing zones on two-lane highways (2,3) Minimum length of passing zone (ft) Design speed (mph) Based on MUTCD criteria Based on d1 + d2 from AASHTO policy Based on 85th percentile d1 + d2 observed in field studies(65) 20 400 505 – 30 400 650 – 40 400 865 – 50 400 1,065 – 55 400 1,155 885 60 400 1,245 – 65 400 1,340 1,185 70 400 1,455 1,335

that could be used for the minimum length of a passing zone, including the implicit MUTCD criteria, the sum of distances dl and d2 based on the assumptions in Green Book policy, and the 85th percentile value of the sum of distances dl and d2 based on field observations (2,3). Sensitivity Analyses Based on Truck Characteristics The design criteria for minimum passing sight distance and minimum passing zone length are sensitive to three major vehicle characteristics: vehicle length, acceleration/ deceleration capabilities, and driver eye height. Sensitivity analyses of these variables are presented below. These sen- sitivity analyses are based on the 1990 FHWA Truck Char- acteristics study (2,3), but have been updated to account for changes in truck characteristics and changes in Green Book and MUTCD criteria since those original sensitivity analyses were performed. Passing Sight Distance The existing design and marking criteria for minimum passing sight distance are based on consideration of passen- ger cars as both the passing and passed vehicles. The sensi- tivity analysis presented below considers three other passing scenarios: a passenger car passing a truck, a truck passing a passenger car, and a truck passing another truck. Passenger Car Passing Truck. Neither the AASHTO nor the MUTCD models can be used to examine the sensitivity of passing sight distance requirements to vehicle length. How- ever, a major advantage of the Glennon model is that the lengths of the passing and passed vehicles appear explicitly in the model. Therefore, this model has been used to compare the passing sight distance requirements for passenger cars and trucks. The lengths of the vehicles in the sensitivity analyses that follow are based on the length of the AASHTO passenger car design vehicle (6 m [19 ft]) and the length of a relatively long truck (23 m [75 ft]). In computing passing sight distance requirements with the Glennon model, presented above in Equations 33 and 34, the deceleration rate, d, used by a passenger car in aborting a pass is assumed to be 2.4 m/s2 [8 ft/s2]. This is a relatively conservative deceleration rate for a passenger car on a dry pavement, but it approaches a maximum deceleration rate in braking on a poor, wet road. The sensitivity analysis considered two alternative sets of assumptions concerning the speeds of the passing and passed vehicles. The first set consists of the standard AASHTO assumptions that the passed vehicle travels at the average run- ning speed of the highway (see Table 41) and that the speed differential, m, between the passing and passed vehicles is a 72 constant 16 km/h (10 mph) at all design speeds. The second set of assumptions was that proposed by Glennon, based on field data (67, 75). Glennon proposed that the passing vehicle should be assumed to travel at the design speed of the highway, but that the speed differential, m, between the passing and passed vehi- cles should be a function of design speed as shown in Table 46. Table 47 presents the passing sight distance requirements for a passenger car passing a truck using the Glennon model and Glennon’s assumptions concerning vehicle speeds, presented above. (An alternative analysis with the standard AASHTO assumptions concerning vehicle speeds yielded very similar results.) For comparative purposes, the passing sight distance requirements for a passenger car passing another passenger car are presented in three different ways: (1) based on AASHTO policy, (2) based on the MUTCD warrants, and (3) based on the Glennon model. Table 47 shows that the passing sight distance require- ments for passenger cars obtained from the Glennon model are very similar to the MUTCD criteria. The passing sight dis- tance requirements for a passenger car passing a truck are 8 to 76 m [25 to 250 ft] higher than for a passenger car pass- ing a passenger car, depending on speed. The Green Book sight distance requirements are much longer than any of the other criteria, because of their very conservative assumptions. Truck Passing Passenger Car. The passing sight distance requirements for a truck passing a passenger car can be addressed through a slight modification of the Glennon model. It is unlikely that a truck would be able to sustain a speed dif- ference as large as a passenger car in performing a passing maneuver. No data are available on the speed differences actu- ally used by trucks in passing, but, for purposes of this analy- sis, it will be assumed that trucks can maintain only one-half of the speed difference used by passenger cars. This assump- tion has been implemented in the following analysis by keep- ing the speed of the passed and opposing vehicles constant and decreasing the speed of the passing vehicle. Given that the speeds of the passing and opposing vehicles are no longer equal, a revised version of the Glennon model was derived and used for this analysis. This revised model for passing maneuvers by trucks is equivalent to Equations 33 and 34 with 0.5 (Vp + Vo) substituted for the V term, where Vp = speed of the passing vehicle (mph) Vo = speed of the opposing vehicle (mph) TABLE 46 Speed differentials between passing and passed vehicles for particular design speeds (75) Design speed (mph) Speed differential (mph) 30 12 40 11 50 10 60 9 70 8

A truck is also not likely to use a deceleration rate as high as 2.4 m/s2 [0.25 g or 8 ft/s2] in aborting a pass except in an emergency situation. Therefore, a deceleration rate of 1.5 m/s2 [0.15 g or 5 ft/s2], which would be a comfortable decelera- tion rate on a dry pavement, has been assumed. Table 48 presents the passing sight distance requirements for a 23-m [75-ft] truck passing a 6-m [19-ft] passenger car under the assumptions discussed above. The passing sight distance requirements for a truck passing a passenger car are 8 to 130 m [25 to 425 ft] more than for a passenger car pass- ing a passenger car, depending on speed. 73 Truck Passing Truck. The passing sight distance require- ments for a truck passing a truck have also been examined and are also presented in Table 48. Both vehicles are assumed to be 23 m [75 ft] in length. The passing sight distance require- ments for a truck passing another truck were found to be 8 to 206 m [25 to 675 ft] longer than for a passenger car passing a passenger car, depending on speed. Comparison of Results. Figure 39 compares the passing sight distance requirements determined in the sensitivity analysis with the current AASHTO and MUTCD policies. TABLE 47 Sight distance requirements for passing by passenger cars based on Glennon model (75) Required passing sight distance (ft) Design or prevailing speed (mph) AASHTO policy MUTCD criteria Passenger car passing passenger car Passenger car passing truck 20 800 – 325 350 30 1,100 500 525 575 40 1,500 600 700 800 50 1,800 800 875 1,025 60 2,100 1,000 1,025 1,250 70 2,500 1,200 1,200 1,450 Figure 39. Required passing sight distance for passenger cars and trucks in comparison with current criteria (2,3). TABLE 48 Sight distance requirements for passing by trucks based on revised Glennon model Required passing sight distance (ft) Design or prevailing speed (mph) AASHTO policy MUTCD criteria Truck passing passenger car Truck passing truck 20 800 – 350 350 30 1,100 500 600 675 40 1,500 600 875 975 50 1,800 800 1,125 1,275 60 2,100 1,000 1,375 1,575 1,875 70 2,500 1,200 1,625

The figure indicates that the MUTCD criteria are in good agreement with the requirements for a passenger car passing another passenger car. The other passing scenarios—passen- ger car passing truck, truck passing passenger car, and truck passing truck—each require progressively more sight dis- tance, but all are substantially less than the current AASHTO Green Book criteria. Figure 40 compares the minimum pass- ing zone lengths for the same scenarios. The development and interpretation of these curves is addressed in the discus- sion of minimum passing zone length that follows. Effect of Driver Eye Height at Crest Vertical Curves. Where passing sight distance is restricted by a vertical curve, the truck driver has an advantage over a passenger car driver due to greater eye height. However, as in the case of stop- ping sight distance, the truck driver has no such advantage where passing sight distance is restricted by a horizontal sight obstruction. Table 49 presents the required minimum vertical curve lengths to maintain passing sight distance over a crest as determined in the FHWA Truck Characteristics study (2, 3) for the four passing scenarios addressed in Tables 47 and 48. Table 49 has been updated to use eye heights of 1,080 mm [3.5 ft] for a passenger car driver and 2,330 mm [7.6 ft] for a truck driver based on the design recommendations of the 2001 Green Book. Table 49 indicates that increased driver eye height par- tially, but not completely, offsets the greater passing sight distance requirements of trucks. At all speeds above 48 km/h [30 mph], a longer minimum vertical curve length is required 74 to maintain adequate passing sight distance for passing maneu- vers involving trucks than for a passenger car passing another passenger car. However, except at high speeds and large algebraic differences in grades (e.g., sharp crests), a truck can safely pass a passenger car on any vertical curve where a pas- senger car can safely pass a truck. Minimum Passing Zone Length There are currently no design or operational criteria for minimum passing zone length, other than the default 120-m [400-ft] guideline set by the MUTCD. One possible criterion for minimum passing zone length is the distance required for a vehicle traveling at or near the design speed of the high- way to pass a slower vehicle. Recent debate over the role of trucks in passing sight distance criteria has largely ignored the longer passing distances and, thus, longer passing zone lengths required for passing maneuvers involving trucks. A sensitivity analysis of passing distances has been con- ducted based on the following assumptions: • The distance required to complete a pass is the sum of the initial maneuver distance (dl) and the distance trav- eled in the left lane (d2). • The passing driver does not begin to accelerate in prepa- ration for the passing maneuver until the beginning of the passing zone is reached. • The initial maneuver distance (dl) for passes by both passenger cars and trucks can be determined using the AASHTO relationship presented in Equation 31. The Figure 40. Required passing zone length to complete a pass at or near the highway design speed.

passing vehicle is assumed to accelerate at a constant rate, a, until the desired speed differential, m, relative to the passed vehicle is reached. Thus, tl can be calculated as m/a. • The acceleration rate, a, and initial maneuver time, t l, for passes by passenger cars as a function of design speed can be approximated by the AASHTO estimates in Table 41. Due to the lower performance capabilities of trucks, their acceleration rates during the initial maneu- ver are assumed to be one-half of those used by passen- ger cars. • The distance traveled in the left lane (d2) can be esti- mated as follows: (35) This relationship is used in preference to the AASHTO expression for d2 because it explicitly contains the lengths of the passing and passed vehicles (Lp and LI) and the speed difference between the vehicles, m. It would be desirable to calibrate Equation 35 with field data. • Equation 35 is based on the premise that the passing vehicle initially trails the passed vehicle by a 1-s gap and returns to its normal lane leading the passed vehicle by a 1-s gap. The passing vehicle is assumed to main- d V 2.93(V - m) + L L m a m 2 P I = + −      0 73 2. 75 tain an average speed differential equal to m during its occupancy of the left lane; the latter assumption is con- sistent with AASHTO policy, but is more restrictive than the Glennon model, which assumes only that a speed dif- ferential equal to m is reached before the passing vehi- cle reaches the critical position (75). • Passenger cars are assumed to accelerate when passing and to maintain an average speed equal to the design speed of the highway and maintain the same average speed differences used to derive Table 47. When pass- ing, trucks are assumed to maintain only one-half of the speed difference of passenger cars, consistent with the assumptions used to derive Table 48. • The assumed lengths of passenger cars and trucks are 6 and 23 m [19 and 75 ft], respectively. The sensitivity analysis results for the distance required to complete a pass are presented in Table 50 for the four pass- ing scenarios considered previously—passenger car passing passenger car, passenger car passing truck, truck passing pas- senger car, and truck passing truck. The required passing dis- tances for these four scenarios are illustrated in Figure 24. Except at very low speeds, all of the passing distances are very much larger than the MUTCD minimum passing zone length of 122 m [400 ft]. Table 50 and Figure 40 show that, in order to complete a passing maneuver at speeds of 100 km/h [60 mph] or more TABLE 49 Minimum vertical curve length (ft) to maintain required passing sight distance Design speed (mph) Algebraic difference in grade (%) 20 30 40 50 60 70 Passenger car passing passenger cara 2 80 200 350 550 760 1,030 4 160 400 700 1,100 1,510 2,060 6 230 600 1,050 1,650 2,260 3,090 8 310 790 1,400 2,190 3,010 4,120 10 380 990 1,750 2,740 3,760 5,150 Passenger car passing trucka 2 90 240 460 760 1,120 1,510 4 180 480 920 1,510 2,240 3,010 6 270 710 1,380 2,260 3,350 4,510 8 350 950 1,830 3,010 4,470 6,010 10 440 1,190 2,290 3,760 5,590 7,510 Truck passing passenger carb 2 60 170 360 600 890 1,240 4 120 340 720 1,190 1,770 2,470 6 180 510 1,080 1,780 2,650 3,700 8 230 680 1,430 2,370 3,540 4,940 10 290 850 1,790 2,960 4,420 6,170 Truck passing truckb 2 60 220 450 760 1,160 1,650 4 120 430 890 1,520 2,320 3,290 6 180 640 1,340 2,280 3,480 4,930 8 230 860 1,780 3,040 4,640 6,570 10 290 1,070 2,220 3,800 5,800 8,210 a Based on sight distance requirements from Table 47 for passenger car driver eye height of 1,080 mm [3.5 ft]. b Based on sight distance requirements from Table 48 for truck driver eye height of 2,330 mm [7.6 ft].

76 cars in those zones. Clearly, this would reduce the level of service on two-lane highways. The increased driver eye height of trucks partially, but not completely, offsets the increased passing sight distance requirements when the truck is the passing vehicle. However, except at very sharp crests on high-speed highways, a truck can safely pass a passenger car on any crest where a passen- ger car can safely pass a truck. No cost-effectiveness analysis of the potential for revising passing sight distance criteria to accommodate trucks was conducted in the Truck Characteristics study because of the lack of data on the operational effects of implementing the revised criteria. The criteria, presented in Tables 47 and 48, address design situations involving a passenger car passing a truck, a truck passing a passenger car, and a truck passing a truck, in contrast to the current criteria, which are based on a passenger car passing a passenger car. Adoption of any of these alternative passing sight distance criteria for marking passing and no-passing zones on two-lane highways would be premature without an operational analysis of the extent to which the revised criteria would degrade the level of service for passenger cars. There are no current criteria for passing zone lengths, except for the default 120-m [400-ft] guideline set by the MUTCD. For all design speeds above 48 km/h [30 mph], the distance required for one vehicle to pass another at or near that design speed is substantially longer than 120 m [400 ft], indicating a need for longer passing zones. The required passing dis- tances and passing zone lengths are increased substantially when the passing vehicle, the passed vehicle, or both, are trucks. However, this analysis is based on assumptions appro- priate for delayed passing maneuvers, which are seldom made by trucks. DECISION SIGHT DISTANCE Current Geometric Design Criteria Decision sight distance is the distance required for a driver to detect an unexpected or otherwise difficult-to-perceive information source or hazard in a roadway environment that may be visually cluttered, recognize the hazard or its threat potential, select an appropriate speed and path, and initiate under the stated assumptions, trucks require passing zones at least 610 m [2,000 ft] long. There are relatively few such passing zones on two-lane highways and, yet, trucks regularly make passing maneuvers. The explanation of this apparent paradox is that, given that there are very few locations where a truck can safely make a delayed pass, truck drivers seldom attempt them. Most passing maneuvers by trucks on two-lane highways are flying passes that require less passing sight dis- tance and less passing zone length than delayed passes. Thus, there may be no need to change current passing sight distance criteria to accommodate a truck passing a passenger car or a truck passing a truck as shown in Table 48. It makes little sense to provide enough passing sight distance for delayed passes by trucks when passing zones are not generally long enough to permit such maneuvers. Summary of Findings The review and sensitivity analysis conducted for the FHWA Truck Characteristics study found that there is very close agreement between the current MUTCD criteria for passing sight distance and the sight distance requirements for a passenger car passing another passenger car based on an analytical model recently developed by Glennon (75). Appli- cation of the Glennon model indicates that successively longer passing sight distances are required for a passenger car pass- ing a truck, a truck passing a passenger car, and a truck pass- ing a truck. There is no general agreement as to which of these passing situations is the most reasonable basis for designing and operating two-lane highways. All of the pass- ing sight distance criteria derived here are shorter than the Green Book design criteria, which are based on very conser- vative assumptions. The analysis results indicate that, if a passenger car pass- ing a passenger car is retained as the design situation, only minor modifications are needed to the MUTCD passing sight distance criteria. If a more critical design situation is selected (e.g., a passenger car passing a truck), passing sight distances up to 76 m [250 ft] longer than the current MUTCD criteria would be required. It is important to recognize that such a change in passing zone marking criteria would completely eliminate some existing passing zones and shorten others, even though passenger cars can safely pass other passenger TABLE 50 Passing zone length required to complete a pass for various passing scenarios (2,3) Minimum length of passing zone (ft) Speed difference (mph) used by passing vehicle Design speed (mph) Passing vehicle speed (V) (mph) Passenger car Truck car passing passenger car PassengerPassenger car passing truck Truck passing passenger car Truck passing truck 20 20 13 6.5 150 225 275 350 30 30 12 6 350 475 600 724 40 40 11 5.5 600 825 975 1,175 50 50 10 5 975 1,250 1,450 1,750 60 60 9 4.5 1,475 1,850 2,025 2,450 70 70 8 4 2,175 2,650 2,900 3,400

and complete the selected maneuver safely and efficiently (1). Decision sight distance is intended to give drivers an addi- tional margin for error and to provide them sufficient length to complete their selected maneuver at the same or reduced speed, rather than to stop. Therefore, the recommended val- ues of decision sight distance are substantially greater than the recommended stopping sight distance criteria. Locations where it may be desirable to provide decision sight distance include interchanges and intersection locations where unusual or unexpected maneuvers are required; changes in cross sec- tion, such as toll plazas and lane drops; and areas of “visual noise” where multiple sources of information, such as road- way elements, traffic, traffic control devices, and advertising signs, compete for the driver’s attention. The concept of decision sight distance was first intro- duced in the 1984 Green Book based on research by McGee et al. (80). The original decision sight distance concept con- sidered only a single maneuver, a lane change to avoid an obstacle, such as a vehicle or a traffic queue, on the roadway ahead. The decision sight distance design values were defined empirically from estimates of the premaneuver (i.e., detec- tion and recognition and decision and response initiation) and maneuver times required to make a lane change at var- ious speeds. The decision sight distance was changed in the 1990 Green Book to include multiple scenarios that might be encountered by a driver approaching a decision point. Specifically, decision sight distance criteria are now defined for five traffic scenarios or avoidance maneuvers. These are as follows: • Avoidance Maneuver A: Stop on rural road; • Avoidance Maneuver B: Stop on urban road; • Avoidance Maneuver C: Speed/path/direction change on rural road; • Avoidance Maneuver D: Speed/path/direction change on suburban road; and • Avoidance Maneuver E: Speed/path/direction change on urban road. The decision sight distances for avoidance maneuvers A and B are determined as follows: 77 ping sight distance to allow the driver additional time to detect and recognize the roadway or traffic situation and initiate a response. For a stop on a rural road (Avoidance Maneuver A), the estimated premaneuver time is 3.0 s. For the more com- plex situation represented by a stop on an urban road (Avoid- ance Maneuver B), the estimated premaneuver time is 9.1 s. The decision sight distances for Avoidance Maneuvers C, D, and E are determined as follows: Metric US Customary (36) where where t = pre-maneuver time, s; t = pre-maneuver time, s; V = design speed, km/h; V = design speed, mph; a = driver deceleration, m/s2 a = driver deceleration, ft/s2 d = 1.47Vt + 1.075 V a 2 d = 0.278Vt + 0.039 V a 2 Metric US Customary d = 0.278Vt d = 1.47Vt (37) where where t = total pre-maneuver and t = total pre-maneuver and maneuver time, s; maneuver time, s; V = design speed, km/h V = design speed, mph Equation 36 is the same model used in the Green Book for stopping sight distance (see Equation 25). However, in appli- cation to decision sight distance, the first term (premaneuver time) is increased above the brake reaction time used for stop- Equation 37 is based on the assumption that in making a path or direction change, the driver will be traveling at the design speed of the roadway for a specified premaneuver and maneuver time. There is no explicit consideration of the pos- sibility that the appropriate maneuver might be a speed change but, if the maneuver appropriate to the traffic situation is a reduction in speed, then the decision sight distances provided by Equation 37 will be conservative. In Equation 37, the parameter, t, represents the total pre- maneuver-plus-maneuver time. The total premaneuver-plus- maneuver time varies between 10.2 and 11.2 s for rural roads, between 12.1 and 12.9 s for suburban roads, and between 14.0 and 14.5 s for urban roads, with lower values used at higher speeds. The Green Book does not specify the alloca- tion of time between the premaneuver and maneuver periods and also does not specify any particular maneuver to be made. Rather, it is presumed that the values of t used are sufficient for whatever maneuver may be required. The decision sight distance criteria recommended in the Green Book are presented in Table 51. Vertical curve lengths to provide these levels of decision sight distance are based on a 1,080-mm [3.5-ft] driver eye height and a 600-mm [2-ft] object height, just as for stopping sight distance. The Green Book decision sight distance criteria are meant to be guidelines rather than absolute requirements. The Green Book emphasizes the importance of traffic control devices, such as advance signing, where the full decision sight dis- tance cannot be provided. Critique of Geometric Design Policy The Green Book criteria for decision sight distance are based primarily on consideration of passenger cars and do not explicitly consider trucks. However, the premaneuver and maneuver times considered are sufficiently long that it is

78 be cost-effective. A similar analysis indicates that changes to the decision sight distance criteria in the 2001 Green Book to better accommodate trucks would still not be cost-effective. INTERSECTION SIGHT DISTANCE Current Geometric Design Criteria Intersection sight distance is provided to allow drivers at, or on the approach to, an intersection to perceive the presence of potentially conflicting vehicles. This should occur in suf- ficient time for motorists to stop or adjust speed, as appro- priate, to avoid colliding in the intersection. The methods for determining the sight distances needed by drivers approach- ing intersections are based on the same principles as stopping sight distance, but incorporate modified assumptions based on observed driver behavior at intersections. The driver of a vehicle approaching an intersection should have an unobstructed view of the entire intersection, includ- ing any traffic control devices, and sufficient lengths along the intersecting highway to permit the driver to anticipate and avoid potential collisions. The sight distance needed under various assumptions of physical conditions and driver behav- ior is directly related to vehicle speeds and to the resultant dis- tances traversed during perception-reaction time and braking. Sight distance is also provided at intersections to allow the drivers of stopped vehicles a sufficient view of the intersect- ing highway to decide when to enter the intersecting highway or to cross it. If the available sight distance for an entering or crossing vehicle is at least equal to the appropriate stopping sight distance for the major road, then drivers have sufficient sight distance to anticipate and avoid collisions. However, in some cases, this may require a major-road vehicle to stop or slow to accommodate the maneuver by a minor-road vehicle. To enhance traffic operations, intersection sight distances likely that these criteria may accommodate trucks as well as passenger cars. For Avoidance Maneuvers A and B, the model used for decision sight distance is the same as that used for stopping sight distance. The premaneuver portion of the design sight distance criteria provides more reaction time than the stop- ping sight distance criteria. This should accommodate truck, as well as passenger car, drivers, especially given that truck drivers have an eye height advantage that lets them see stop conditions hidden by crest vertical curves before passenger car drivers. The deceleration rate used in determining the decision sight distance criteria for Avoidance Maneuvers A and B is the same value used in determining stopping sight distance criteria. A formal sensitivity analysis of decision sight distance requirements to accommodate trucks for Avoidance Maneu- vers C, D, and E will be difficult because the Green Book does not distinguish explicitly between premaneuver and maneu- ver time and because the specific maneuvers to be accommo- dated are not specified. Given that Avoidance Maneuvers C, D, and E involve speed/path/direction changes, rather than braking to a stop, the longer braking distances of trucks may be less of an issue than for situations where a stop is required. On the other hand, trucks are substantially larger and less maneuverable than passenger cars and may require more maneuver time in some situations (e.g., lane changes). The greater eye height of truck drivers is a potential advantage for Avoidance Maneuvers C, D, and E because a truck driver may be able to see over the vehicle immediately ahead and may be able to perceive traffic situations requiring an avoidance maneuver before a passenger car driver would. The FHWA Truck Characteristics study included a cost- effectiveness analysis of potential changes to the decision sight distance policy in the 1984 Green Book to better accommodate trucks. This analysis concluded that such changes would not TABLE 51 Design values for decision sight distance (1) Metric US Customary Decision sight distance (m) Decision sight distance (ft) Avoidance maneuver Avoidance maneuver Design speed (km/h) A B C D E Design speed (mph) A B C D E 50 70 155 145 170 195 30 220 490 450 535 620 60 95 195 170 205 235 35 275 590 525 625 720 70 115 235 200 235 275 40 330 690 600 715 825 80 140 280 230 270 315 45 395 800 675 800 930 90 170 325 270 315 360 50 465 910 750 890 1030 100 200 370 315 355 400 55 535 1030 865 980 1135 110 235 420 330 380 430 60 610 1150 990 1125 1280 120 265 470 360 415 470 65 695 1275 1050 1220 1365 130 305 525 390 450 510 70 780 1410 1105 1275 1445 75 875 1545 1180 1365 1545 80 970 1685 1260 1455 1650 Avoidance Maneuver A: Stop on rural road—t = 3.0 s. Avoidance Maneuver B: Stop on urban road—t = 9.1 s. Avoidance Maneuver C: Speed/path/direction change on rural road—t varies between 10.2 and 11.2 s. Avoidance Maneuver D: Speed/path/direction change on suburban road—t varies between 12.1 and 12.9 s. Avoidance Maneuver E: Speed/path/direction change on urban road—t varies between 14.0 and 14.5 s.

that exceed stopping sight distances are desirable along the major road. Prior to the 2001 Green Book, intersection sight distance policies were presented based on a kinematic or acceleration- deceleration model. Research by Harwood et al. (81) docu- mented conceptual inconsistencies in these models and for- mulated a revised approach to intersection sight distance criteria based on gap acceptance. A gap-acceptance model, calibrated with field data, was used for all intersection sight distance cases, except for intersections with no traffic control on any of the approaches (Case A). Sight Triangles Two types of clear sight triangles are considered in inter- section design: approach sight triangles and departure sight triangles. Approach Sight Triangles Each quadrant of an intersection should contain a triangu- lar area free of obstructions that might block an approaching driver’s view of potentially conflicting vehicles. The length of the legs of this triangular area, along both intersecting road- ways, should be such that the drivers can see any potentially conflicting vehicles in sufficient time to slow or stop before colliding within the intersection. Figure 41a shows typical clear sight triangles to the left and to the right for a vehicle approaching an uncontrolled or yield-controlled intersection. Departure Sight Triangles A second type of clear sight triangle provides sight distance sufficient for a stopped driver on a minor-road approach to depart from the intersection and enter or cross the major road. Figure 41b shows typical departure sight triangles to the left and to the right of the location of a stopped vehicle on the minor road. Departure sight triangles should be provided in each quadrant of each intersection approach controlled by stop or yield signs and for some signalized intersection approaches. Identification of Sight Obstructions Within Sight Triangles The profiles of the intersecting roadways should be designed to provide the recommended sight distances for drivers on the intersection approaches. Within a sight triangle, any object at a height above the elevation of the adjacent roadways that would obstruct the driver’s view should be removed or low- ered, if practical. Such objects may include buildings, parked vehicles, highway structures, roadside hardware, hedges, trees, bushes, unmowed grass, tall crops, walls, fences, and the ter- 79 rain itself. Particular attention should be given to the eval- uation of clear sight triangles at interchange ramp/crossroad intersections where features such as bridge railings, piers, and abutments are potential sight obstructions. The determination of whether an object constitutes a sight obstruction should consider both the horizontal and vertical alignment of both intersecting roadways, as well as the height and position of the object. In making this determination, it should be assumed that the driver’s eye is 1,080 mm [3.5 ft] above the roadway surface and that the object to be seen is 1,080 mm [3.5 ft] above the surface of the intersecting road. This object height is based on a vehicle height of 1,330 mm [4.35 ft], which represents the 15th percentile of vehicle heights in the current passenger car population less an allowance of 250 mm [10 in]. This allowance represents a near-maximum value for the portion of a passenger car height that needs to be visible for another driver to recognize it as the object. The use of an object height equal to the driver eye height makes intersection sight distances reciprocal (i.e., if one driver can see another vehicle, then the driver of that vehicle can also see the first vehicle). Where the sight-distance value used in design is based on a single-unit or combination truck as the design vehicle, it is also appropriate to use the eye height of a truck driver in checking sight obstructions. The value for a truck driver’s eye height recommended in the Green Book is 2,330 mm [7.6 ft] above the roadway surface. Intersection Sight Distance Cases The recommended dimensions of the sight triangles vary with the type of traffic control used at an intersection because different types of control impose different legal constraints on drivers and, therefore, result in different driver behavior. Procedures to determine sight distances at intersections are provided in the Green Book for the following cases: • Case A—Intersections with no control; • Case B—Intersections with stop control on the minor road; • Case B1—Left turn from the minor road; • Case B2—Right turn from the minor road; • Case B3—Crossing maneuver from the minor road; • Case C—Intersections with yield control on the minor road; • Case C1—Crossing maneuver from the minor road; • Case C2—Left or right turn from the minor road; • Case D—Intersections with traffic signal control; • Case E—Intersections with all-way stop control; and • Case F—Left turns from the major road. The following discussion addresses Cases B, C, D, E, and F. Case A is omitted because it is applicable only to very

low-volume intersections at which the appropriate design vehicle is unlikely to be a truck. Case B—Intersections With Stop Control on the Minor Road Departure sight triangles for intersections with stop con- trol on the minor road are considered for three situations: • Case B1—Left turns from the minor road; • Case B2—Right turns from the minor road; and • Case B3—Crossing the major road from a minor-road approach. 80 Intersection sight distance criteria for stop-controlled intersections are longer than stopping sight distance to ensure that the intersection operates smoothly. Minor-road vehicle operators can wait until they can proceed safely without forc- ing a major-road vehicle to stop. Case B1—Left Turn From the Minor Road The Green Book states that departure sight triangles for traffic approaching from either the right or the left, like those shown in Figure 41b, should be provided for left turns from the minor road onto the major road for all stop-controlled approaches. The length of the leg of the departure sight tri- Figure 41. Intersection sight triangles.

angle along the major road in both directions is the recom- mended intersection sight distance for Case B1. The vertex (decision point) of the departure sight triangle on the minor road should be 4.4 m [14.4 ft] from the edge of the major-road traveled way. This represents the typical posi- tion of the minor-road driver’s eye when a vehicle is stopped relatively close to the major road. Field observations of vehi- cle stopping positions found that, where necessary, drivers will stop with the front of their vehicles 2.0 m [6.5 ft] or less from the edge of the major-road traveled way. Measurements of passenger cars indicate that the distance from the front of the vehicle to the driver’s eye for the current U.S. passenger car population is nearly always 2.4 m [8 ft] or less (81). The Green Book states that, where practical, it is desirable to increase the distance from the edge of the major-road trav- eled way to the vertex of the clear sight triangle from 4.4 m to 5.4 m [14.4 to 17.8 ft]. This increase allows 3.0 m [10 ft] from the edge of the major-road traveled way to the front of the stopped vehicle, providing a larger sight triangle. The length of the sight triangle along the minor road (distance “a” in Figure 41b) is the sum of the distance from the major road plus one-half of the lane width for vehicles approaching from the left, or one-and-one-half lane width for vehicles approach- ing from the right. Field observations of the gaps in major-road traffic actu- ally accepted by drivers turning onto the major road have shown that the values in Table 52 provide sufficient time for the minor-road vehicle to accelerate from a stop and com- plete a left turn without unduly interfering with major-road traffic operations. The time gap acceptance time does not vary with approach speed on the major road. Studies have indicated that a constant value of time gap, independent of approach speed, can be used as a basis for intersection sight distance determinations. Observations have also shown that major-road drivers will reduce their speed to some extent when minor-road vehicles turn onto the major road. Where the time gap acceptance values in Table 52 are used to deter- mine the length of the leg of the departure sight triangle, most major-road drivers should not need to reduce speed to less than 70 percent of their initial speeds (81). 81 The intersection sight distance in both directions should be equal to the distance traveled at the design speed of the major road during a period of time equal to the time gap. In apply- ing Table 52, it is usually assumed that the minor-road vehi- cle is a passenger car. However, where substantial volumes of heavy vehicles enter the major road, such as from a ramp ter- minal, tabulated values for single-unit or combination trucks are provided. Table 52 includes appropriate adjustments to the gap times for the number of lanes on the major road and for the approach grade of the minor road. The Green Book states that the intersection sight distance along the major road (dimension b in Figure 41b) is deter- mined by the following: Metric US Customary ISD = 0.278Vmajor tg ISD = 1.47Vmajor tg (38) where where ISD = intersection sight distance ISD = intersection sight distance (length of the leg of sight (length of the leg of sight triangle along the major triangle along the major road) (m) road) (ft) Vmajor = design speed of major Vmajor = design speed of major road (km/h) road (mph) tg = time gap for minor road tg = time gap for minor road vehicle to enter the major vehicle to enter the major road (s) road (s) TABLE 52 Time gap for Case B1—left turn from stop (1) Design vehicle Time gap (s) at design speed of major road (tg) Passenger car 7.5 Single-unit truck 9.5 Combination truck 11.5 NOTE: Time gaps are for a stopped vehicle to turn right or left onto a two-lane highway with no median and grades 3 percent or less. The table values require adjustment as follows: For multilane highways: For left turns onto two-way highways with more than two lanes, add 0.5 seconds for passenger cars or 0.7 seconds for trucks for each additional lane, from the left, in excess of one, to be crossed by the turning vehicle. For minor road approach grades: If the approach grade is an upgrade that exceeds 3 percent; add 0.2 seconds for each percent grade for left turns. The Green Book recommends that sight distance design for left turns at divided-highway intersections should consider multiple design vehicles and median width. If the design vehi- cle used to determine sight distance for a divided-highway intersection is larger than a passenger car, then sight distance for left turns will need to be checked for that selected design vehicle and for smaller design vehicles as well. If the divided- highway median is wide enough to store the design vehicle with a clearance to the through lanes of approximately 1 m [3 ft] at both ends of the vehicle, no separate analysis for the departure sight triangle for left turns is needed on the minor- road approach for the near roadway to the left. In most cases, the departure sight triangle for right turns (Case B2) will

provide sufficient sight distance for a passenger car to cross the near roadway to reach the median. Possible exceptions are addressed in the discussion of Case B3. If the design vehicle can be stored in the median with ade- quate clearance to the through lanes, a departure sight triangle to the right for left turns should be provided for that design vehicle turning left from the median roadway. Where the median is not wide enough to store the design vehicle, a depar- ture sight triangle should be provided for that design vehicle to turn left from the minor-road approach. The median width should be considered in determining the number of lanes to be crossed. The median width should be converted to equivalent lanes. For example, a 7.2-m [24-ft] median should be considered as two additional lanes to be crossed in applying the multilane highway adjustment for time gaps in Table 52. Furthermore, a departure sight triangle for left turns from the median roadway should be provided for the largest design vehicle that can be stored on the median 82 roadway with adequate clearance to the through lanes. If a divided highway intersection has a 12-m [40-ft] median width and the design vehicle for sight distance is a 22-m [74-ft] combination truck, departure sight triangles should be provided for the combination truck turning left from the minor-road approach and through the median. In addition, a departure sight triangle should also be provided to the right for a 9-m [30-ft] single-unit truck turning left from a stopped position in the median. Figure 42 compares the intersection sight distances by type of design vehicle for Case B1. Case B2—Right Turns from the Minor Road The Green Book states that a departure sight triangle for traffic approaching from the left like that shown in Figure 41b should be provided for right turns from the minor road Figure 42. Intersection sight distance—Case B1—left turn from stop (1).

onto the major road. The intersection sight distance for right turns is determined in the same manner as for Case B1, except that the time gaps (tg) in Table 52 are adjusted. Field obser- vations indicate that, in making right turns, drivers gener- ally accept gaps that are slightly shorter than those accepted in making left turns (81). The time gaps in Table 52 can be decreased by 1.0 s for right-turn maneuvers without undue interference with major-road traffic. These adjusted time gaps for the right turn from the minor road are shown in Table 53. Figure 43 compares the design values for the design vehicles for each of the time gaps in Table 53. When the minimum recommended sight distance for a right-turn maneuver cannot be provided, even with the reduction of 1.0 s from the values in Table 53, the Green Book recommends that consideration should be given to installing regulatory speed signing or other traffic control devices on the major-road approaches. Case B3—Crossing Maneuver from the Minor Road In most cases, the departure sight triangles for left and right turns onto the major road, as described for Cases B1 and B2, will also provide more than adequate sight distance for minor-road vehicles to cross the major road. However, the Green Book notes that, in the following situations, it is advis- able to check the availability of sight distance for crossing maneuvers: • Where left and/or right turns are not permitted from a particular approach and the crossing maneuver is the only legal maneuver; • Where the crossing vehicle would cross the equivalent width of more than six lanes; or • Where substantial volumes of heavy vehicles cross the highway and steep grades that might slow the vehicle 83 while its back portion is still in the intersection are present on the departure roadway on the far side of the intersection. The formula for intersection sight distance in Case B1 is used again for the crossing maneuver except that time gaps (tg) are obtained from Table 53. At divided highway inter- sections, depending on the relative magnitudes of the median width and the length of the design vehicle, intersection sight distance may need to be considered for crossing both road- ways of the divided highway or for crossing the near lanes only and stopping in the median before proceeding. The appli- cation of adjustment factors for median width and grade are discussed under Case B1. Case C—Intersections With Yield Control on the Minor Road Drivers approaching yield signs are permitted to enter or cross the major road without stopping, if there are no poten- tially conflicting vehicles on the major road. The sight dis- tances needed by drivers on yield-controlled approaches exceed those for stop-controlled approaches. For four-leg intersections with yield control on the minor road, two separate pairs of approach sight triangles like those shown in Figure 41a should be provided. One set of approach sight triangles is needed to accommodate cross- ing the major road and a separate set of sight triangles is needed to accommodate left and right turns onto the major road. Both sets of sight triangles should be checked for potential sight obstructions. For three-leg intersections with yield control on the minor road, only the approach sight triangles to accommodate left- and right-turn maneuvers need be considered, because the crossing maneuver does not exist. Case C1—Crossing Maneuver From the Minor Road The Green Book design values for the length of the leg of the approach sight triangle along the minor road to accommo- date the crossing maneuver from a yield-controlled approach (distance “a” in Figure 41a) is given in Table 54. The distances in Table 54 are based on the same assumptions as those for Case A except that, based on field observations, minor-road vehicles that do not stop are assumed to decelerate to 60 per- cent of the minor-road design speed, rather than 50 percent. Sufficient travel time for the major-road vehicle should be provided to allow the minor-road vehicle: (1) to travel from the decision point to the intersection, while decelerating at the rate of 1.5 m/s2 [5 ft/s2] to 60 percent of the minor-road design speed; and then (2) to cross and clear the intersec- tion at that same speed. The intersection sight distance along TABLE 53 Time gap for Case B2—right turn from stop and Case B3—crossing maneuver (1) Design vehicle Time gap (s) at design speed of major road (tg) Passenger car 6.5 Single-unit truck 8.5 Combination truck 10.5 NOTE: Time gaps are for a stopped vehicle to turn right onto or cross a two-lane highway with no median and grades 3 percent or less. The table values require adjustment as follows: For multilane highways: For crossing a major road with more than two lanes, add 0.5 seconds for passenger cars and 0.7 seconds for trucks for each additional lane to be crossed and for narrow medians that cannot store the design vehicle. For minor road approach grades: If the approach grade is an upgrade that exceeds 3 percent, add 0.1 seconds for each percent grade.

the major road to accommodate the crossing maneuver (dis- tance b in Figure 41a) should be computed with Equation 39. The value of tg should equal or exceed the appropriate travel time for crossing the major road from a stop-controlled approach, as shown in Table 53. The design values for the time gap (tg) shown in Table 54 incorporate these crossing times for two-lane highways and are used to develop the length of the leg of the sight triangle along the major road in Table 55. Case C2—Left or Right Turn from the Minor Road The Green Book states that length of the leg of the approach sight triangle along the minor road to accommodate left and 84 right turns without stopping (distance a in Figure 41a) should be 25 m [82 ft]. This distance is based on the assumption that drivers making left and right turns without stopping will slow to a turning speed of 16 km/h [10 mph]. The leg of the approach sight triangle along the major road (distance b in Figure 41a) is similar to the major-road leg of the departure sight triangle for stop-controlled intersections in Cases B1 and B2. However, the Green Book states that the time gaps in Table 52 should be increased by 0.5 s to the val- ues shown in Table 56. The appropriate lengths of the sight triangle leg are shown in Figure 44 for the various design vehicle categories. The minor-road vehicle needs 3.5 s to travel from the decision point to the intersection. This repre- sents additional travel time that is needed at a yield-controlled Figure 43. Intersection sight distance—Case B2—right turn from stop and Case B3—crossing maneuver (1).

intersection, but is not needed at a stop-controlled intersec- tion (Case B). 85 the turning vehicle accelerates from 16 km/h (10 mph) rather than from a stop condition. The net 0.5-s increase in travel time for a vehicle turning from a yield-controlled approach is the difference between the 3.5-s increase in travel time and the 3.0-s reduction in travel time. The Green Book states that departure sight triangles like those provided for stop-controlled approaches (see Cases B1, B2, and B3) should also be provided for yield-controlled approaches to accommodate minor-road vehicles that stop at the yield sign to avoid conflicts with major-road vehicles. However, given that approach sight triangles for turning maneuvers at yield-controlled approaches are larger than the departure sight triangles used at stop-controlled intersec- tions, no specific check of departure sight triangles at yield- controlled intersections should be needed. Yield-controlled approaches generally need greater sight distance than stop-controlled approaches, especially at four- leg yield-controlled intersections where the sight distance needs of the crossing maneuver should be considered. If sight distance sufficient for yield control is not available, use of a stop sign instead of a yield sign should be considered. In addi- tion, at locations where the recommended sight distance can- not be provided, consideration should be given to installing regulatory speed signing or other traffic control devices at the intersection on the major road to reduce the speeds of approaching vehicles. Case D—Intersections with Traffic Signal Control At signalized intersections, the first vehicle stopped on one approach should be visible to the driver of the first vehicle TABLE 54 Case C1—crossing maneuvers from yield-controlled approaches—length of minor-road leg and travel times (1) Metric US Customary Minor-road approach Travel time (tg) (seconds) Minor-road approach Travel time (tg) (seconds) Design speed (km/h) Length of leg1 (m) Travel time ta1,2 (seconds) Calculated value Design value3,4 Design speed (mph) Length of leg1 (ft) Travel time ta1,2 (seconds) Calculated value Design value3,4 20 20 3.2 7.1 7.1 15 75 3.4 6.7 6.7 30 30 3.6 6.2 6.5 20 100 3.7 6.1 6.5 40 40 4.0 6.0 6.5 25 130 4.0 6.0 6.5 50 55 4.4 6.0 6.5 30 160 4.3 5.9 6.5 60 65 4.8 6.1 6.5 35 195 4.6 6.0 6.5 70 80 5.1 6.2 6.5 40 235 4.9 6.1 6.5 80 100 5.5 6.5 6.5 45 275 5.2 6.3 6.5 90 115 5.9 6.8 6.8 50 320 5.5 6.5 6.5 100 135 6.3 7.1 7.1 55 370 5.8 6.7 6.7 110 155 6.7 7.4 7.4 60 420 6.1 6.9 6.9 120 180 7.0 7.7 7.7 65 470 6.4 7.2 7.2 130 205 7.4 8.0 8.0 70 530 6.7 7.4 7.4 75 590 7.0 7.7 7.7 80 660 7.3 7.9 7.9 1 For minor-road approach grades that exceed 3 percent, multiply the distance or the time in this table by the appropriate adjustment factor from Green Book Exhibit 9-53. 2 Travel time applies to a vehicle that slows before crossing the intersection but does not stop. 3 The value of tg should equal or exceed the appropriate time gap for crossing the major road from a stop-controlled approach. 4 Values shown are for a passenger car crossing a two-lane highway with no median and grades 3 percent or less. Metric US Customary (39) b = 0.278Vmajor tg b = 1.47Vmajor tg where where tg = travel time to reach and tg = travel time to reach and clear the major road (s) clear the major road (s) b = length of leg of sight b = length of leg of sight triangle along the major triangle along the major road (m) road (ft) ta = travel time to reach the ta = travel time to reach the major road from the major road from the decision point for a decision point for a vehicle that does not vehicle that does not stop (s) (use appropriate stop (s) (use appropriate value for the minor-road value for the minor-road design speed from design speed from Exhibit 9-60 adjusted for Exhibit 9-60 adjusted for approach grade, where approach grade, where appropriate) appropriate) w = width of intersection to be w = width of intersection to be crossed (m) crossed (ft) La = length of design vehicle La = length of design vehicle (m) (ft) Vminor = design speed of minor Vminor = design speed of minor road (km/h) road (mph) Vmajor = design speed of major Vmajor = design speed of major road (km/h) road (mph) t t w LVg a a = + + 0 88. minor t t w LVg a a = + + 0 167. minor However, the acceleration time after entering the major road is 3.0 s less for a yield sign than for a stop sign because

stopped on each of the other approaches. Left-turning vehi- cles should have sufficient sight distance to select gaps in oncoming traffic and complete left turns. Apart from these sight conditions, the Green Book states that generally there are no other approach or departure sight triangles needed for signalized intersections. Signalization may be an appropriate crash countermeasure for higher volume intersections with restricted sight distance that have experienced a pattern of sight-distance related crashes. However, if the traffic signal is to be placed on two-way flashing operation (i.e., flashing yellow on the major-road approaches and flashing red on the minor-road approaches) under off-peak or nighttime conditions, then the appropriate departure sight triangles for Case B, both to the left and to the right, should be provided for the minor-road approaches. In addition, if right turns on a red signal are to be permitted from any approach, then the appropriate departure sight triangle to the left for Case B2 should be provided to accommodate right turns from that approach. The Green Book criteria for intersection sight distance Case D reflect the differences between passenger cars and trucks in that those differences are considered explicitly in Case B. 86 Case E—Intersections with All-Way Stop Control At intersections with all-way stop control, the Green Book states that the first stopped vehicle on one approach should be visible to the drivers of the first stopped vehicles on each of the other approaches. There are no other sight distance cri- teria applicable to intersections with all-way stop control and, indeed, all-way stop control may be the best option at a limited number of intersections where sight distance for other control types cannot be attained. There are no differ- ences between passenger cars and trucks in the intersection sight distance criteria for Case E. Case F—Left Turns From the Major Road All locations along a major highway from which vehicles are permitted to turn left across opposing traffic, including intersections and driveways, should have sufficient sight dis- tance to accommodate the left-turn maneuver. Left-turning drivers need sufficient sight distance to decide when it is safe to turn left across the lane(s) used by opposing traffic. Sight distance design should be based on a left turn by a stopped TABLE 55 Length of sight triangle leg along major road—Case C1—crossing maneuver at yield-controlled intersections (1) Metric US Customary Minor-road design speed (km/h) Minor-road design speed (mph) 20 30-80 90 100 110 120 130 15 20-50 55 60 65 70 75 80 Major road design speed (km/h) Stopping sight distance (m) Design values (m) Major road design speed (mph) Stopping sight distance (ft) Design values (ft) 20 20 40 40 40 40 45 45 45 15 80 150 145 150 155 160 165 170 175 30 35 60 55 60 60 65 65 70 20 115 200 195 200 205 215 220 230 235 40 50 80 75 80 80 85 90 90 25 155 250 240 250 255 265 275 285 295 50 65 100 95 95 100 105 110 115 30 200 300 290 300 305 320 330 340 350 60 85 120 110 115 120 125 130 135 35 250 345 335 345 360 375 385 400 410 70 105 140 130 135 140 145 150 160 40 305 395 385 395 410 425 440 455 465 80 130 160 145 155 160 165 175 180 45 360 445 430 445 460 480 490 510 525 90 160 180 165 175 180 190 195 205 50 425 495 480 495 510 530 545 570 585 100 185 200 185 190 200 210 215 225 55 495 545 530 545 560 585 600 625 640 110 220 220 200 210 220 230 240 245 60 570 595 575 595 610 640 655 680 700 120 250 240 220 230 240 250 260 270 65 645 645 625 645 660 690 710 740 755 130 285 260 235 250 260 270 280 290 70 730 690 670 690 715 745 765 795 815 75 820 740 720 740 765 795 820 850 875 80 910 790 765 790 815 850 875 910 930 TABLE 56 Time gap for Case C2—left or right turn (1) Design vehicle Time gap (tg) seconds Passenger car 8.0 Single-unit truck 10.0 Combination truck 12.0 NOTE: Time gaps are for a vehicle to turn right or left onto a two-lane highway with no median. The table values require adjustments for multilane highways as follows: For left turns onto two-way highways with more than two lanes, add 0.5 seconds for passenger cars or 0.7 seconds for trucks for each additional lane, from the left, in excess of one, to be crossed by the turning vehicle. For right turns, no adjustment is necessary.

vehicle, because a vehicle that turns left without stopping would need less sight distance. The Green Book criteria for sight distance along the major road to accommodate left turns is the distance traversed at the design speed of the major road in the travel time for the design vehicle as shown in Table 57. Critique of Geometric Design Criteria Because the intersection sight distance criteria in the 2001 Green Book are based on relatively recent research that explic- itly considered the sight distance needs of trucks, there does not appear to be any need to reevaluate the conceptual or the- oretical basis of these criteria at this time. These criteria should be reevaluated in the future to reflect highway agency expe- rience with their implementation. 87 RAILROAD-HIGHWAY GRADE CROSSING SIGHT DISTANCE Current Geometric Design Criteria Sight distance is provided at railroad-highway grade cross- ings to accommodate two specific scenarios: • Case A—sight distance for a moving vehicle approach- ing the grade crossing on the highway and • Case B—sight distance for a vehicle stopped on the highway approach. These cases are equivalent to the approach and departure sight triangles for intersections shown in Figure 41 and are of primary interest at railroad-highway grade crossings with- out train-activated warning devices. Sight distance design Figure 44. Intersection sight distance—Case C2—yield-controlled left or right turn (1).

criteria for these cases are presented in the Green Book, but have been adapted from two other publications (82, 83). As in the case of a highway intersection, several events can occur at a railroad-highway grade intersection without train- activated warning devices. Two of these events, which relate to determining the sight distance in the Case A scenario, are as follows: • The vehicle operator can observe the approaching train in a sight line that will allow the vehicle to pass through the grade crossing prior to the train’s arrival at the crossing. • The vehicle operator can observe the approaching train in a sight line that will permit the vehicle to be brought to a stop prior to encroachment in the crossing area. Both of these maneuvers for Case A are shown in Figure 45, based on Green Book Exhibit 9-103. The sight triangle con- sists of the two major legs (i.e., the sight distance, dH, along the highway and the sight distance, dT, along the railroad tracks). Case A of Table 58, based on Green Book Exhibit 9-104, indicates values of the sight distances for various speeds of the vehicle and the train. These distances are developed from Equation 40. This equation incorporates a driver deceleration of 3.4 m/s2 [11.2 ft/s2] for consistency with the revised stopping sight distance criteria in the 2001 Green Book. 88 The Green Book states that corrections should be made for skew crossings and highway grades that are other than flat. Case B in Table 58 contains various values of departure sight distance for a range of train speeds. When a vehicle has stopped at a railroad crossing, the next maneuver is to depart from the stopped position. The vehicle operator should have sufficient sight distance along the tracks to accelerate the vehicle and clear the crossing prior to the arrival of a train, even if the train comes into view just as the vehicle starts, as shown in Figure 46, based on Green Book Exhibit 9-105. These values are obtained from the following equation: Metric US Customary where where A = constant = 0.278 A = constant = 1.47 B = constant = 0.039 B = constant = 1.075 dH = sight-distance leg along the dH = sight-distance leg along the highway allows a vehicle highway allows a vehicle proceeding to speed Vv proceeding to speed Vv to cross tracks even though to cross tracks even though a train is observed a train is observed d V V A V t BV a D L W T T V v v = + + + +[ ]( ) 2 2 d V V A V t BV a D L W T T V v v = + + + +[ ]( ) 2 2 d AV t BV a D dH v v e= + + + 2 d AV t BV a D dH v v e= + + + 2 (40) TABLE 57 Time gap for Case F—left turns from the major road (1) Design vehicle Time gap (s) at design speed of major road (tg) Passenger car 5.5 Single-unit truck 6.5 Combination truck 7.5 Adjustment for multilane highways: For left-turning vehicles that cross more than one opposing lane, add 0.5 seconds for passenger cars and 0.7 seconds for trucks for each additional lane to be crossed. at a distance dT from at a distance dT from the the crossing or to stop crossing or to stop the the vehicle without vehicle without encroachment encroachment of the of the crossing area (ft) crossing area (m) dT = sight-distance leg along the dT = sight-distance leg along the railroad tracks to permit the railroad tracks to permit the maneuvers described as for maneuvers described as dH (ft) for dH (m) Vv = speed of the vehicle (mph) Vv = speed of the vehicle (km/h) VT = speed of the train (mph) VT = speed of the train (km/h) t = perception/reaction time, t = perception/reaction time, which is assumed to be 2.5 s which is assumed to be (This is the same value used 2.5 s (This is the same in Chapter 3 to determine the value used in Chapter 3 to stopping sight distance.) determine the stopping a = driver deceleration, which is sight distance.) assumed to be 11.2 ft/s2. a = driver deceleration, which (This is the same value used is assumed to be 3.4 m/s2 in Chapter 3 to determine (This is the same value stopping sight distance.) used in Chapter 3 to D = distance from the stop line or determine stopping sight front of the vehicle to the distance.) nearest rail, which is D = distance from the stop line assumed to be 15 ft or front of the vehicle to the de = distance from the driver to nearest rail, which is the front of the vehicle, which assumed to be 4.5 m is assumed to be 10 ft de = distance from the driver to L = length of vehicle, which is the front of the vehicle, assumed to be 65 ft which is assumed to be W = distance between outer rails 3.0 m (for a single track, this value L = length of vehicle, which is is 5 ft) assumed to be 20 m W = distance between outer rails (for a single track, this value is 1.5 m)

The Green Book states that corrections should be made for skewed crossings and for highway grades other than flat. The Green Book states that sight distances of the order shown in Table 58 are desirable at any railroad grade cross- ing not controlled by active warning devices, but that their attainment is difficult and often impractical, except in flat, open terrain. In other than flat terrain, the Green Book states that it may be appropriate to rely on speed control signs and devices and to predicate sight distance on a reduced vehicle speed of oper- ation. Where sight obstructions are present, it may be appro- priate to install active traffic control devices that will bring all highway traffic to a stop before crossing the tracks and will warn drivers automatically in time for an approaching train. The Green Book states that the driver of a stopped vehicle at a crossing should see enough of the railroad track to be able to cross it before a train reaches the crossing, even though the train may come into view immediately after the vehicle starts 89 to cross. The length of the railroad track in view on each side of the crossing should be greater than the product of the train speed and the time needed for the stopped vehicle to start and cross the railroad. The sight distance along the railroad track may be determined in the same manner as it is for a stopped vehicle crossing a preference highway, which is covered pre- viously in this chapter. In order for vehicles to cross two tracks from a stopped position, with the front of the vehicle 4.5 m [15 ft] from the closest rail, sight distances along the railroad should be determined from Equation 41 with a proper adjustment for the W value. Critique of Geometric Design Criteria Since the sight distance criteria for highway-railroad grade crossings have been revised in the 2001 Green Book to reflect the revised stopping sight distance criteria, the sensitivity analysis performed in the FHWA Truck Characteristics study is no longer current and a new sensitivity analysis has been performed. This sensitivity analysis was performed to com- pare the sight distance requirements based on the 2001 Green Book criteria and sight distances derived for trucks with anti- lock braking systems. This sensitivity analysis considered only the Case A scenario (i.e., sight distance for a moving vehicle approaching the grade crossing on the highway). Sight dis- tances were derived for three vehicle lengths. Results of the analysis are provided in Table 59. In general, the sight dis- tances derived for vehicles with antilock braking systems are slightly higher than the sight distances derived from the cur- rent stopping sight distance criteria, but the differences are small. Thus, the current sight distance criteria for railroad- highway grade crossings appear to sufficiently accommodate trucks, so there is no need to update these criteria at this time. INTERSECTION AND CHANNELIZATION GEOMETRICS Current Geometric Design Criteria A key control in the design of at-grade intersections and ramp terminals is the turning radius and path of a selected design vehicle. The following portions of the Green Book incorporate design criteria for intersections and turning road- ways that are tied directly to the turning ability of selected design vehicles: • Curvature of turning roadways and curvature at inter- sections (Green Book Chapter 3, p. 203) • Widths of turning roadways at intersections (Chapter 3, p. 223–226) • Design of roundabouts (Chapter 9, p. 581) • Minimum edge-of-traveled-way designs for turning road- ways (Chapter 9, p. 587–614) • Curb return radii (Chapter 9, p. 623–625) Metric US Customary (41) where where A = constant = 0.278 A = constant = 1.47 dT = sight distance leg along dT = sight distance leg along railroad tracks to permit the railroad tracks to permit the maneuvers described as maneuvers described as for for dH (m) dH (ft) VT = speed of train (km/h) VT = speed of train (mph) VG = maximum speed of vehicle VG = maximum speed of vehicle in first gear, which is in first gear, which is assumed to be 2.7 m/s assumed to be 8.8 fps a1 = acceleration of vehicle in a1 = acceleration of vehicle in first gear, which is first gear, which is assumed to be 0.45 m/s2 assumed to be 1.47 ft/s2 L = length of vehicle, which is L = length of vehicle, which is assumed to be 20 m assumed to be 65 ft D = distance from stop line to D = distance from stop line to nearest rail, which is nearest rail, which is assumed to be 4.5 m assumed to be 15 ft J = sum of perception and time J = sum of perception and time to activate clutch or to activate clutch or automatic shift, which is automatic shift, which is assumed to be 2.0 s assumed to be 2.0 s W = distance between outer rails W = distance between outer rails for a single track, this value for a single track, this value is 1.5 m is 5 ft or distance vehicle travels or distance vehicle travels while accelerating to while accelerating to maximum speed in first gear maximum speed in first gear V a G 2 1 2 2 8 8 2 1 47 26 3= =[ ]( . )( )( . ) . ftVaG21 22 2 72 0 45 8 1= =[ ]( . )( )( . ) . m d V a a G = 2 12 d V a a G = 2 12 d AV V a L D W d V J T T G a G = + + + − +[ ] 1 2 d AV V a L D W d V J T T G a G = + + + − +[ ] 1 2

• Turning roadways with corner islands (Chapter 9, p. 638–643) • Control radii for minimum turning paths at median openings (Chapter 9, p. 694–704) • Minimum designs for U-turns at median openings (Chap- ter 9, p 715) 90 The minimum turning radii for the current Green Book design vehicles are presented in Table 20, based on Green Book Exhibit 2-2. The Green Book establishes the minimum turning path for design trucks based on the boundaries of the outer trace of the front overhang and the sharpest turning radius of the right inner rear wheel. Minimum turning radius Figure 45. Case A: moving vehicle to safely cross or stop at railroad crossing (1).

is defined as the path of the outer front wheel, following a cir- cular arc, at a speed of less than 16 km/h (10 mph), and is lim- ited by the vehicle steering mechanism. Minimum inside radius is the path traced by the right rear wheel. Because a truck has a long wheelbase, its rear wheels do not follow the same path as its front wheels during a turn. The differences in these paths are referred to as offtracking. Off- tracking amounts vary directly with the wheelbase of a unit and inversely with the radius of turn. Swept path width, the difference in paths of the outside front tractor tire and the inside rear trailer tire, is a more appropriate parameter for design consideration. Swept path width determinations delin- eate the boundaries of the space occupied by the vehicle nego- tiating its turn. Offtracking and swept path widths are defined and discussed more fully in Chapter 4 of this report. Critique of Geometric Design Criteria Design Vehicle Changes The recommendation to drop the WB-15 [WB-50] design vehicle from the Green Book will require changes to text and exhibits in Green Book Chapter 9. The recommended changes are presented in Appendix F. Double and Triple Left-Turn Lanes The use of double left-turn lanes, and even triple left-turn lanes, is becoming more common due to increasing demand levels. Under certain conditions, double left-turn lanes accompanied with a separate left-turn signalization phase can accommodate up to approximately 180 percent of the 91 volume that can be served by a single left-turn lane with the same available green time. The Green Book states that where sufficient right-of-way, space for a long-radius turn, and a wide cross street are available, installation of double left-turn lanes may be a practical design to serve a heavy left-turn movement. The Green Book also indicates that the desirable turning radius for a double left-turn lane is 27 m [90 ft]. Exhibit 9-13A in the Green Book illustrates an inter- section configuration with double left-turn lanes for one of the left-turning movements. In this illustration, the double left-turn lanes are located within the median of the divided highway and are separated from the through lanes by either an elongated island or by pavement markings. Given that left-turn maneuvers are accomplished simultaneously from both lanes, the median opening and crossroad pavement should be sufficiently wide to receive the two side-by-side traffic streams. The Green Book provides guidance on ways to accom- modate left-turn maneuvers of various design vehicles. Exhibit 9-76 shows the paths of several design vehicles positioned as they would govern median end design for vehicles making a left turn to both leave and enter a divided highway. Exhibits 9-77 through 9-83 provide guidance on control radii from minimum practical design of median openings and indicate how each control radius design affects larger vehicles and occasional movements other than those for which the design is developed. Exhibits 9-85 and 9-87 provide additional guidance on design of median openings, and other exhibits and sections of the Green Book provide general guidance to accommodate left-turn maneu- vers at intersections. However, with the exception of indi- cating a desirable turning radius for a double left-turn lane and providing an illustration of an intersection with a double TABLE 58 Required design sight distance for combination of highway and train vehicle speeds; 20-m [65-ft] truck crossing a single set of tracks at 90 percent (1) Metric US Customary Case A Moving vehicle Case A Moving vehicle Train speed (km/h) Case B Departure from stop Vehicle speed (km/h) Train speed (mph) Case B Departure from stop Vehicle speed (mph) 0 10 20 30 40 50 60 70 80 90 100 110 130 0 10 20 30 40 50 60 70 80 Distance along railroad from crossing, dT(m) Distance along railroad from crossing, dT(ft) 10 45 39 24 21 19 19 19 19 20 21 21 22 23 120 24 10 240 146 106 99 100 105 111 118 126 20 91 77 49 41 38 38 38 39 40 41 43 45 47 48 20 480 293 212 198 200 209 222 236 252 30 136 116 73 62 57 56 57 58 60 62 64 67 70 73 30 721 439 318 297 300 314 333 355 378 40 181 154 98 82 103 77 75 116 140 163 186 209 233 256 279 302 326 116 135 155 174 193 213 251 232 271288 268 263 267 249 244 247 230 225 226 211 207 206 192 188 185 172 169 164 153 150 144 134 131 123 115 113 76 77 80 83 86 89 93 97 40 961 585 424 396 401 419 444 473 504 50 227 193 122 96 94 95 97 100 103 107 112 121 50 1201 732 530 494 501 524 555 591 630 60 272 232 147 113 120 124 129 134 145 60 1441 878 636 593 601 628 666 709 756 70 317 270 171 132 140 145 150 156 169 70 1681 1024 742 692 701 733 777 828 882 80 362 309 196 151 160 165 172 179 194 80 1921 1171 848 791 801 838 888 946 1008 90 408 347 220 170 179 186 193 201 218 90 2162 1317 954 890 901 943 999 1064 1134 100 453 386 245 189 199 207 215 223 242 110 498 425 269 208 219 227 236 246 266 120 544 463 294 227 239 248 258 268 290 130 589 502 318 246 259 269 279 290 315 140 634 540 343 265 279 289 301 313 339 Distance along highway from crossing, dH(m) Distance along highway from crossing, dH(ft) 16 26 39 54 71 90 112 137 163 192 223 256 292 71 137 222 326 449 591 753 933

92 Figure 46. Case B: departure of vehicle from stopped position to cross single railroad track (1).

left-turn lane, the Green Book does not go into further detail on the design of double left-turn lanes. The primary factor to consider in designing double left- turn lanes is vehicle offtracking or swept path width. When vehicles negotiate the turn side by side, the vehicles should not encroach on the adjacent travel lane. Because many fac- tors affect the control turning radius of double left-turn lanes, it is necessary to provide guidance on the range of offtrack- ing or swept path width of design vehicles for various turn- ing radii. The offtracking and resultant swept path widths of several design vehicles were determined for 90-deg turns with centerline turning radii of 15.2, 22.9, 30.5, and 45.7 m (50, 75, 100, and 150 ft) using AutoTURN software. It is rec- ommended that an exhibit be added to the Green Book that indicates the swept path width of several design vehicles for centerline turning radii of 22.9, 30.5, and 45.7 m (75, 100, and 150 ft). This type of exhibit will provide flexibility in designing adequate turning paths for double left-turn lanes by allowing for interpolation of swept path widths for a range of turning radii. Roundabouts In Green Book Chapter 9, there is a brief introduction to roundabouts (p. 578–583); however, no quantitative dis- 93 cussion of truck performance at roundabouts is included. No sources were found in the literature that deal specifi- cally with the issue of truck stability or rollover at round- abouts. The FHWA Roundabout Guide (84) discusses in detail the geometric design of roundabouts considering large vehicles. The discussion includes design vehicles to be considered, and references the Green Book for obtaining dimensions and turning path requirements for a variety of common highway vehicles. The Roundabout Guide indicates that for single- lane roundabouts, the size of the inscribed circle is largely dependent on the turning requirements of the design vehicle. Table 60, from the Roundabout Guide, provides recommended maximum entry design speeds for specific categories of round- abouts. These were obtained from international studies as the optimum design speeds to minimize crashes. Furthermore, the Roundabout Guide provides recommended inscribed circle diameter ranges for various site categories and design vehicles (see Table 61). With respect to superelevation, the Roundabout Guide rec- ommends, for the circulatory roadway, a cross slope of 2 per- cent away from the central island. This is recommended, among other reasons, to increase the visibility of the central island and to promote low circulating speeds. Vehicles making through- and left-turning movements however must negotiate the roundabout at negative superelevation. High speeds TABLE 59 Sensitivity analysis for sight distance along railroad from crossing (dT) and along highway from crossing (dH) Case A (Moving Vehicle) Vehicle Length = 68.5 ft (WB-62) Vehicle Length = 73.5 ft (WB-67) Vehicle Length = 77.5 ft (WB-71) Train Speed (mph) Vehicle Speed (mph) 20 30 40 50 60 70 20 30 40 50 60 70 20 30 40 50 60 70 Sight Distance Along Railroad From Crossing, dT (ft)—Current AASHTO Policy (2001) 10 108 100 101 105 111 118 110 102 102 106 112 119 112 103 103 107 113 120 20 215 200 202 210 222 237 220 203 204 212 224 238 224 206 206 214 225 239 30 323 300 302 315 334 355 330 305 306 318 336 357 336 309 309 321 338 359 40 430 399 403 421 445 473 440 406 408 425 448 476 448 411 412 428 451 478 50 538 499 504 526 556 592 550 508 510 531 560 595 560 514 515 535 564 598 60 645 599 605 631 667 710 660 609 612 637 672 714 672 617 618 642 676 718 70 753 699 705 736 779 828 771 711 714 743 784 833 785 720 721 749 789 837 80 861 799 806 841 890 946 881 812 816 849 897 952 897 823 824 856 902 957 90 968 899 907 946 1,001 1,065 991 914 918 955 1,009 1,071 1,009 926 927 962 1,015 1,076 Sight Distance Along Highway From Crossing, dH (ft)—Current AASHTO Policy (2001) 137 221 325 447 589 750 137 221 325 447 589 750 137 221 325 447 589 750 Sight Distance Along Railroad From Crossing, dT (ft)—Antilock Brake System 10 107 101 106 111 117 125 110 102 107 112 117 125 112 104 108 113 118 126 20 214 201 211 223 233 249 219 205 214 225 235 251 223 207 216 226 236 252 30 321 302 317 334 350 374 329 307 321 337 352 376 335 311 324 339 354 378 40 428 403 423 445 466 498 438 409 428 449 470 501 446 415 432 452 472 503 50 535 504 529 556 583 623 548 512 535 561 587 626 558 519 540 565 590 629 60 642 604 634 668 700 747 657 614 642 674 705 752 669 622 648 679 709 755 70 749 705 740 779 816 872 767 717 749 786 822 877 781 726 756 792 827 881 80 856 806 846 890 933 997 876 819 856 898 939 1,002 892 830 864 905 945 1,007 90 963 906 951 1,002 1,049 1,121 986 921 963 1,011 1,057 1,128 1,004 933 972 1,018 1,063 1,133 Sight Distance Along Highway From Crossing, dH (ft)—Antilock Brake System 136 224 344 478 621 793 136 224 344 478 621 793 136 224 344 478 621 793

through the roundabout can result in loss-of-load incidents for trucks; however, it is indicated in the Roundabout Guide that drivers generally expect to travel at slower speeds and will accept the higher side force caused by a reasonable superelevation rate. In summary, it is recom- mended that the Green Book section on roundabouts be expanded to incorporate the design guidelines developed in the Roundabout Guide, particularly those shown in Tables 60 and 61. CRITICAL LENGTH OF GRADE Current Geometric Design Criteria The Green Book presents the current warrant for the addi- tion of a truck climbing lane in terms of a critical length of grade. A climbing lane is not warranted if the grade does not exceed this critical length. If the critical length is exceeded, then a climbing lane is desirable and should be considered. The final decision to install a truck climbing lane may depend on several factors, but basically is determined by the reduc- tion in level of service that would occur without the addition. This reduction, in turn, is a function of the traffic volume, the percentage of trucks, the performance capabilities of the trucks, the steepness of the grade, and the length of grade remaining beyond the critical length. The critical length of grade, itself, is established by the “gradeability” of trucks. Subjectively, the critical length of grade is the “maximum length of a designated upgrade on which a loaded truck can operate without an unreasonable reduction in speed.” The Green Book considers the critical length of grade to be dependent on three factors: 94 1. The weight and power of the representative truck used as the design vehicle, which determine its speed main- tenance capabilities on grades; 2. The expected speed of the truck as it enters the critical length portion of the grade; and 3. The minimum speed on the grade below which interfer- ence to following vehicles is considered unreasonable. Based on these factors, the Green Book defines the critical length of grade as the length of grade that would produce a speed reduction of 15 km/h (10 mph) for a 120 kg/kW (200 lb/hp) truck. The 120 kg/kW (200 lb/hp) truck is intended for use for average conditions in the United States. Figure 47 illustrates speed-distance curves for deceleration of a 120 kg/kW (200 lb/hp) truck on an upgrade, as pre- sented in the Green Book. The use of a truck with a higher weight-to-power ratio is justified at sites with extremely low-powered or heavily loaded trucks in the traffic stream (e.g., in coal mining regions or near gravel quarries). Through the 1994 edition of the Green Book, critical length of grade was based on a 180-kg/kW (300-lb/hp) truck, rather than a 120-kg/kW (200-lb/hp) truck. Critique of the Geometric Design Criteria For the most part, the logical approach followed by the Green Book is well thought out. The procedures to be applied are straightforward and reasonable. Moreover, the AASHTO criteria for Factors 2 and 3 also seem reasonable. Factor 1, on the other hand, was, until recently, determined using truck performance data that were out of date. The revision from 180 to 120 kg/kW (300 to 200 lb/hp) was TABLE 60 Recommended maximum entry design speed (84) Site category Recommended maximum entry design speed Mini-Roundabout 25 km/h [15 mph] Urban Roundabout 25 km/h [15 mph] Urban Single Lane 35 km/h [20 mph] Urban Double Lane 40 km/h [25 mph] Rural Single Lane 40 km/h [25 mph] Rural Double Lane 50 km/h [30 mph] TABLE 61 Diameter of inscribed circle for roundabouts of specific site categories and design vehicles (84) Site Category Typical Design Vehicle Inscribed Circle Diameter Range* Mini-Roundabout Single-Unit Truck 13 – 25 m [45 – 80 ft] Urban Compact Single-Unit Truck/Bus 25 – 30 m [80 – 100 ft] Urban Single Lane WB-15 (WB-50) 30 – 40 m [100 – 130 ft] Urban Double Lane WB-15 (WB-50) 45 – 55 m [150 – 180 ft] Rural Single Lane WB-20 (WB-67) 35 – 40 m [115 – 130 ft] Rural Double Lane WB-20 (WB-67) 55 – 60 m [180 – 200 ft] * Assumes 90° angles between entries and no more than four legs.

95 Figure 47. Speed-distance curves for a typical heavy truck of 120 kg/kW [200 lb/hp] for deceleration of upgrades (1).

based on judgment, rather than actual field data and, there- fore, merits closer review. Specific comments on the AASHTO criteria are presented below. Unreasonable Interference with Following Vehicles The amount of speed reduction used as the criterion for Fac- tor 3 in determining the critical length of grade is based on its expected effect on the accident involvement rate of trucks. It is argued, based on known effects of speed differences between vehicles on accident rates, that any speed difference will increase accident rates to some extent. The amount of this increase that is “reasonable” has been determined through engineering judgment. The 1965 Blue Book used a 24-km/h (5-mph) speed reduction for critical length of grade; this was changed to the more conservative 16-km/h (10-mph) speed reduction in the 1984 Green Book and has been retained since. Speed at Entrance to the Critical Length of Grade The Green Book points out, properly, that the speed of trucks on a grade depends, in part, on their speed on entering the grade. It is reasonable to use the average running speed if the entrance is on level terrain. The chart for critical length of grade presented in Figure 48, based on Green Book Exhibit 3- 63, is based on a truck speed entering the grade of 110 km/h (70 mph). However, if the upgrade in question is immediately preceded by a previous upgrade, the truck speed may already be depressed, which should be accounted for. Similarly, it is commonly known that truck drivers will accelerate somewhat on a downgrade immediately preceding an upgrade, to get a “running start” at it. In that case, the critical length of grade will be longer than with a level entrance. It would be desir- able to provide designers with the capability to readily con- sider more than one value of entrance speed. Design Vehicle Field study results presented in Appendix D indicate that the 85th-percentile truck weight-to-power ratios range from 102 to 126 kg/kW (170 to 210 lb/hp) for the truck population on freeways and 108 to 168 kg/kW (180 to 280 lb/hp) for the truck population on two-lane highways. The available data suggest that truck performance is better for the freeway truck population than for the two-lane highway truck population and is better for the truck population in Western states than in Eastern states. Final Climbing Speeds The most common measure used to quantify truck perfor- mance on grades is the final climbing speed. This is the ulti- 96 mate, slowest speed (the crawl speed) that the truck would be reduced to if the grade were sufficiently long. It is often reported in the literature or used in making comparisons between different vehicles. It is a useful measure for exam- ining capacity, for example, on very long grades where trucks are actually reduced to their final climbing speeds. However, the important parameter in determining the criti- cal length of grade is the distance required for the first 15 km/h (10 mph) of speed reduction on the grade. However, the final climbing speed or crawl speed of a truck can be used to estimate the truck’s weight-to-power ratio and thereby determine the distance required for a 15-km/h (10- mph) speed reduction. The relationship between truck speed profiles on specified grades and truck weight-to-power ratios can be made most readily with truck performance equations like those used in the TWOPAS computer simulation model (85,86). The research has developed a Microsoft Excel spreadsheet, known as the Truck Speed Performance Model (TSPM), to apply the TWOPAS performance equations for trucks. This spread- sheet can be used to plot the speed-distance profile for a truck based on the following: • Truck weight-to-power ratio, • Vehicle profile of the roadway (percent grade and points of change), and • Initial speed of the truck at the foot of the grade. Aerodynamic drag forces on the truck are accounted for based on the elevation of the site above sea level. Figure 49 presents an example of a truck speed profile on a grade developed with the TSPM spreadsheet. This spread- sheet is recommended for use as a design tool because, unlike Figures 47 and 48 used in the current Green Book, the TSPM spreadsheet is sensitive to the site-specific truck entrance speed, the estimated site-specific weight-to power ratios of trucks, and the actual vertical profile of the site, rather than an assumed constant grade. Figures 47 and 48 may be retained in the Green Book as examples, but the TSPM spreadsheet will provide a more useful tool for considering actual site conditions. DOWNGRADES Any vehicle, when traveling on a downgrade, loses poten- tial energy because of its loss of elevation. This loss is equal to the product of its weight and its elevation descent. If there were no losses such as aerodynamic or rolling drag, and no braking, all of this energy would be converted to an increase in kinetic energy, expressed as 0.5 MV2, where M is the vehi- cle mass and V is its speed. Fortunately, aerodynamic and rolling losses absorb some of this energy, but not all. (In pas- senger cars, these drag forces often can absorb most of the potential energy change, perhaps augmented by some mod- est braking on all but the most severe grades.)

97 Figure 48. Critical lengths of grade for design, assumed typical heavy truck of 120 kg/kW [200 lb/hp], entering speed = 110 km/h [70 mph] (1).

The major dissipater of excess energy in trucks is normally its brakes. The energy absorbed by the brakes is converted into heat, raising the temperature of the brake linings, brake drums/disks, and wheel assemblies. Their temperatures are commonly raised to 500 or 600°F or more. This heat, in turn, is dissipated to the surrounding air, primarily via convection, through fins and other means designed to be effective heat dissipaters. However, if the heat is not dissipated rapidly enough, and the brake temperature rises above some thresh- old, the brakes are said to become overheated, and they can no longer absorb energy at the same rate. Under these circum- stances, the truck will begin to gain speed. The truck driver must anticipate this situation, by select- ing a lower gear ratio. This helps in two ways. In a lower gear ratio, the truck engine can absorb more energy per unit dis- tance traveled. In addition, by selecting a lower gear ratio, the truck will be traveling at a lower speed, V, and thus reduce its needs to absorb energy as rapidly. FHWA has funded research regarding means of providing warning information to drivers (87,88). Trucks, when “in gear,” can absorb large amounts of energy because of engine drag. Many truck operators who frequently travel in hilly or mountainous terrain use special engine brakes such as the Jacobs engine brake, known as a Jake Brake. These devices enable the engine’s valve timing to be modified so these devices act as large air compressors, absorbing even more power. However, they can only operate through the drive wheels connected to the engine. They are quite effective on trucks such as tractor-semitrailer configurations, where two of the five axles are driven. However, they are much less effec- 98 tive on twin-trailer configurations (e.g., 2S-1-2 combinations) where only one of the five axles is driven. There are two major impacts of truck downhill perfor- mance on highway design. First, where trucks should use lower gears, and thus lower “crawl” speeds, they may be traveling significantly slower than the rest of the traffic. If such regions are very long, or if there are not significant passing opportunities on two-lane roads for the other down- grade traffic, consideration might be given to adding a downgrade passing lane. The second potential impact is to provide for trucks whose drivers did not initially select a low enough gear ratio to enable them to maintain vehicle control on the downgrade. If a driver, early on the down- grade section, wishes to change to a lower gear ratio, he can brake to reduce speed, then downshift the transmission. However, if the brakes are already overheated from overusage, they may not be able to slow the truck further, so downshifting is no longer possible. In this situation, the driver can only hope that horizontal curvature and other traffic enable him to avoid an accident by steering the vehi- cle as it gains speed; another option is for the driver to intentionally leave the roadway to avoid becoming a “run- away.” To provide assistance to drivers in this situation, emergency escape ramps are sometimes added by the high- way agency. The Green Book provides information on the design of emergency escape ramps, but not on specific war- rants for specific criteria for placement of such ramps. Guidance on the issues of avoiding runaway trucks and providing emergency escape ramps is addressed by Allen et al. (89) and by Abdelwahab and Morrall (90). In particular, Allen et al. (89) provide a recommended procedure for TRUCK SPEED PROFILE FOR ROUTE 3 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 0.0 1000.0 2000.0 3000.0 4000.0 5000.0 6000.0 7000.0 Distance (ft) Sp ee d (m ph ) Figure 49. Example of truck speed profile plot from TSPM spreadsheet.

analysis of truck performance on downgrades that was rec- ommended for incorporation in FHWA’s Interactive High- way Safety Design Model (IHSDM). This procedure is based on four speed criteria: 1. The maximum speed at which the specified truck can descend the specified grade without losing braking ability; 2. The maximum speed at which the specified truck can descend the specified grade without rolling over on a horizontal curve; 3. The maximum speed at which the specified truck can descend the specified grade without losing the ability to brake safely to a stop using a deceleration rate of 3.4 m/s2 (11.1 ft/s2) or more; and 4. The maximum speed at which the specific truck can descend the specified grade without losing the ability to slow to the appropriate desired speed for any horizon- tal curve. Criteria 1 and 2 are safety criteria that represent the thresholds at which accidents are expected. Speeds higher than the speed for Criterion 1 would be expected to result in loss of braking control (i.e., a runaway truck). Speeds higher than Criterion 2 would be expected to result in a truck rollover. Criteria 3 and 4 are more conservative and represent thresholds for good design that do not approach impending loss of control. Criterion 3 ensures that a truck will be able to brake to a stop using a deceleration rate of not more than 3.4 m/s2 (11.1 ft/s2), the deceleration rate assumed in the current Green Book design criteria for stopping sight dis- tance design (1). Criterion 4 ensures that the truck will not only not roll over on a horizontal curve, but also will be able to traverse each curve on the grade at the speed that drivers normally select for such curves when they are not on a downgrade. The recommended truck operating speed for the grade is the lesser of the speeds determined for Criteria 3 and 4. The appropriateness of the recommended truck operating speed can also be judged by the magnitude of its margin of safety with respect to the loss-of-control speed (i.e., the lower of the speeds determined with Criteria 1 and 2). To judge the acceptability of the downgrade design, the designer must assess whether, with appropriate warning signs, it is reasonable to expect truckers to slow to the rec- ommended truck operating speed before leaving the tope of the grade. Appropriate models can then be used to eval- uate the location on the downgrade at which loss of safety margin, based on Criterion 3 or 4, would be expected and the location at which loss of control, based on Criterion 1 or 2, would be expected for various entering truck speeds. The recommended methodology for downgrade analysis to determine potential locations for emerging escape ramps is as follows (89): 99 • Step 1—Select a suitable truck for use as the design vehicle for downgrade analysis. If recreational vehicles are present in substantial numbers on the downgrade (e.g., 5 percent of the traffic stream or more), a suitable recreational vehicle should also be selected for analysis. • Step 2—Determine the speeds designated by Criteria 1 through 4. Determine the recommended truck operating speed and the margin of safety to the loss-of-control speed. • Step 3—Assess whether the recommended truck operat- ing speed will be maintained by the vast majority of truck drivers. This assessment could be made with formal risk assessment logic based on further research, or it could be left to the judgment of the designer. • Step 4—Modify the geometrics of the downgrade if nec- essary and feasible. This could involve using less steep slopes, flattening horizontal curves, or both. • Step 5—If the recommended truck operating speed is deemed too low and it is physically or economically infeasible to modify the geometrics of the downgrade, the loss-of-control locations and the speed profiles fol- lowing loss of control can be used to identify potential sites for emergency escape ramps. The speed profile data can also be used to anticipate potential truck entry speeds to the emergency escape ramp. The truck entry speed is an important design parameter in determining the required length of the ramp. While the procedures recommended by Allen et al. (89) for locating emergency escape ramps would be a desirable addi- tion to the Green Book, speed prediction models for imple- menting the procedure have not yet been developed. Allen et al. (89) present a plan for modifying the existing TWOPAS and VDANL models to provide suitable speed profiles for trucks on downgrades. However, these recommended model revisions have not yet been implemented. Therefore, the inclu- sion in the Green Book of the procedure presented above would be premature. ACCELERATION LANES Current Geometric Design Criteria Acceleration lanes are speed-change lanes that provide suf- ficient distance for vehicles to accelerate to near highway speeds before entering the through lanes of a highway. Accel- eration lane length is measured from the point where the left edge of the traveled way of the ramp joins the traveled way of the through roadway to the beginning of the downstream taper. Table 62 presents the Green Book design values for accel- eration lane length. Table 63 presents adjustment factors to those values that are applied to provide longer acceleration lanes on upgrades. The Green Book states that, to aid truck acceleration, high-speed entrance ramps should desirably be located on descending grades and that longer acceleration

100 TABLE 62 Minimum acceleration lengths for entrance terminals with flat grades of 2 percent or less (1) TABLE 63 Speed change lane adjustment factors as a function of grade (1) Metric US Customary Acceleration lanes Acceleration lanes Design speed of highway (km/h) Ratio of length on grade to length of level for design speed of turning curve (km/h)a Design speed of highway (mph) Ratio of length on grade to length of level for design speed of turning curve (mph)a 40 50 60 70 80 All speeds 20 30 40 50 All speeds 3 to 4% upgrade 3 to 4% downgrade 3 to 4% upgrade 3 to 4% downgrade 60 1.3 1.4 1.4 – – 0.7 40 1.3 1.3 – – 0.7 70 1.3 1.4 1.4 1.5 – 0.65 45 1.3 1.35 – – 0.675 80 1.4 1.5 1.5 1.5 1.6 0.65 50 1.3 1.4 1.4 – 0.65 90 1.4 1.5 1.5 1.5 1.6 0.6 55 1.35 1.45 1.45 – 0.625 100 1.5 1.6 1.7 1.7 1.8 0.6 60 1.4 1.5 1.5 1.6 0.6 110 1.5 1.6 1.7 1.7 1.8 0.6 65 1.45 1.55 1.6 1.7 0.6 120 1.5 1.6 1.7 1.7 1.8 0.6 70 1.5 1.6 1.7 1.8 0.6 5 to 6% upgrade 5 to 6% downgrade 5 to 6% upgrade 5 to 6% downgrade 60 1.5 1.5 – – – 0.6 40 1.5 1.5 – – 0.6 70 1.5 1.6 1.7 – – 0.6 45 1.5 1.6 – – 0.575 80 1.5 1.7 1.9 1.8 – 0.55 50 1.5 1.7 1.9 – 0.55 90 1.6 1.8 2.0 2.1 2.2 0.55 55 1.6 1.8 2.05 – 0.525 100 1.7 1.9 2.2 2.4 2.5 0.5 60 1.7 1.9 2.2 2.5 0.5 110 2.0 2.2 2.6 2.8 3.0 0.5 65 1.85 2.05 2.4 2.75 0.5 120 2.3 2.5 3.0 3.2 3.5 0.5 70 2.0 2.2 2.6 3.0 0.5 a Ratio from this table multiplied by the length in Table 54 gives length of speed change lane on grade.

lanes should be provided on elevated freeways where entrance ramps must necessarily incorporate upgrades. Critique of Geometric Design Criteria An evaluation of Table 62 was conducted using the truck speed profile model (TSPM), described in Appendix E, to determine the weight-to-power ratios implied by the design values. To simplify the following discussion, all quantities are presented in U.S. customary units only. Since Table 62 per- tains to grades of 2 percent or less, separate analyses were con- ducted for level (0 percent) grades and grades of 2 percent. Table 64 indicates the maximum weight-to-power ratio of a truck capable of achieving the given conditions as specified in Table 62, assuming a 0 percent grade. For example, Table 62 specifies that given the design speed of the highway is 30 mph and vehicles enter the acceleration lane from a stopped condi- tion, the minimum acceleration lane length is 180 ft. Table 62 also specifies that vehicles are assumed to accelerate to a speed 101 of 23 mph over this 180-ft distance. Table 64 indicates that a truck with a maximum weight-to-power ratio of 105 lb/hp is capable of accelerating from an initial speed of 0 mph to a final speed of 23 mph over a distance of 150 ft on a level (0 percent) grade. Similarly, given a highway design speed of 30 mph, an initial speed of 14 mph, a final speed of 23 mph, and an accel- eration lane length of 140 ft (as specified in Table 62) on a 0 percent grade, Table 64 indicates that a truck with a maximum weight-to-power ratio of 140 lb/hp can achieve these given conditions. Table 65 indicates the maximum weight-to-power ratios of vehicles able to achieve the given conditions assum- ing minimum acceleration lane lengths (as specified in Table 62) and a constant grade of 2 percent. Table 64 indicates that trucks with weight-to-power ratios in the range of 100 to 145 lb/hp have sufficient acceleration capabilities to achieve the given speeds within the minimum acceleration lengths, assuming a 0 percent grade. However, if the acceleration lanes have grades even as low as 2 percent, Table 65 indicates that trucks with weight-to-power ratios in TABLE 64 Maximum weight-to-power ratios for minimum acceleration lengths (0 percent grades) Maximum weight-to-power ratio (lb/hp) capable of reaching Va given V ′ a for 0 percent grades over acceleration lengths as specified in Table 62 Highway Stop condition 15 20 25 30 35 40 45 50 And initial speed, V a (mph) Design speed, V (mph) Speed reached, Va (mph) 0 14 18 22 26 30 36 40 44 30 23 105 140 – – – – – – – 35 27 110 120 130 – – – – – – 40 31 105 115 120 125 120 – – – – 45 35 120 120 125 135 135 135 – – – 50 39 120 120 120 120 120 125 145 – – 55 43 120 120 115 120 120 120 120 130 – 60 47 110 115 115 115 110 115 110 120 130 65 50 110 110 110 110 110 115 110 110 110 70 53 105 105 105 105 105 105 105 105 105 75 55 105 105 100 100 105 105 105 105 100 ′ TABLE 65 Maximum weight-to-power ratios for minimum acceleration lengths (2 percent grades) Maximum weight-to-power ratio (lb/hp) capable of reaching Va given V′ a for 2 percent grades over acceleration lengths as specified in Table 62 Highway Stop condition 15 20 25 30 35 40 45 50 And initial speed, V′a (mph) Design speed, V (mph) Speed reached, Va (mph) 0 14 18 22 26 30 36 40 44 30 23 65 110 – – – – – – – 35 27 65 100 100 – – – – – – 40 31 80 85 95 95 100 – – – – 45 35 90 95 95 100 100 100 – – – 50 39 90 90 90 95 95 95 110 – – 55 43 90 90 85 90 90 90 90 100 – 60 47 85 85 85 85 85 85 85 85 90 65 50 85 85 80 80 80 80 80 80 80 70 53 80 80 80 80 80 80 80 80 75 75 55 80 80 80 80 80 80 80 80 75

the range of 65 to 110 lb/hp have sufficient acceleration capa- bilities to achieve the given speeds within the minimum accel- eration lengths. Considering that the 2001 Green Book indi- cates a 200-lb/hp truck is representative of the size and type of vehicle normally used for design control of major high- ways and that current field data indicate that on the free- ways the 85th percentile weight-to-power ratios of trucks falls within a fairly narrow range around 170 to 210 lb/hp (see Appendix D), this analysis indicates that the underlying assumptions for estimating the minimum acceleration lengths in Table 62 do not necessarily account for the performance capabilities of heavily loaded vehicles. It appears that the cur- rent Green Book criteria can accommodate an average truck, but not a heavily loaded truck. The TSPM once again was used to determine the minimum acceleration lengths required to enable a 180-lb/hp vehicle to reach the given conditions as specified in Table 62. Table 66 presents the minimum acceleration lengths assuming a 0 per- cent grade for the acceleration lane. Table 66 indicates that a 180 lb/hp truck can accelerate from an initial speed of 0 mph to a final speed of 23 mph over a distance of 275 ft on a level (0 percent) grade. The minimum acceleration lengths given in Table 66 are, on average, about 1.8 times greater than the min- imum acceleration lengths given in Table 62. Although the sensitivity analysis presented here indicates a potential need to increase acceleration lengths to accommo- date heavily loaded trucks better, no accident data indicate that trucks have difficulties with acceleration lanes designed according to the current criteria. In addition, some of the lengths given in Table 66 are rather long, with the extreme case requiring a minimum acceleration length on the order of 0.6 mi. Therefore, no change to the current Green Book cri- teria is recommended at this time. However, future research should investigate truck-related accidents near acceleration lanes. If this future research should find that trucks have diffi- culties with acceleration lanes as currently designed, the 102 design values in Table 62 should be increased to reflect the greater lengths as provided in Table 66, or a compromise should be reached for economic purposes. In addition, Table 63 would also require modification. DECELERATION LANES The Green Book design criteria for deceleration lanes are intended to provide sufficient distance for vehicles to slow from the speed of the major roadway to appropriate speed for any horizontal curve that may be located on the ramp. Such speed changes are normally made with controlled decelera- tion rates of which trucks are clearly capable. There is con- cern that trucks may skid or roll over on ramp curves if the truck is traveling substantially faster than the design speed of the curve (see subsequent discussion of horizontal curve design). However, there is no indication that driver choice of faster operating speeds is the result of short deceleration lanes or is correctable by using longer deceleration lanes. There- fore, no changes in the current Green Book design criteria for deceleration lane length are recommended. LANE WIDTH Current Geometric Design Criteria The Green Book encourages the use of 3.6-m [12-ft] lanes for all but the lowest volume highways. In particular, on rural arterials, lane widths less than 3.6 m [12 ft] are normally used only for roads with design speeds less than 100 km/h [60 mph] and average daily traffic (ADT) less than 1,500 veh/day or design speeds less than 80 km/h [50 mph] and ADTs less than 2,000 veh/day (see Green Book Exhibit 7-3). For urban arteri- als, the AASHTO Green Book states that 3.0-m [10-ft] lanes should be used only in highly restricted areas having little or TABLE 66 Minimum acceleration lengths for a 180 lb/hp truck Acceleration length, L (ft), necessary for entrance curve to enable a 180 lb/hp truck to reach Va given V′ a for a 0 percent grade Highway Stop condition 15 20 25 30 35 40 45 50 And initial speed, V′ a (mph) Design speed, V (mph) Speed reached, Va (mph) 0 14 18 22 26 30 36 40 44 30 23 275 160 – – – – – – – 35 27 400 300 230 – – – – – – 40 31 590 475 400 310 170 – – – – 45 35 800 700 630 540 400 240 – – – 50 39 1100 1020 950 850 720 560 200 – – 55 43 1510 1400 1330 1230 1100 920 580 240 – 60 47 2000 1900 1830 1740 1600 1430 1070 760 330 65 50 2490 2380 2280 2230 2090 1920 1560 1220 800 70 53 3060 2960 2900 2800 2670 2510 2140 1810 1260 75 55 3520 3430 3360 3260 3130 2960 2590 2290 1850

no truck traffic. However, both 3.3- and 3.6-m [11- and 12-ft] lane widths are used extensively on urban arterials. The AASHTO Green Book does encourage wider lanes to accommodate trucks on some turning roadways at intersec- tions and some horizontal curves. These issues are discussed later in this section. Critique of Geometric Design Criteria The lane width criteria in the AASHTO Green Book were established without reference to any explicit vehicle width specification. However, it is implicit in the criteria that the need for 3.3- and 3.6-m [11- and 12-ft] lanes is based on the consideration of truck width. Two older studies have addressed the operational effects of wider vehicles and the implications of these effects for high- way design. A joint NHTSA-FHWA assessment conducted in 1973 compared the operational effects of 2.4- and 2.6-m [8.0- and 8.5-ft] wide buses on two-lane, four-lane, six-lane, and eight-lane highways based on research reported in the literature (91,92). This research found no effect of bus width on the lateral placement of adjacent cars, regardless of high- way type and ambient wind conditions. There was a shift in the lateral position of cars by 300 to 460 mm [12 to 18 in.] when a bus was present, but the magnitude of this shift did not vary between 2.4- and 2.6-m [8.0- and 8.5-ft] wide buses. A 1982 FHWA study of the effects of truck width on the positions of adjacent vehicles found no adverse effects of increased truck width either in passing maneuvers or at nar- row bridges (93). The passing maneuver studies were con- ducted on a two-lane highway with lane widths that varied from 3.2 to 3.6 m [10.5 to 12 ft]. Vehicle widths of 2.4, 2.6, 2.7, and 2.9 m [8.0, 8.5, 9.0, and 9.5 ft] were varied by chang- ing the width of a fabricated wood and aluminum box on the trailer. The lateral position of the passing vehicle moved fur- ther to the left as the truck width increased, but there was no effect of truck widths on shoulder encroachments in passing maneuvers, which were observed consistently in about 6 per- cent of the passes. In studies at a narrow bridge on a two-lane highway with 3.5-m [11.5-ft] lanes, there was no effect of truck width on the speed or lateral placement of oncoming vehicles. Research has shown a definite relationship between lane width and safety on two-lane roads (94, 95). However, there is no indication in this research that the observed effect relates directly to truck widths. Rearward amplification, dis- cussed in Chapter 5, refers to amplification of the magnitude of steering corrections in the rear trailers of multitrailer truck combinations. There is no indication that rearward amplifi- cation of sufficient magnitude to require lane widths greater than 3.3 to 3.6 m [11 to 12 ft] occurs with sufficient fre- quency that wider lanes are needed. 103 HORIZONTAL CURVE RADIUS AND SUPERELEVATION This section of the report examines the role of truck con- siderations in the design of horizontal curves. Pavement widening on horizontal curves is addressed in the next section. Current Geometric Design Criteria The current design criteria for horizontal curves are estab- lished in the AASHTO Green Book. Under the AASHTO pol- icy, a vehicle on a horizontal curve is represented as a point mass. From the basic laws of physics, the lateral acceleration of a point mass traveling at constant speed on a circular path can be represented by the relationship: (42) where a = lateral acceleration (g) V = vehicle speed (mph) R = radius of curve (ft) The lateral acceleration experienced by the vehicle is expressed in units of the acceleration of gravity (g), which are equal to 9.8 m/s2 [32.2 ft/s2]. On a superelevated curve, the superelevation offsets a portion of the lateral accelera- tion, such that (43) where anet = unbalanced portion of lateral acceleration (g) e = superelevation (ft/ft) The unbalanced portion of the lateral acceleration vehicle is a measure of the forces acting on the vehicle that tend to make it skid off the road or overturn. The side frictional demand of the vehicle is mathematically equivalent to the unbalanced lat- eral acceleration (anet). For this reason, Equation 43 appears in the AASHTO Green Book in the following form: (44) where f = side friction demand The tendency of the vehicle to skid off the road must be resisted by tire/pavement friction. The vehicle will skid off the road, unless the tire/pavement friction coefficient exceeds the side friction demand. However, it is also critical for safe f V15R e 2 = − a V 15R enet 2 = − a V 15R 2 =

vehicle operations that vehicles not rollover on horizontal curves. The tendency of the vehicle to overturn must be resisted by the roll stability of the vehicle. The vehicle will roll over unless the rollover threshold of the vehicle exceeds the unbalanced lateral acceleration (anet). Selection of Radius and Superelevation The objective of Green Book criteria for horizontal curve design is to select the radius and superelevation so that the unbalanced lateral acceleration is kept within tolerable limits. The Green Book criteria limit the unbalanced lateral accelera- tion for horizontal curves to a maximum of 0.175 g at 24 km/h [15 mph] decreasing to a maximum of 0.08 g at 129 km/h [80 mph]. This limitation is based on the results of research performed from 1936 through 1949 that established 0.17 g as the maximum unbalanced lateral acceleration at which dri- vers felt comfortable. Thus, these AASHTO criteria are based on maintaining comfort levels for passenger car drivers. The AASHTO criteria are not based explicitly on estimates of available tire/pavement friction levels or vehicle rollover thresholds, although it was assumed implicitly that available friction levels and rollover thresholds were higher than the specified driver comfort levels. The Green Book provides design charts for maximum superelevation rates (emax) from 4 to 12 percent. Highway agencies have established their own policies concerning the maximum superelevation rate that will be used on horizon- tal curves under their jurisdiction. Most highway agencies use maximum superelevation rates of either 6 or 8 percent. States that experience snow and ice conditions typically use lower superelevation rates. For any particular maximum superelevation rate and maximum side friction demand, the minimum radius of curvature can be determined as follows: (45) where Rmin =minimum radius of curvature (ft) Vd = design speed of curve (mph) emax = specified maximum superelevation rate (ft/ft) fmax = specified maximum side friction demand Table 67, based on Green Book Exhibit 3-14, presents the minimum radius of curvature for specific combinations of maximum superelevation rate and maximum side friction demand considered in the Green Book. In the design of a horizontal curve under the Green Book policy, the first major decision is to select its radius of curva- ture. Next, the selected radius is checked to ensure that it is not less than Rmin for the design speed of the highway. Finally, if the selected radius is greater than Rmin, a superelevation less than emax is selected using Exhibits 3-21 through 3-25 of the Green Book. R V15(e f d 2 max max min )= + 104 Transition Design Most horizontal curves are circular curves that directly adjoin tangent roadway sections at either end with no tran- sition curve. Thus, a vehicle entering a curve theoretically encounters an instantaneous increase in lateral acceleration from a minimal level of the tangent section to the full lateral acceleration required to track the particular curve. The oppo- TABLE 67 Minimum radius for design of rural highways, urban freeways, and high- speed urban streets using limiting values of e and f (1) US Customary Design Speed (mph) Maximum e (%) Limiting Values of f Total (e/100 + f) Calculated Radius (ft) Rounded Radius (ft) 15 4.0 0.175 0.215 70.0 70 20 4.0 0.170 0.210 127.4 125 25 4.0 0.165 0.205 203.9 205 30 4.0 0.160 0.200 301.0 300 35 4.0 0.155 0.195 420.2 420 40 4.0 0.150 0.190 563.3 565 45 4.0 0.145 0.185 732.2 730 50 4.0 0.140 0.180 929.0 930 55 4.0 0.130 0.170 1190.2 1190 60 4.0 0.120 0.160 1505.0 1505 15 6.0 0.175 0.235 64.0 65 20 6.0 0.170 0.230 116.3 115 25 6.0 0.165 0.225 185.8 185 30 6.0 0.160 0.220 273.6 275 35 6.0 0.155 0.215 381.1 380 40 6.0 0.150 0.210 509.6 510 45 6.0 0.145 0.205 660.7 660 50 6.0 0.140 0.200 836.1 835 55 6.0 0.130 0.190 1065.0 1065 60 6.0 0.120 0.180 1337.8 1340 65 6.0 0.110 0.170 1662.4 1660 70 6.0 0.100 0.160 2048.5 2050 75 6.0 0.090 0.150 2508.4 2510 80 6.0 0.080 0.140 3057.8 3060 15 8.0 0.175 0.255 59.0 60 20 8.0 0.170 0.250 107.0 105 25 8.0 0.185 0.245 170.8 170 30 8.0 0.160 0.240 250.8 250 35 8.0 0.155 0.235 348.7 350 40 8.0 0.150 0.230 465.3 465 45 8.0 0.145 0.225 502.0 500 50 8.0 0.140 0.220 760.1 760 55 8.0 0.130 0.210 963.5 965 60 8.0 0.120 0.200 1204.0 1205 65 8.0 0.110 0.190 1487.4 1485 70 8.0 0.100 0.180 1820.9 1820 75 8.0 0.090 0.170 2213.3 2215 80 8.0 0.080 0.160 2675.6 2675 15 10.0 0.175 0.275 54.7 55 20 10.0 0.170 0.270 99.1 100 25 10.0 0.165 0.265 157.8 160 30 10.0 0.160 0.280 231.5 230 35 10.0 0.155 0.255 321.3 320 40 10.0 0.150 0.250 428.1 430 45 10.0 0.145 0.245 552.9 555 50 10.0 0.140 0.240 696.8 695 55 10.0 0.130 0.230 879.7 880 60 10.0 0.120 0.220 1094.6 1095 65 10.0 0.110 0.210 1345.8 1345 70 10.0 0.100 0.200 1838.8 1840 75 10.0 0.090 0.190 1980.3 1980 80 10.0 0.080 0.180 2378.3 2380 15 12.0 0.175 0.295 51.0 50 20 12.0 0.170 0.290 92.3 90 25 12.0 0.165 0.285 146.7 145 30 12.0 0.160 0.280 215.0 215 35 12.0 0.155 0.275 298.0 300 40 12.0 0.150 0.270 396.4 395 45 12.0 0.145 0.265 511.1 510 50 12.0 0.140 0.260 643.2 845 55 12.0 0.130 0.250 809.4 810 60 12.0 0.120 0.240 1003.4 1005 65 12.0 0.110 0.230 1228.7 1230 70 12.0 0.100 0.220 1489.8 1490 75 12.0 0.090 0.210 1791.7 1790 80 12.0 0.080 0.200 2140.5 2140 NOTE: In recognition of safety considerations, use of emax = 4.0% should be limited to urban conditions.

site occurs as a vehicle leaves a horizontal curve. In fact, there is a gradual rather than an instantaneous change in lateral acceleration, because drivers steer a spiral or transition path as they enter or leave a horizontal curve. The design of the superelevation transition section is used to partially offset the changes in lateral acceleration that do occur. First, a super- elevation runout section is used on the tangent section to remove the adverse crown slope. Next, a superelevation runoff section is provided in which the pavement is rotated around its inside edge to attain the full required superelevation; typical design practice is to place two-thirds of the superelevation runoff on the tangent approach and one-third on the curve. The Green Book encourages the use of spiral transition curves to provide a smooth transition between tangents and circular curves. In a spiral transition curve, the degree of curvature varies linearly from zero at the tangent end to the degree of the circular arc at the circular curve end. The length of the spiral transition curve can be made the same as the superelevation runoff, so that the degree of curvature and pavement cross slope change together. Critique of Geometric Design Criteria Consideration of Friction Demand The point mass representation of a vehicle that forms the basis for Equations 42, 43, and 44 is not based on any par- 105 ticular set of vehicle characteristics and is theoretically as applicable to trucks as to passenger cars. However, in light of the differences between passenger cars and trucks in size, number of tires, tire characteristics, and suspension charac- teristics, the suitability of the equations for trucks was recently reexamined. A 1985 FHWA study by MacAdam et al. (96) found that, given that the basic laws of physics apply to both passenger cars and trucks, the point mass representation in Equation 44 can be used to determine the net side friction demand of both passenger cars and trucks. However, they found that while the friction demands at the four tires of a passenger car are approximately equal, the friction demands at the various tires of a tractor-trailer truck vary widely, as illustrated in Figure 50. The net result of this tire-to-tire variation in friction demand is that trucks typically demand approximately 10 percent higher side friction than passenger cars. The FHWA Truck Charac- teristics study termed this higher side friction demand the effective side friction demand of trucks. The point mass representation of a vehicle has another weakness, however, that applies to both passenger cars and trucks. Equation 42 is based on the assumption that vehicles traverse curves following a path of constant radius equal to the radius of the curve. However, field studies have shown that all vehicles oversteer at some point on a horizontal curve. At the point of oversteering, the vehicle is following a path radius that is less than the radius of the curve (97). Thus, at Figure 50. Example of variation in side friction demand between wheels of a truck on a horizontal curve (96).

some point on each curve, the friction demand of each vehicle will be slightly higher than suggested by Equation 44. Over- steering by passenger cars is not considered in the AASHTO design policy for horizontal curves, but it is probably not critical because the AASHTO maximum lateral acceleration requirements are based on driver comfort levels rather than the available pavement friction. No data are available on the amount of oversteering by trucks relative to passenger cars. Consideration of Rollover Threshold As demonstrated above, AASHTO criteria for horizontal curve design do not explicitly consider vehicle rollover thresh- olds. The rollover threshold for passenger cars may be as high as 1.2 g, so a passenger car will normally skid off a road long before it would roll over. Thus, the consideration of rollover threshold is not critical for passenger cars. However, tractor- trailer trucks have relatively high centers-of-gravity and con- sequently tend to have low rollover thresholds. Furthermore, because of suspension characteristics, the rollover threshold of tractor-trailer trucks is substantially less than it would be if a truck were a rigid body. Recent research, summarized in Chapter 5 of this report, has determined that the rollover thresholds of most trucks are greater than or equal to 0.35 g. Given that AASHTO design policy permits lateral acceleration as large as 0.17 g on hor- izontal curves, the margin of safety for trucks is typically at least 0.18 g. As discussed above, oversteer will generally result in a lateral acceleration greater than fmax at some point on the curve for vehicles traveling at the design speed. As an example of truck operations on horizontal curves, Figure 51 presents the distribution of nominal side friction 106 demand for trucks from combined data on four curves in the Chicago area as part of a NHTSA study (98). The radii of the four curves range from 67 to 256 m [220 to 840 ft] and the superelevations range from 0.02 to 0.088. The distribution in Figure 51 was developed by measuring truck speeds on the curve and calculating the lateral acceleration for each truck from the known radius and superelevation using Equation 43. The figure illustrates that trucks generating lateral acceler- ations above 0.30 g are observed, and the lateral accelerations for some trucks range as high as 0.40 g. No generalizations should be drawn from these data, because they represent only four particular horizontal curves, but they do illustrate that levels of side friction demand capable of producing rollovers for some trucks can occur. Sensitivity Analyses Sensitivity analyses have been conducted to determine whether the existing horizontal curve design criteria are ade- quate to accommodate trucks. The adequacy of the existing criteria was evaluated with respect to both their ability to keep vehicles from skidding off the road and their ability to keep vehicles from rolling over. These sensitivity analyses involved explicit comparisons between the margins of safety against skidding and rollover for passenger cars and trucks. There have been particular concerns about vehicles traveling faster than the design speed, particularly on freeway ramps. The sensitivity analysis presented here is an update of the analysis performed for the FHWA Truck Characteristics study (2, 3), which resulted in the recent changes in Green Book design policy for horizontal curves. Figure 51. Nominal lateral accelerations of trucks based on their observed speeds on selected horizontal curves in the Chicago area (98).

Margin of Safety Against Skidding Current design criteria for horizontal curves are intended to maintain the vehicle lateral acceleration within driver comfort levels that are below the lateral acceleration at which the vehicle would skid on a wet pavement. The vehicle’s lateral acceleration is resisted by superelevation and tire-pavement friction. Table 68 shows that current design criteria provide a margin of safety of 0.30 to 0.41 g against a passenger car skidding off the road on a minimum radius curve on wet pavement when traveling at the design speed. The margin of safety is the magnitude of the additional lateral acceleration that the vehicle could undergo without skidding. Tire-pavement friction on a given pavement is lower for truck tires than for passenger car tires. Olson et al. estimate that truck tires have coefficients of friction that are only about 70 percent of those of passenger car tires (28). In addi- tion, the 1985 FHWA study discussed above has shown that trucks generate friction demands approximately 10 percent higher than passenger cars when traversing a curve (96). Thus, Table 68 shows that the margin of safety against a truck skid- ding off the road on a wet pavement is less than for a pas- senger car. The margin of safety against skidding for a truck traveling at the design speed on a minimum radius curve on a wet pavement ranges from 0.15 to 0.22 g. On dry pavements, tire-pavement friction is much higher than on wet pavement. Locked-wheel pavement friction coef- ficients of 0.65 or more are typical for passenger cars on dry surfaces. Thus, peak friction levels would be even higher by a factor of 1.45. Peak friction levels for trucks were assumed to be 56 percent of the values for passenger cars. As shown in Table 68, the margin of safety for both passenger cars and trucks on dry surfaces is much higher than on wet surfaces. A simple example will show how the margin of safety against skidding is calculated using the data in the first row of Table 68. This row represents a horizontal curve with a design speed of 20 mph and a maximum superelevation of 4.0 percent. Under the Green Book policy, a horizontal curve with a design speed of 32 km/h [20 mph] can be designed with a maximum tolerable lateral acceleration of 0.17 g. An equiv- alent statement is that the maximum side friction demand for a vehicle traveling at the design speed on a curve with maxi- mum superelevation is 0.17 g. The minimum radius of curva- ture for this situation can be determined as follows: (46) The assumed pavement friction coefficient at 32 km/h [20 mph] for locked-wheel braking by a passenger car tire on a wet pavement is not specified in the current Green Book, but has been estimated in previous AASHTO policies for stopping sight distance as 0.40 (48). The peak friction coef- ficient available for cornering on a wet pavement is computed as follows: R (20)15( .04 + 0.17) ft 2 min = =0 127 107 0.40(1.45) = 0.58 A peak friction coefficient of 0.58 means that a vehicle can generate up to 0.58 g of unbalanced lateral acceleration with- out skidding. Therefore, the margin of safety against skid- ding for a passenger car on a wet pavement traveling at the design speed under assumed design conditions can be com- puted as the difference between the maximum lateral accel- eration that can be developed without exceeding the avail- able friction (0.58 g) and the friction demand (0.17 g): 0.58 − 0.17 = 0.41 The pavement friction coefficient under dry conditions was estimated as 0.65, as described above. Under dry condi- tions, the peak friction available for cornering is computed as follows: 0.65(1.45) = 0.94 Therefore, the margin of safety against skidding under dry conditions is as follows: 0.94 − 0.17 = 0.77 The calculations of the margin of safety against skidding for a truck are similar. As discussed above, the maximum demand friction for a truck is 10 percent higher than for a passenger car, based on the results of a 1985 FHWA study (96). Thus, when a truck is traversing a horizontal curve at the design speed under design conditions at the maximum tolerable lateral acceleration of 0.17 g, the effective maxi- mum friction demand is as follows: 0.17(1.1) = 0.19 Since research has shown that truck tires can generate only about 70 percent of the friction of passenger car tires, the peak friction available under wet conditions for a truck is as follows: 0.58(0.70) = 0.41 and the margin of safety under wet conditions is as follows: 0.41 − 0.19 = 0.22 Similarly, under dry conditions, the available peak friction for a truck tire is as follows: 0.94(0.70) = 0.66 and the margin of safety under dry conditions is as follows: 0.66 − 0.19 = 0.47

108 TABLE 68 Margins of safety against skidding on horizontal curves Passenger car Truck Design Speed (mph) Maximum super- elevation e Maximum tolerable lateral acceleration (g) Maximum demand f Minimum radius (ft) Available f (wet) Margin of safety (wet) Margin of safety (dry) Maximum tolerable lateral acceleration (g) Minimum radius (ft) Maximum demand f Available f (wet) Margin of safety (wet) Margin of safety (dry) 20 4.0 0.17 0.17 127 0.58 0.41 0.77 0.17 127 0.19 0.41 0.22 0.47 30 4.0 0.16 0.16 300 0.51 0.35 0.78 0.16 300 0.18 0.36 0.18 0.48 40 4.0 0.15 0.15 561 0.46 0.31 0.79 0.15 561 0.17 0.32 0.16 0.50 50 4.0 0.14 0.14 926 0.44 0.30 0.80 0.14 926 0.15 0.30 0.15 0.51 60 4.0 0.12 0.12 1,500 0.42 0.30 0.82 0.12 1,500 0.13 0.29 0.16 0.53 20 6.0 0.17 0.17 116 0.58 0.41 0.77 0.17 116 0.19 0.41 0.22 0.47 30 6.0 0.16 0.16 273 0.51 0.35 0.78 0.16 273 0.18 0.36 0.18 0.48 40 6.0 0.15 0.15 508 0.46 0.31 0.79 0.15 508 0.17 0.32 0.16 0.50 50 6.0 0.14 0.14 833 0.44 0.30 0.80 0.14 833 0.15 0.30 0.15 0.51 60 6.0 0.12 0.12 1,333 0.42 0.30 0.82 0.12 1,333 0.13 0.29 0.16 0.53 70 6.0 0.10 0.10 2,042 0.41 0.31 0.84 0.10 2,042 0.11 0.29 0.18 0.55 80 6.0 0.08 0.08 3,048 0.40 0.32 0.86 0.08 3,048 0.09 0.28 0.19 0.57 20 8.0 0.17 0.17 107 0.58 0.41 0.77 0.17 107 0.19 0.41 0.22 0.47 30 8.0 0.16 0.16 250 0.51 0.35 0.78 0.16 250 0.18 0.36 0.18 0.48 40 8.0 0.15 0.15 464 0.46 0.31 0.79 0.15 464 0.17 0.32 0.16 0.50 50 8.0 0.14 0.14 758 0.44 0.30 0.80 0.14 758 0.15 0.30 0.15 0.51 60 8.0 0.12 0.12 1,200 0.42 0.30 0.82 0.12 1,200 0.13 0.29 0.16 0.53 70 8.0 0.10 0.10 1,815 0.41 0.31 0.84 0.10 1,815 0.11 0.29 0.18 0.55 80 8.0 0.08 0.08 2,667 0.40 0.32 0.86 0.08 2,667 0.09 0.28 0.19 0.57 20 10.0 0.17 0.17 99 0.58 0.41 0.77 0.17 99 0.19 0.41 0.22 0.47 30 10.0 0.16 0.16 231 0.51 0.35 0.78 0.16 231 0.18 0.36 0.18 0.48 40 10.0 0.15 0.15 427 0.46 0.31 0.79 0.15 427 0.17 0.32 0.16 0.50 50 10.0 0.14 0.14 694 0.44 0.30 0.80 0.14 694 0.15 0.30 0.15 0.51 60 10.0 0.12 0.12 1,091 0.42 0.30 0.82 0.12 1,091 0.13 0.29 0.16 0.53 70 10.0 0.10 0.10 1,633 0.41 0.31 0.84 0.10 1,633 0.11 0.29 0.18 0.55 80 10.0 0.08 0.08 2,370 0.40 0.32 0.86 0.08 2,330 0.09 0.28 0.19 0.57 20 12.0 0.17 0.17 92 0.58 0.41 0.77 0.17 92 0.19 0.41 0.22 0.47 30 12.0 0.16 0.16 214 0.51 0.35 0.78 0.16 214 0.18 0.36 0.18 0.48 40 12.0 0.15 0.15 395 0.46 0.31 0.79 0.15 395 0.17 0.32 0.16 0.50 50 12.0 0.14 0.14 641 0.44 0.30 0.80 0.14 641 0.15 0.30 0.15 0.51 60 12.0 0.12 0.12 1,000 0.42 0.30 0.82 0.12 1,000 0.13 0.29 0.16 0.53 70 12.0 0.10 0.10 1,485 0.41 0.31 0.84 0.10 1,485 0.11 0.29 0.18 0.55 80 12.0 0.08 0.08 2,133 0.40 0.32 0.86 0.08 2,133 0.09 0.28 0.19 0.57 NOTE: Adapted from Reference 2 to incorporate 2001 Green Book criteria for horizontal curve design.

The margins of safety for trucks in Table 67 are large enough to provide safe truck operations if there are no major deviations from the basic assumptions used in horizontal curve design. The effects of deviations from the basic assump- tions are considered below. Margin of Safety Against Rollover Table 69 presents an analysis of the margin of safety against rollover provided by current horizontal curve design criteria. The margin of safety is the magnitude of the additional lat- eral acceleration that the vehicle could undergo without rolling over. The table shows the rollover margin of safety for pas- senger cars with roll over thresholds of 1.20 g and for trucks with rollover thresholds from 0.35 to 0.40 g. The margin of safety against rollover for passenger cars traveling at the design speed ranges from 1.03 to 1.10 g. At all design speeds, the margin of safety against rollover for a passenger car is much higher than the margin of safety against skidding on either a wet or dry pavement. Thus, rollover is not a major concern for passenger cars because, unless they col- lide with another vehicle or object, passenger cars will skid rather than roll over. In contrast to the related issue of skid- ding off the road, the margin of safety against rollover is not dependent on whether the pavement is wet or dry. Chapter 5 of this report establishes that a conservative value of truck rollover threshold appropriate for use in design is 0.35 g. The margin of safety for a truck with a rollover threshold of 0.35 g ranges from 0.18 to 0.27 g. This margin of safety is adequate to prevent rollover for trucks traveling at or below the design speed. The margin of safety against rollover increases with increasing design speed, while the margin of safety against skidding decreases. Comparison of Tables 68 and 69 indicates that rollover is a particular concern for trucks. Under the assumed design conditions for horizontal curves, a truck will roll over before it will skid on a dry pavement. Under the assumed design conditions on a wet pavement, a truck will roll over before it skids at design speeds of 64 to 80 km/h [40 to 50 mph] and below; above that speed, a truck will skid before it rolls over. The effects of deviations from the basic assumptions are con- sidered below. Deviations from Assumed Design Conditions The margins of safety against skidding and rollover are a measure of the extent to which real-world drivers, vehicles, and highways can deviate from the assumed conditions with- out resulting in a skid or a rollover. Deviations from assumed conditions that can increase the likelihood of skidding include the following: • Vehicles traveling faster than the design speed, • Vehicles turning more sharply than the curve radius, 109 • Lower pavement friction than assumed by the Green Book, and • Poorer tires than assumed by the Green Book. Traveling faster than the design speed and turning more sharply than the curve radius would also increase the likeli- hood of rollovers. In addition, the likelihood of a rollover would also be increased for a truck with a rollover threshold less than the assumed value of 0.35 g. It would seem logical that the practice of providing less than full superelevation at the point of curvature (PC) would also increase the likelihood of rollovers, but this is not always the case. Horizontal curves without spiral transitions are typ- ically designed with 2/3 of the superelevation runoff on the tangent in advance of the PC and 1/3 of the superelevation runoff on the curve itself. Thus, only 2/3 of the design super- elevation is available at the PC, and this lack of full super- elevation at the PC would appear to have the potential to off- set up to approximately 0.03 g of the available margin of safety. However, the Green Book assumes, and field and sim- ulation studies confirm, that even on horizontal curves with- out spiral transitions, drivers tend to steer a spiral path. Thus, where maximum superelevation is not available, the driver is usually not steering a minimum-radius path. Computer simulation studies of trucks traversing horizon- tal curves reported in the FHWA Truck Characteristics study (2, 3) found that developing full superelevation on the tangent approach to a conventional circular curve actually developed slightly more lateral acceleration than development of super- elevation with the 2/3 to 1/3 rule. While the difference in lat- eral acceleration is small—at most 0.03 g—it is in the wrong direction, so development of full superelevation on the tan- gent is not a desirable approach to reducing truck rollovers. The same study found a small decrease in lateral accelera- tion—typically less than 0.01 g—when spiral transitions were used to develop the superelevation. Thus, the use of spi- ral transitions is desirable but, because of the small reduction in lateral acceleration, the use of spirals is unlikely to provide a major reduction in rollover accidents. Field data show that vehicles traversing a curve do not precisely follow the curve. Thus, while the path may have a larger radius than the curve at the PC, it will also have a smaller radius than the curve at some point in the curve. Sim- ulation results show that the maximum lateral acceleration occurs several hundred feet after entering a curve. However, simulation results also show that the maximum excursion of lateral acceleration above the value obtained from the stan- dard curve formula is approximately 0.02 g, which would offset a small portion of the margins of safety against rolling and skidding. Field studies for passenger cars suggest that this is a reasonable average value, but more extreme values can occur. Truck drivers may have lower excursions of lat- eral acceleration than passenger car drivers, but there are no data on this issue.

110 TABLE 69 Margins of safety against rollover on horizontal curves Passenger car Truck Rollover margin of safety Design Speed (mph) Maximum e Maximum tolerable lateral acceleration Minimum radius (ft) Rollover margin of safety RT = 1.20 g Maximum tolerable lateral acceleration Minimum radius (ft) RT = 0.35 g RT = 0.40 g 20 4.0 0.17 127 1.03 0.17 127 0.18 0.23 30 4.0 0.16 300 1.04 0.16 300 0.19 0.24 40 4.0 0.15 561 1.05 0.15 561 0.20 0.25 50 4.0 0.14 926 1.06 0.14 926 0.21 0.26 60 4.0 0.12 1,500 1.08 0.12 1,500 0.23 0.28 20 6.0 0.17 116 1.03 0.17 116 0.18 0.23 30 6.0 0.16 273 1.04 0.16 273 0.19 0.24 40 6.0 0.15 508 1.05 0.15 508 0.20 0.25 50 6.0 0.14 833 1.06 0.14 833 0.21 0.26 60 6.0 0.12 1,333 1.08 0.12 1,333 0.23 0.28 70 6.0 0.10 2,042 1.10 0.10 2,042 0.25 0.30 80 6.0 0.08 3,048 1.12 0.08 3,048 0.27 0.32 20 8.0 0.17 107 1.03 0.17 107 0.18 0.23 30 8.0 0.16 250 1.04 0.16 250 0.19 0.24 40 8.0 0.15 464 1.05 0.15 464 0.20 0.25 50 8.0 0.14 758 1.06 0.14 758 0.21 0.26 60 8.0 0.12 1,200 1.08 0.12 1,200 0.23 0.28 70 8.0 0.10 1,815 1.10 0.10 1,815 0.25 0.30 80 8.0 0.08 2,667 1.12 0.08 2,667 0.27 0.32 20 10.0 0.17 99 1.03 0.17 99 0.18 0.23 30 10.0 0.16 231 1.04 0.16 231 0.19 0.24 40 10.0 0.15 427 1.05 0.15 427 0.20 0.25 50 10.0 0.14 694 1.06 0.14 694 0.21 0.26 60 10.0 0.12 1,091 1.08 0.12 1,091 0.23 0.28 70 10.0 0.10 1,633 1.10 0.10 1,633 0.25 0.30 80 10.0 0.08 2,370 1.12 0.08 2,330 0.27 0.32 20 12.0 0.17 92 1.03 0.17 92 0.18 0.23 30 12.0 0.16 214 1.04 0.16 214 0.19 0.24 40 12.0 0.15 395 1.05 0.15 395 0.20 0.25 50 12.0 0.14 641 1.06 0.14 641 0.21 0.26 60 12.0 0.12 1,000 1.08 0.12 1,000 0.23 0.28 70 12.0 0.10 1,485 1.10 0.10 1,485 0.25 0.30 80 12.0 0.08 2,133 1.12 0.08 2,133 0.27 0.32 NOTE: Adapted from Reference 2 to incorporate 2001 Green Book criteria for horizontal curve design.

The Green Book criteria for tire-pavement friction are based on a poor, wet pavement and (apparently) on worn tires. Table 68 has provided an adjustment to these values for the differences between passenger cars and trucks. The assump- tions appear to be conservative for design purposes. In fact, an interesting aspect of this factor discussed below is what happens when the likelihood of skidding is reduced because tire pavement-friction is higher than the design value. The review of the potential for safety problems created by deviations from the design assumptions indicates that travel- ing faster than the design speed of the curve is the single greatest concern. This is a particular concern on freeway ramps for two reasons. First, freeway ramps generally have lower design speeds than major roadways, which means that they have lower margins of safety against rollover (but higher margins of safety against skidding). Second, traveling faster than the design speed is especially likely on off-ramps, where vehicles traveling at higher speeds enter the ramp from the major roadway. Table 70 compares the speeds at which skidding or rollover would occur for passenger cars and trucks traversing minimum radius curves designed in accordance with current Green Book criteria. The table shows that, on a dry pavement, a passen- ger car will skid at a lower speed than it will roll over, and a truck with rollover threshold of 0.35 g will roll over at a lower speed than it will skid. On a wet pavement, a passen- ger car will still skid at a lower speed than it will roll over. A truck, on the other hand, will roll over before it skids at design speeds of 32 km/h [20 mph] or less under the assumed values for pavement friction on wet pavements. At higher speeds, a truck generally will skid before it will roll over. However, if a wet pavement has above-minimum friction, the truck may still roll over at a lower speed than it will skid. PAVEMENT WIDENING ON HORIZONTAL CURVES Current Geometric Design Criteria The Green Book presents the current criteria for pavement widening on horizontal curves to accommodate offtracking of trucks. Offtracking is the phenomenon, common to all vehicles although much more pronounced with large trucks, in which the rear wheels do not track precisely behind the front wheels when the vehicle negotiates a horizontal curve. The Green Book criteria call for widening of curves accord- ing to tabulated criteria that depend on the pavement width on the tangent, the design speed, and the degree of curve. The pavement-widening criteria are presented in Green Book Exhibits 3-51 and 35-2. These exhibits note that pavement- widening is not needed when the widening value is less than 0.6 m [2 ft]. The tabulated values apply to the WB-15 [WB-50] design vehicle; adjustments for other design vehicles are pro- vided. The Green Book tables apply only to two-lane roads 111 (one- or two-way); the values given are to be adjusted upward for three- or four-lane roads. The Green Book also details how the widening should be accomplished. In other words, it notes whether the added width should be on the inside or outside of the curve, how it should be transitioned, and how the center line should be adjusted. Critique of Geometric Design Criteria The current design criteria for pavement widening on hor- izontal curves was updated to reflect recommended changes to the Green Book design vehicles. Green Book Exhibits 3-51 and 3-52 are affected primarily because of the recommenda- tion to eliminate the WB-15 [WB-50] as a design vehicle, and the values shown for traveled way widening are based on the WB-15 [WB-50] design vehicle. The values in Green Book Exhibits 3-51 and 3-52 were adjusted accordingly to reflect the WB-19 [WB-62] as the base vehicle, and the pre- vious column for the WB-19 [WB-62] design vehicle was removed (see Appendix F). CROSS-SLOPE BREAKS Current Geometric Design Criteria The following represents a brief summary of the Green Book criteria for cross-slope rates: • On tangent or long-radius curved alignment with nor- mal crown and turf shoulders, the maximum shoulder slope rates result in algebraic differences of 6 to 7 per- cent between the pavement and the shoulder. • For desirable operation, all or part of the shoulder on the outside of a horizontal curve should be sloped upward at about the same rate or at a lesser rate than the super- elevated pavement. • The cross-slope break at the edge of the paved surface is limited to a maximum of approximately 8 percent. • To alleviate severe cross-slope breaks, the use of a con- tinuously rounded shoulder cross section may be used on the outside of superelevated pavements. Critique of Geometric Design Criteria Cross-Slope Breaks A 1982 FHWA study investigated the operational effects of cross-slope breaks on highway curves (99). Using the Highway-Vehicle-Object Simulation Model (HVOSM), vehi- cle traversals were simulated for various combinations of pavement and shoulder slopes for a range of horizontal cur- vature. The objective criterion was to limit lateral accelera- tion to a level that was stable at the tire-pavement interface

112 TABLE 70 Vehicle speed at impending skidding or rollover on horizontal curves Passenger car speed (mph) Truck speed (mph) @ rollover Design speed (mph) Maximum e Maximum tolerable lateral acceleration Minimum radius (ft) Passenger car available cornering f @ impending skid (wet) @ impending skid (dry) @ rollover RT = 1.20 g @ impending skid (wet) @ impending skid (dry) RT = 0.35 g RT = 0.40 g 20 4.0 0.17 127 0.58 34.4 43.2 48.6 27.9 34.9 27.3 29.0 30 4.0 0.16 300 0.51 49.7 66.4 74.7 40.5 53.7 41.9 44.5 40 4.0 0.15 561 0.46 64.9 90.8 102.1 52.9 73.4 57.3 60.8 50 4.0 0.14 926 0.44 81.7 116.7 131.2 66.7 94.3 73.6 78.2 60 4.0 0.12 1,500 0.42 101.7 148.5 167.0 83.1 120.0 93.7 99.5 20 6.0 0.17 116 0.58 33.4 41.7 46.8 27.3 33.9 26.7 28.3 30 6.0 0.16 273 0.51 48.3 64.0 71.8 39.7 52.0 41.0 43.4 40 6.0 0.15 508 0.46 62.9 87.3 98.0 51.8 70.9 55.9 59.2 50 6.0 0.14 833 0.44 79.0 111.8 125.5 65.2 90.8 71.6 75.8 60 6.0 0.12 1,333 0.42 98.0 141.4 158.7 80.9 114.9 90.5 95.9 70 6.0 0.10 2,042 0.41 120.0 175.0 196.5 99.1 142.2 112.1 118.7 80 6.0 0.08 3,048 0.40 145.0 213.8 240.0 119.1 173.7 136.9 145.0 20 8.0 0.17 107 0.58 32.5 40.5 45.3 26.8 33.0 26.3 27.8 30 8.0 0.16 250 0.51 47.0 61.8 69.3 38.9 50.5 40.2 42.4 40 8.0 0.15 464 0.46 61.3 84.3 94.4 50.9 68.8 54.7 57.8 50 8.0 0.14 758 0.44 76.9 107.7 120.6 64.0 87.9 69.9 73.9 60 8.0 0.12 1,200 0.42 94.9 135.5 151.8 79.1 110.6 88.0 93.0 70 8.0 0.10 1,815 0.41 115.5 166.6 186.7 96.3 136.1 108.2 114.3 80 8.0 0.08 2,667 0.40 138.6 202.0 226.3 115.7 164.9 131.2 138.6 20 10.0 0.17 99 0.58 31.8 39.3 43.9 26.4 32.2 25.9 27.2 30 10.0 0.16 231 0.51 46.0 60.0 67.1 38.4 49.2 39.5 41.6 40 10.0 0.15 427 0.46 59.9 81.6 91.2 50.2 67.0 53.7 56.6 50 10.0 0.14 694 0.44 75.0 104.0 116.3 62.9 85.4 68.4 72.1 60 10.0 0.12 1,091 0.42 92.2 130.5 145.9 77.5 107.0 85.8 90.5 70 10.0 0.10 1,633 0.41 111.8 159.6 178.7 94.0 130.9 105.0 110.7 80 10.0 0.08 2,370 0.40 133.3 192.3 215.0 112.3 157.7 126.5 133.3 20 12.0 0.17 92 0.58 31.1 38.2 42.7 26.0 31.5 25.5 26.8 30 12.0 0.16 214 0.51 45.0 58.3 65.1 37.8 48.1 38.8 40.9 40 12.0 0.15 395 0.46 58.6 79.2 88.4 49.5 65.3 52.8 55.5 50 12.0 0.14 641 0.44 73.4 101.0 112.7 62.0 83.2 67.2 70.7 60 12.0 0.12 1,000 0.42 90.0 126.1 140.7 76.2 103.9 84.0 88.3 70 12.0 0.10 1,485 0.41 108.7 153.7 171.5 92.1 126.6 102.3 107.6 80 12.0 0.08 2,133 0.40 129.0 184.2 205.5 109.5 151.8 122.6 129.0 NOTE: Adapted from Reference 6 to incorporate 2001 Green Book criteria for horizontal curve design.

and tolerable to the driver. A 1971 Dodge Coronet was the passenger car used in the simulations. The study results indicated that a four-wheel traversal and entry to a cross-slope break produce a more extreme response than a two-wheel traversal. The dynamic effects were found to be most sensitive to shoulder cross slope and to exceed reasonable driver discomfort levels for the design conditions that reduce the conditions associated with higher cross-slope breaks. It was determined that relatively large negative slopes are tolerable on very narrow shoulders. As shoulder width increases, permissible shoulder slopes should decrease to maintain the established maximum driver discomfort level. Specifically, the study found that maximum driver discom- fort occurred when all four wheels were on the shoulder, not when the vehicle crosses the break. The FHWA study identified two unanswered questions regarding the sensitivity of trucks to cross-slope break tra- versals (99): 1. Do professional (truck) drivers exhibit higher tolerable levels of driver discomfort? 2. Do shoulder traversals by trucks occur often enough to justify the truck as the “design” vehicle for cross-slope break recommendations? No further data were found in the literature to shed any addi- tional light on these issues. Centerline Crowns In another portion of the same FHWA study discussed above, the dynamic effects of centerline crowns on expected vehicle maneuvers were evaluated for the purpose of rec- ommending maximum centerline crown designs as a func- 113 tion of vehicle type and design speed (100). The controlling operational maneuver was the passing situation. Research was limited to tangent roadway sections. Vehicle types con- sidered included compact and midsize passenger cars, loaded and empty tractor-trailer truck combinations, and single-unit trucks. The pertinent truck-related findings include the following: • A loaded or empty tractor-trailer truck generates lower tire friction demand than automobiles on 2-percent cross slopes. • Driver discomfort levels and vehicle roll angle are also less for trucks than automobiles on 2-percent cross slopes at high speed (approximately 121 km/h [75 mph]. • An empty tractor-trailer produces similar tire friction demands (approximately 0.30 g), but has significantly lower driver discomfort and roll angle values. The implication of the findings is that cross slopes should be kept to a minimum on high-speed highways. The primary reason is that the simulation of nominally critical passing behavior produced vehicle dynamic responses on the order of 0.28 to 0.34 g for cross slopes of 2 percent for all vehicle types. VERTICAL CLEARANCES The Green Book criteria for vertical clearance are gener- ally 4.3 m [14 ft] on local roads and collectors and 4.9 m [16 ft] on arterials and freeways. The design vehicles specified in the Green Book have a maximum height of 4.1 m [13.5 ft]. Most trucks have heights less than 4.1 m [13.5 ft], so verti- cal clearance is generally not an issue for overhead structures designed in accordance with the Green Book.

Next: Chapter 7 - Conclusions and Recommendations »
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 505: Review of Truck Characteristics as Factors in Roadway Design presents guidance to roadway geometric designers on how to accommodate large trucks on the U.S. highway system.

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