Determining Guidelines for Ramp and Interchange Spacing (2011) / Chapter Skim
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Chapter 2 Information Gathering
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-1 Chapter 2 INFORMATION GATHERING The project team collected information to identify the origins of the current design standards and practice for ramp and interchange spacing and to determine how various spacing and interchange configurations can impact a facility's safety and operations. The primary information gathering activities that the project team completed include: • Conducting and summarizing domestic and international research on ramp and interchange spacing, operations, and safety; • Reviewing and summarizing human factors considerations, such as sign sequencing and message units; a review of sample information from five state agency freeway signing handbooks; and a summary of the underlying philosophy for providing guidance information and driver information processing; • Summarizing design vehicle evolution, such as documenting changes in passenger car performance characteristics (i.e., acceleration, braking, transmission type, etc.)
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-2 In 1957, Owens of the Automotive Safety Foundation stated that "one mile between interchanges is a desirable minimum with one-half mile an absolute minimum" (1)
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-3 Exhibit 2-1 General Interchange Spacing Recommendations by J.E. Leisch, 1959 (3)
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-4 Exhibit 2-2 Specific Interchange Spacing Recommendations by J
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-5 Exhibit 2-3 Methods of Increasing the Number of Ramps by J.E. Leisch, 1959 (3)
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-6 Over the years, guidelines from these early studies and others have been incorporated into various versions of policy by the American Association of State Highway and Transportation Officials (AASHTO) and the American Association of State Highway Officials (AASHO)
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-7 Similar to AASHTO, the ITE Freeway Handbook acknowledges that required weaving distances, as outlined in the 2000 Highway Capacity Manual (HCM) , may be longer than 1,600 to 2,000 feet.
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-8 deceleration lane when one was provided prior to the exit gore and behaved as if the ramp were of a taper design (11)
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-9 on arterial operations and the road network as a whole. A summary of each is provided below.
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-10 The findings of this study that are relevant to NCHRP 3-88 include the operational conditions posed by closely spaced on-ramps and the benefits of metered on-ramp flow. The authors did not provide any minimum or desirable spacing values based upon the results of their study 2.1.1.3.3 Chicago study (2)
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-11 The analysis of annual transportation cost found that in the high density area, optimum spacing of interchanges was 1 mile and optimum spacing of grade separations was one-half mile. For the low density area, optimum spacing of interchanges was 2 miles and optimum spacing of grade separations was onehalf or 1 mile.
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-12 2.1.2 Primary Resource Documents The AASHTO Green Book and the HCM are two commonly used resources for freeway and interchange planning and design. The AASHTO Green Book recommends minimum interchange spacing dimensions, and the HCM quantifies the impact of interchange spacing on traffic operations.
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-13 Exhibit 2-5 Ramp Terminal Spacing Guidelines, AASHO Red Book (1957)
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-14 mile per hour design speed category was added to the table. The document also noted that, for consecutive exits, the minimum spacing for adequate signing is 1,000 feet on a full freeway and 600 feet between an exit on a full freeway and an exit on a C-D road.
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-15 The 1973 Red Book recommended a minimum of 1,000 feet between successive exits on a full freeway and 800 feet between an exit on a full freeway and an exit on a collector-distributor road, as shown in Exhibit 2-7 (21)
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-16 Exhibit 2-8 Minimum Ramp Terminal Spacing, J
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-17 2.1.2.1.2 Interchange Spacing Interchange spacing guidance first appeared in the 1984 Green Book. The text has remained virtually the same in all Green Books since, including the 2004 Green Book, which states: "Minimum spacing of arterial interchanges (distance between intersecting streets with ramps)
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-18 conflict with one another, because not all interchanges are on the Interstate Highway System. 2.1.2.1.4 AASHTO Summary AASHTO policies on ramp and interchange spacing in the 2004 Green Book have not changed significantly since the first Green Book was published in 1984.
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-19 base free-flow speed was reduced by 7.5 miles per hour. Base free-flow speed adjustments were not specified for densities of more than 2.0 interchanges per mile.
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-20 Exhibit 2-10 Definition of Weaving Segment Length, 1985 through 2000 HCMs (25, 26, 27, 14) The weaving model of the 1985 HCM was based upon several research projects conducted in the early 1980s (25)
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From page 31... ...
NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-21 2.1.2.2.3 Freeway Merge and Diverge Segments The 1965 HCM was the first edition to analyze operations at ramp junctions on freeways. LOS was based upon volume and number of lanes.
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-22 areas extend 1,500 feet upstream of diverge areas and 1,500 feet downstream from merge areas. To minimize the impacts of ramps and interchanges on freeway operations, these analysis procedures suggest the following designs: • Total ramp density should remain low.
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-23 seen in Exhibit 2-14, this version offers "desirable" and "adequate" ramp spacing dimensions in addition to "absolute" minimums. This version is similar to the table Jack E
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From page 34... ...
Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-24 area suburbanizes, additional interchanges can be added to create 3-mile spacing. 2.1.2.4 PRIMARY DOCUMENT SUMMARY Minimum interchange spacing guidelines from four national-level documents are presented below in Exhibit 2-15.
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-25 AASHTO values, and others call for greater spacing. Summaries and highlights from the sampled state documents are provided below.
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-26 In addition to the HDM, California issues Design Information Bulletins (DIB)
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-27 less than 1 mile may be developed in urban areas with collector-distributor roads. For ramps, the RDM contains a diagram that provides the minimum physical nose to physical nose spacing guidelines for freeways, C-D roads, and arterials.
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-28 2.1.3.5 OREGON Oregon's Highway Design Manual (HDM) addresses spacing in two separate sections (37)
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From page 39... ...
NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-29 has another figure for interchanges with multilane crossroads, but distance "A" (the end of acceleration taper to start of deceleration taper measurement) is unchanged.
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-30 minimum rural interchange spacing range of 3 to 25 miles. No state sampled has interchange spacing values less than those in the 2004 AASHTO Green Book (i.e., 1 mile in urban areas and 2 miles in rural areas)
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-31 South Africa define spacing in terms of crossroad-to-crossroad distance, similar to the AASHTO Policies in the United States, thus choosing to define interchange spacing. The United Kingdom's definition of a nose is shown in Exhibit 2-21.
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-32 • Spacing between roadways is too coarse of a measure to use as a basis for determining interchange spacing because of the differences among interchange forms. For example, ramp spacing with a Parclo-A followed by a Parclo-B interchange will be greater than ramp spacing with consecutive diamonds.
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-33 Exhibit 2-22 Definition of "Yellow Line Break Point Distance" Used in Gauteng Province, South Africa (2005)
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-34 2.1.5 Signing Considerations Guide signs are used by drivers as a navigational aid while they travel. On freeways, guide signs identify upcoming exits in advance of and at the ramp itself.
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-35 In addition to being clear and predictable, signing must not be so extensive that it presents drivers with more information than they are able to process. Motorists should be presented with the maximum amount of useful visual information in a manner that is uniform but also prioritizes information that is most important (47)
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-36 Exhibit 2-25 Message unit limits, reprinted in Texas Freeway Signing Handbook (2008)
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-37 2.1.5.2 SIGN SPACING 2.1.5.2.1 FHWA's Manual of Uniform Traffic Control Devices (MUTCD)
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-38 Exhibit 2-27 Signing of a major or intermediate interchange, example 1, MUTCD (2009)
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-39 Exhibit 2-28 Signing of a major or intermediate interchange, example 2, MUTCD (2009)
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-40 Major interchanges are interchanges with other expressways, other freeways, high-volume multilane highways, principal urban arterials, or certain major rural routes. Minor interchanges have a sum of exit volumes of less than 100 vehicles per day in the design year.
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-41 Exhibit 2-29 Interchange Sequence Signs, MUTCD (2009)
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-42 2.1.5.2.2 State Sign Guidance A number of states publish their own version of the MUTCD or other sign guidance documents. These documents are not intended to override FHWA's MUTCD but rather supplement the information.
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-43 Although overlapping a series of signs would not necessarily violate the MUTCD's minimum recommended spacing of 800 feet between advance guide signs, it would violate driver expectancy by simultaneously presenting two sign sequences. In situations such as this, the MUTCD recommends the use of interchange sequence signs.
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-44 Literature reviewed for NCHRP 3-88 indicates that acceleration capabilities of vehicles have increased since the 1930s. A 2000 study by Long documented that automobiles in the early 1990s had higher maximum acceleration rates than those from the 1930s (53)
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-45 trucks had changed little since 1984 but that weight-to-horsepower ratios of low-performing trucks continued a decreasing trend (54)
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-46 increases) and at the ramp level (e.g., speed decreases as weaving length decreases)
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-47 Many of the studies reviewed had higher-level objectives than a detailed look at interchange- and ramp-related issues. These objectives included, for example, applying and testing new model estimation techniques or developing general freeway safety models.
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From page 58... ...
Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-48 Exhibit 2-30 Illustration of interchange area concept Torbic et al. incorporated two types of freeway segment models into the Interchange Safety Analysis Tool (ISAT)
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From page 59... ...
NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-49 • 6-lane urban; and • 8-lane urban. All of the freeway segments models had the following functional form: SLADTaN b ××= )
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-50 less safe3 0% 20% 40% 60% 80% 100% 120% 140% 160% 180% 200% 0 50,000 100,000 150,000 200,000 250,000 Average Daily Traffic (veh/day) P e rc e n t d if fe re n c e i n e x p e c te d a c c id e n ts a 4-lane rural, total 4-lane rural, fatal + injury 6-lane rural, fatal + injury 4-lane urban, fatal + injury 6-lane urban, fatal + injury 8-lane urban, fatal + injury 6-lane rural, total 4-lane urban, total 6-lane urban, total 8-lane urban, total .
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-51 medium to large urban areas4 Models for fatal plus injury accidents, injury accidents and property-damageonly (PDO) accidents were estimated for the following freeway types: (60, 58)
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-52 0% 200% 400% 600% 800% 1000% 1200% 1400% 0 20,000 40,000 60,000 80,000 100,000 120,000 140,000 Average Daily Traffic (veh/day) P e rc e n t d if fe re n c e i n e x p e c te d a c c id e n ts a , b NC, 4-lane, total NC, 4-lane, fatal + injury NC, >4-lane, total NC,> 4-lane, fatal + injury TN, 4-lane, fatal + injury TN, 4-lane, total TN, >4-lane, total TN,> 4-lane, fatal + injury a %100×− outside outsidewithin N NN b Exhibit 2-32 Comparison of safety for freeway segments within interchange areas to freeway segments outside of interchange areas using North Carolina and Tennessee models For this comparison, lengths of segments inside and outside of interchange areas were set equal to the mean length of the interchange segments (0.57 for both North Carolina models; 1.0 for Tennessee 4-lane; 0.88 for Tennessee >4-lane.
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From page 63... ...
NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-53 Exhibit 2-33 summarizes freeway accident rates by adjacent interchange unit and area type provided in a synthesis by Twomey et al.
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From page 64... ...
Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-54 • accident type: single-vehicle or multiple vehicle • accidents by traffic conditions: peak or off-peak • accidents by pavement condition: dry or wet • accidents by light condition: daytime or dark • accident severity: PDO or injury Models within each category were estimated simultaneously using seemingly unrelated negative binomial regression, which accounted for error correlation between the accident types in each category. The presence of an entrance or exit ramp within a defined influence area increased the expected accident frequency for all eight accident types.
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From page 65... ...
NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-55 Interstates (not including the Pennsylvania Turnpike)
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-56 increase with ramp proximity (i.e., shorter distances from ramps) at a higher rate in urban areas than in rural areas.
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From page 67... ...
NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-57 Cirillo examined the relationship between accident rates and weaving area lengths using Interstate data from 20 states (70)
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-58 Exhibit 2-36 Analysis of accident rates by weaving areas length reported by Cirillo, 1970 (70) Bared et al.
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From page 69... ...
NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-59 example, shorter segment lengths are likely associated with an increased presence of auxiliary lanes between the entrance and exit ramps of two consecutive crossroads; a feature not captured in the data. 0 20 40 60 80 100 120 140 160 180 200 0 0.5 1 1.5 2 2.5 3 3.5 Segment Length defined as Interchange Spacing (miles)
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-60 S = interchange spacing (miles) ; and =∑ 3,2,1,,, bbbaRampADTADT same as previously defined.
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-61 • a Washington freeway dataset consisting of 100 spacing observations representing 144 freeway miles was added and used for model estimation and validation; • mainline traffic was specified as vehicles per lane per day; • ramp volumes were expressed at the ratio of ramp ADT to mainline ADT for the California models; • cross section variables representing median width, median type and HOV lane presence were included in some models; and • the definition for interchange spacing was changed to represent the distance between crossroads of consecutive interchanges. Model estimation results are summarized in Exhibit 2-39.
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-62 Exhibit 2-39 Summary of reported models in Pilko et al., 2007 (69)
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-63 Freeway type Expected unit % increase in total accidents Expected unit % increase in injury accidents 1 Expected unit % increase in fatal accidents 4- and 6-lane rural +24% +24% n/a 4-lane urban 2 +32% +27% n/a 6-lane urban 2 +20% n/a n/a2 2 1 The expected increase in accidents given the addition of one interchange to a freeway segment. The effect is multiplicative (e.g., the expected increase in accidents given the addition of two interchanges on a 4-lane rural freeway would be 1.24 *
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-64 2.1.7.3 SAFETY SUMMARY A total of twelve studies, and sixteen subsequent papers and reports provided insights relevant to NCHRP 3-88 project objectives. The literature generally fell into two categories: • Studies that provide insight into the safety effects of interchange and ramp presence; and • Studies that provide insight into the safety effects of interchange and ramp spacing.
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-65 segment length (i.e., interchange spacing)
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Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-66 needs in the current practice of ramp and interchange spacing." As outlined in this task, the project team submitted a memorandum to the NCHRP panel for their approval and suggestions for the focus group meeting approach, participants and discussion topics. The team conducted the focus group via telephone, which was cost effective way to engage a wide range of individuals.
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NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-67 • Spacing considerations should consider whether the facility is a system or service interchange. The spacing guidance may need to be separated for each type.
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From page 78... ...
Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-68 2.2.3 Analysis Techniques • State agencies typically use the Highway Capacity Software to conduct preliminary analyses and then CORSIM or another type of detailed analysis software is often needed for complex designs. • Using 20 years forecast volumes in the analysis often leads to failing levels-of-service.
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From page 79... ...
NCHRP 3-88 Interim Report Guidelines for Ramp and Interchange Spacing Chapter 2: Information Gathering 2-69 Additional guidance on the following considerations would be beneficial for designers to make more informed decisions about each specific project characteristic. • Operational impacts of closely spaced ramps with high volumes • Operational impacts of high truck volumes • Variations in acceleration and deceleration lengths • Two lane entrance and exit ramps • Ability to provide appropriate guide signing for all types of users What are the challenges and opportunities of applying the current criteria in contemporary and future practice?
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From page 80... ...
Interim Report NCHRP 3-88 Chapter 2: Information Gathering Guidelines for Ramp and Interchange Spacing 2-70 • Effect of variations in ramp and mainline volumes on appropriate minimum ramp terminal spacing • Effect of truck volume variations • Knowledge of a driver's expectations regarding separation distances (ramp terminal spacing) • Knowledge of what constitutes "adequate signing" for situations involving close ramp spacing • Effect of variations in facility operating speed and suggested minimum ramp terminal spacing • Spacing impacts due to decision sight distance What might be the quantitative information a decision maker or designer would like to have access to before making a decision?
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