National Academies Press: OpenBook

Design Guide for Addressing Nonrecurrent Congestion (2014)

Chapter: 3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS

« Previous: 2 SELECTING DESIGN TREATMENTS TO ADDRESS NONRECURRENT CONGESTION
Page 19
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 19
Page 20
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 20
Page 21
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 21
Page 22
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 22
Page 23
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 23
Page 24
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 24
Page 25
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 25
Page 26
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 26
Page 27
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 27
Page 28
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 28
Page 29
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 29
Page 30
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 30
Page 31
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 31
Page 32
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 32
Page 33
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 33
Page 34
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 34
Page 35
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 35
Page 36
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 36
Page 37
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 37
Page 38
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 38
Page 39
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 39
Page 40
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 40
Page 41
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 41
Page 42
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 42
Page 43
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 43
Page 44
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 44
Page 45
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 45
Page 46
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 46
Page 47
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 47
Page 48
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 48
Page 49
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 49
Page 50
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 50
Page 51
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 51
Page 52
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 52
Page 53
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 53
Page 54
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 54
Page 55
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 55
Page 56
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 56
Page 57
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 57
Page 58
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 58
Page 59
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 59
Page 60
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 60
Page 61
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 61
Page 62
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 62
Page 63
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 63
Page 64
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 64
Page 65
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 65
Page 66
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 66
Page 67
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 67
Page 68
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 68
Page 69
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 69
Page 70
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 70
Page 71
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 71
Page 72
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 72
Page 73
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 73
Page 74
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 74
Page 75
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 75
Page 76
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 76
Page 77
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 77
Page 78
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 78
Page 79
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 79
Page 80
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 80
Page 81
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 81
Page 82
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 82
Page 83
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 83
Page 84
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 84
Page 85
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 85
Page 86
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 86
Page 87
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 87
Page 88
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 88
Page 89
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 89
Page 90
Suggested Citation:"3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS." National Academies of Sciences, Engineering, and Medicine. 2014. Design Guide for Addressing Nonrecurrent Congestion. Washington, DC: The National Academies Press. doi: 10.17226/22475.
×
Page 90

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.

17 3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS This chapter catalogs design treatments that can be considered for use in addressing nonrecurrent congestion. A detailed summary of each of the design treatments is pro- vided that includes the following information: • Description and objective • Typical applications • Design criteria • How treatment reduces nonrecurrent congestion • Factors influencing treatment effectiveness • Cost The design treatments are classified on the basis of similarities with respect to function or location on the roadway. Chapter 4 presents a catalog of the secondary treatments. MEDIANS Emergency Crossovers Description and Objective An emergency crossover is a median opening on a divided highway segment for cross- ing by emergency, law enforcement, maintenance, and traffic service vehicles. Other types of median crossovers are addressed elsewhere in this chapter: • Crossovers for contraflow (see discussion under Contraflow Lanes for Emergency Evacuations)

18 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Temporary construction crossovers (see discussion under Contraflow Lanes for Work Zones) • Median openings controlled by manual or automated gates (see discussion under Gated Median Barriers) The objective of an emergency crossover is to provide better access for law enforcement and emergency vehicles responding to an incident, thus reducing the time between when an incident occurs and when emergency responders arrive on the scene. The sooner emergency responders can arrive on the scene of an incident, the sooner the incident can be cleared and normal freeway operations can resume. Figure 3.1 illustrates an example of an emergency crossover. Typical Applications Emergency crossovers are typically provided on rural sections of freeways where in- terchanges are spaced farther apart and a well-developed street network, providing options for alternate routes, is not available. At such locations, emergency responders are faced with excessive travel distances to reach an incident. Emergency crossovers may also be considered for rerouting traffic, via a U-turn maneuver, in response to a major lane-blocking incident. When a major incident blocks traffic for a significant amount of time, traffic may back up behind the incident and cause significant delay to motorists. If an emergency crossover is used to reroute traffic to the opposite direction of travel, the queue in the direction of the incident can dis- sipate. These rerouted vehicles can return to the next closest interchange and proceed along an alternate route to their destination. Figure 3.1. Emergency crossover on I-435 in Kansas.

19 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Although emergency crossovers are typically provided on freeways, median open- ings on arterial roadways can also be used to provide better access for emergency responders and to allow for traffic to be rerouted from a major incident. Design Criteria Important design considerations for installation of emergency crossovers on rural free- ways include interchange spacing, median width, and stopping sight distance. The American Association of State Highway and Transportation Officials (AASHTO) Green Book (2011) provides the following design guidance for emergency crossovers: • Emergency crossovers may be provided where interchange spacing exceeds 5 mi. • Between interchanges, emergency crossovers may be spaced at 3- to 4-mi intervals or as needed. • Emergency crossovers generally should not be located closer than 1,500 ft from the end of a speed-change taper of a ramp or to any structure. • Crossovers should be located only where above-minimum stopping sight distance is provided and preferably should not be located on superelevated curves. • The width of the crossover should be sufficient for turning movements and should have a surface capable of supporting emergency vehicles and maintenance equipment. • Emergency crossovers should be depressed below shoulder level to be inconspicu- ous to traffic. The Green Book provides additional guidance for such issues as sideslope and median barrier treatment. Emergency crossovers on controlled-access facilities are designed for authorized vehicle use only, and public use of these crossovers is prohibited unless directed by law enforcement (e.g., during major lane-blocking incidents). How Treatment Reduces Nonrecurrent Congestion Reduces Lane-Blocking Time of an Incident Use of emergency crossovers reduces the time during which disabled, crash-involved, or police vehicles remain in the roadway or on the roadway shoulder by decreasing the response time of emergency personnel. By reducing the lane-blocking time of an incident, the nonrecurrent congestion associated with that incident is reduced. This reduction in nonrecurrent congestion results in increased reliability for the roadway segment. Reduces Demand Volume During an Incident When emergency crossovers are used to allow vehicles to perform U-turn maneuvers to alleviate congestion caused by a major lane-blocking incident, demand is essen- tially reduced on the segment with temporarily reduced capacity by rerouting traffic to alternate facilities.

20 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Factors Influencing Treatment Effectiveness Factors that may influence the effectiveness of emergency crossovers in reducing non- recurrent congestion include the following: • Average reduction in response time for those situations in which the emergency crossover is used • Percentage of incidents (and incident type) for which the emergency crossover will be used to improve response time • Frequency of lane-blocking incidents for which queued traffic may be rerouted Cost Factors affecting the cost of installing an emergency crossover include the following: • Width and topography (cross slope) of the existing median • Existing median control type (curb, cable median barrier, semirigid barrier), if any • Material to be used for crossover surface (concrete, asphalt, gravel) • New signing required In general, emergency crossovers can be constructed for a relatively low cost. However, if significant cut, fill, or grading work is required to build a crossover, costs may be considerably higher. Movable Traffic Barriers Description and Objective A movable traffic barrier (MTB) is a concrete, crash-worthy barrier (similar to a jersey barrier) that can be shifted from one side of a lane to another, as illustrated in Figure 3.2, to allow flexibility in the designated purpose or direction of travel flow for that lane. The MTB is moved using a specially designed vehicle that shifts each section laterally as the vehicle moves forward. The vehicle does not interfere with traffic in adjacent lanes. One mile of MTB can be shifted one lane in less than 15 min, according to manufacturers. Typical Applications Crash-worthy barriers are used to enhance safety by reducing the potential for head-on collisions. An MTB provides similar safety benefits, but it also has the flexibility to change lane designations when needed. MTBs are used to help align ca- pacity with demand to minimize the impacts of recurrent and nonrecurrent congestion. MTBs can also be used to separate work zones from travel lanes. These devices can pro- vide more space in the work zone during off-peak hours, while protecting both the traveling motor- ists and the construction workers. Shifting the Figure 3.2. Movable traffic barrier. Source: Federal Highway Administration website.

21 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION barrier borrows a lane from operation as a travel lane and adds it to the space within the work zone, potentially allowing work to be conducted more efficiently during off- peak hours without adversely affecting travelers during peak hours. Although the most common applications of MTBs are to alleviate recurrent con- gestion due to unbalanced flow during peak periods and to safeguard workers during long-term construction projects, the potential benefits to nonrecurrent congestion are considered here. Nonrecurrent congestion situations such as work zones and major incidents are candidates for MTB application. In these instances, the barrier can be shifted to add a lane in the direction of traffic where the incident is taking place, help- ing to balance capacity and alleviate congestion. Applications of an MTB system to address nonrecurrent congestion include the following: • Providing flexibility in closing and opening lanes adjacent to work zones to accom- modate traffic demand and work zone space needs during different times of the day, potentially reducing the duration of the work zone • Using an MTB already installed for a recurrent congestion application to treat nonrecurrent congestion due to incidents or work zones Design Criteria Some state agencies have authored special provisions for MTBs wherever they are fur- nished, installed, operated, or maintained within the state. Specific design standards related to MTBs may address barrier reflector markers, the barrier transfer machine, end protection, and obstacle markers. Barriers can be designed to facilitate the ability to open a section of the barrier wall in minutes by using hand tools. If desired, a gate may be installed for easier access where emergency turnarounds are needed. The length of the gate may be variable to meet specific needs. Gated median barriers are considered as a separate treatment and are described in a subsequent section of this guide. How Treatment Reduces Nonrecurrent Congestion Reduces Work Zone Impact An MTB may be used during a work zone to provide an additional lane for the direc- tion of travel with higher demand. For example, if a work zone is planned that will block two lanes, the MTB may “borrow” one lane from the opposing direction for the duration of the work zone, which will offset the negative impact of the work zone. The demand and capacity of both directions of travel need to be considered, however, to ensure that traffic operations in both directions are balanced. Reduces Major Incident Impact During a major incident, MTBs may be used to offset the reduction of available lanes in the direction with higher demand. For example, if a major crash blocks multiple lanes in the direction of heavy demand, the MTB could be shifted to provide additional capacity for the duration of the incident. After the crash is cleared and traffic opera- tions have normalized, the lane could be shifted back to normal operation.

22 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION The use of MTBs in response to incidents is a function of the anticipated or observed severity of the incident-related congestion to the time required to move the barrier. In other words, if an incident can be cleared in less time than it would take to move the barrier, then it would not be useful to move it. However, the congestion resulting from crashes involving multiple vehicles and affecting multiple lanes could potentially be significantly alleviated through the use of MTBs. It is expected that MTBs would only be employed to address the impacts of these incidents when already in place for other reasons. That is, it would not be expected that an agency would deploy an MTB for the sole purpose of responding to major lane-blocking crashes and incidents. Like the third scenario described above, the exist- ing system is deployed to help reallocate capacity during major incidents (e.g., serious lane-blocking crashes, debris on the roadway, hazardous material spills) that occur on the roadway segment. It is assumed that during these incidents, the reallocation of lane space becomes a priority over maintaining the high-occupancy vehicle (HOV), high- occupancy toll, or express lanes in order to minimize congestion due to the incident. Factors Influencing Treatment Effectiveness Because an MTB may negatively affect the opposing direction of traffic by temporarily borrowing one of the lanes, the following factors may influence the effectiveness of an MTB in reducing nonrecurrent congestion: • How many lanes will be shifted • Traffic volume for each direction • Roadway capacity in each direction In the case of an MTB being used during typical minor work zones (e.g., restriping, pavement repair), factors that may influence its effectiveness include the following: • Number of lanes blocked during a typical minor work zone • Expected number days per year when a minor work zone will be in effect In the case of an MTB being used for major incidents, the factors that may influ- ence its effectiveness include the following: • Average expected duration of a major incident for which an MTB would be used • Expected number of such events per year • Time that it takes to shift the barrier over Generally, using an MTB system to reduce nonrecurrent congestion will be most beneficial in cases for which (1) demand-to-capacity (d/c) ratios are high and minor road work is often needed, or (2) d/c ratios are high and major incidents occur frequently. Cost The higher cost that may be associated with MTBs suggest that they are unlikely to be constructed for the sole purpose of reducing nonrecurrent congestion. Rather, an MTB system might be installed to reduce recurrent congestion, and its potential application to nonrecurrent congestion would be an important ancillary benefit.

23 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Factors that affect the cost of installing and operating an MTB system include the following: • Length of roadway where MTB will be used • Type of barrier-moving machine used • Number of barrier-moving machines needed • Frequency of use of the system • Movable barrier installation costs • Changes to pavement markings • Additional signing required • Ongoing operating costs o Machine operator wages (or additional wages, if an MTB system is already in place but currently used for recurrent congestion only) o Fuel, maintenance, and repairs for barrier-moving machine Gated Median Barriers Description and Objective A gated median barrier consists of adding a gated section within a continuous median barrier. Typically, these gates, which may be manually or automatically operated, pro- vide access to maintenance personnel, emergency responders, and other authorized users. An example of a gated median barrier is shown in Figure 3.3. The objective of a gated median barrier is to provide quicker access for emer- gency responders arriving on the scene of an incident. The faster emergency responders arrive on the scene, the sooner the incident can be cleared and normal operations can resume. Gated median barriers offer this benefit while preventing unauthorized access. Typical Applications Gated median barriers are typically provided on urban sections of freeway, but where interchanges are spaced farther apart. Where interchange spacing is relatively long, emergency responders may be faced with excessive travel distances to reach an inci- dent. Gated median barriers allow emergency responders to access the incident site more directly and quickly. Gated median barriers can also be used to reroute traffic, via a U-turn maneuver, in response to a major lane-blocking incident. If a major incident blocks traffic for a sig- nificant amount of time, traffic may back up behind the incident and cause significant delay to motorists. If a gated median barrier is opened to allow traffic to be rerouted to the opposite direction of travel, the queue in the incident direction can dissipate. These rerouted vehicles can return to the next closest interchange and proceed along an alternate route to their destination.

24 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Design Criteria The Federal Highway Administration (FHWA) report Facilitating Incident Manage- ment Strategies on Freeways (Parham et al. 1999) provides design guidance for gated median barriers: • Weight. The weight of the gate should not exceed 40,000 lb. • Crashworthiness. The gate should meet the crashworthy recommendations pre- sented in NCHRP Report 350 (Ross et al. 1993) for longitudinal barriers. How Treatment Reduces Nonrecurrent Congestion Reduces Incident Duration Gated median barriers reduce the period of time during which disabled, crash-involved, or police vehicles remain in the roadway or on the roadway shoulder by decreasing the response time of emergency personnel. By reducing the lane-blocking time of an incident, the nonrecurrent congestion associated with that incident is reduced. This reduction in nonrecurrent congestion results in an increased reliability for the roadway segment. Factors Influencing Treatment Effectiveness The following factors may influence the effectiveness of gated median barriers at re- ducing nonrecurrent congestion: • Average response time reduction for the instances when the treatment is used • Percentage of each incident type that will use the treatment to improve response time Figure 3.3. Gated median barrier in Atlanta, Georgia.

25 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Cost Factors that affect the cost of installing a gated median barrier include the following: • Type of gated median barrier (manually operated, automated) • Operational and ongoing maintenance costs • Cost of removing existing barrier • Installation of new signing to discourage unauthorized use of turnaround Movable Cable Median Barriers Description and Objective A movable cable median barrier involves construction of a specially designed wire cable barrier system that can be removed to allow median crossovers. The system is constructed such that the cables can be detached from the posts individually and the posts can be removed from their base. This deconstruction can be done over a short segment of the length of the cable to provide a temporary median opening for emer- gency vehicles in the event of a crash or other major incident. This machine- and tool- free process is designed such that the cable median barrier can be reassembled with minimal effort to restore the barrier system to its permanent state. The objective of movable cable median barriers is to provide a temporary access point for emergency vehicles. In the event of a major incident that results in significant queuing, a temporary median opening in the cable median barrier can also be used to reroute traffic, via a U-turn maneuver, to dissipate the queue. These rerouted vehicles can return to the next closest interchange and proceed along an alternate route to their destination. Figure 3.4 illustrates a movable cable median barrier. Typical Applications Movable cable barriers are typically used along roadway segments with a crash his- tory of frequent head-on collisions. When movable cable barriers are used in medians, the capability of creating access points is most effective where the median is easily traversable. Movable cable median barriers may be considered on roadways where a cable barrier exists or is needed and where flexibility is desired in providing access points along the median for emergency vehicles or for rerouting traffic around an incident or work zone. Design Criteria Design criteria for the movable feature of the cables and posts are vendor specific; several companies have developed movable cable median barriers. The key compo- nents of the movable cable median barrier are (1) wires that can be easily detached from the posts and (2) posts that can be easily removed from their foundation.

26 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Figure 3.4. Standard three-strand movable cable median barrier. How Treatment Reduces Nonrecurrent Congestion Reduces Incident Duration Movable cable median barriers reduce the time during which disabled, crash-involved, or police vehicles remain in the roadway or on the roadway shoulder by decreasing the response time of emergency personnel. By reducing the lane-blocking time of an incident, the nonrecurrent congestion associated with that incident is reduced. This reduction in nonrecurrent congestion increases reliability for the roadway segment. Factors Influencing Treatment Effectiveness Factors that may influence the effectiveness of a movable cable median barrier at re- ducing nonrecurrent congestion include the following: • Average reduction in response time when the treatment is used • Percentage of each incident type that will use the treatment to improve response time Cost The following factors can affect the cost of installing a movable cable median barrier along a roadway section: • Type of installation (new construction or retrofit)

27 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Length of proposed barrier section • Existing barrier type (modular concrete, semirigid, nonmovable cable) • Removal costs for the existing median barrier • Installation costs for the movable cable median barrier • Expected frequency of use for emergency access or queue dissipation Extra-Height Median Barriers Description and Objective An extra-height median barrier (EHMB) obscures motorists’ view of the opposite di- rection of travel. This effect is often achieved through design of a barrier taller than driver eye height, but it can also be achieved by the addition of material on top of an existing barrier. EHMBs can be used to reduce nonrecurrent congestion in the follow- ing ways: • Minimize rubbernecking by blocking the view of opposing lanes of traffic during an incident (“gawk screens”) • Prevent crashes caused by headlight glare from opposing traffic at night (often referred to as “glare screens” and commonly used in work zones) • Prevent cross-median collisions involving taller vehicles that may not be stopped by a traditional height barrier (i.e., an EHMB must be crashworthy) Depending on the material used to provide the extra height, a particular installa- tion of an EHMB may serve more than one of these purposes. Rubbernecking causes delay and degrades reliability in two major ways: • Rubbernecking during an incident causes traffic to slow, thereby reducing capacity and increasing congestion on the roadway. • Gawking can lead to secondary collisions when a driver’s attention is focused on an incident and not on the traffic in front of or around him. Although reducing headlight glare can be beneficial for drivers, there is no clear research indicating what effect it might have on reducing nonrecurrent congestion or improving travel time reliability. Reducing cross-median collisions involving trucks is an important safety benefit of tall concrete EHMBs. Various methods of providing extra height to median barriers have been imple- mented. These include installing concrete barriers that are designed to be taller, adding planters and shrubs on top of the barrier, installing metal or plastic paddles or screens on top of the barrier, or using tall shrubs instead of a concrete barrier in the median. An example of an EHMB is illustrated in Figure 3.5. Typical Applications EHMBs may be considered at high-crash locations where vehicles in one direction of travel are visible to drivers in the other direction of travel and where rubbernecking at incidents in the opposite direction of travel is problematic. Application of an EHMB

28 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Figure 3.5. Extra-height median barrier in Atlanta, Georgia. must consider the horizontal and vertical geometry of the roadway (see Design Criteria below). Design Criteria For purposes of calculating vertical design criteria (such as sight distance), the AASHTO Green Book (2011) uses a driver eye height of 42 in. A barrier must certainly be no lower than this height to visually obstruct the opposing traffic lanes, and larger heights are probably needed to effectively perform this function (especially for taller vehicles). The area over which the median barrier blocks the view of opposing lanes is referred to as the influence area. In addition to ensuring that the median barrier is sufficiently tall to reduce visual distraction caused by incidents on opposing lanes of the divided highway, several other factors that will affect treatment effectiveness should be considered: • Roadway geometry. Vertical and horizontal alignment will affect the driver’s sight lines and the influence area of the EHMB. • Opacity of barrier design. Widely spaced “paddles” may reduce headlight glare for approaching vehicles a significant distance away, but they may not eliminate incident-induced gawking, especially as the driver approaches the incident and can look between the paddles. The influence area of the barrier in this case may be significantly reduced. • Crashworthiness. Only some materials used to add height to barriers are intended to reduce cross-median crashes. If reducing nonrecurrent congestion due to severe cross-median crashes is a goal, a design that reduces these types of crashes will be required.

29 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION How Treatment Reduces Nonrecurrent Congestion Reduces Number of Gawking Events EHMBs reduce rubbernecking by obscuring drivers’ view of incidents on the other side of the barrier (the opposite direction of travel). Without an EHMB in place, drivers tend to gawk at incidents that occur in the opposite direction of travel. This gawking usually involves a reduction in speeds as drivers slow down to get a better view of the incident on the opposite side of the roadway. An EHMB can improve reliability by blocking the view of drivers in this situation, such that they are unaware of opposite- direction incidents. By reducing the number of gawking events, EHMBs can reduce the delay associated with the lower speeds during these events and improve reliability. Reduces Incident Frequency When drivers gawk at incidents in the opposite direction, their attention is diverted from the task of operating their vehicle. As a result, crashes can occur in the primary direction that are a direct result of gawking at an opposite-direction incident. These crashes are termed “secondary crashes.” To the extent that gawking at opposite-direction incidents causes secondary crashes, an EHMB will eliminate these secondary crashes. By eliminating these sec- ondary crashes, delay in the primary direction is reduced and reliability is improved. Reduces Incident Severity EHMBs may reduce cross-median crashes involving tall vehicles (such as tractor trailers), which are often severe in nature. Although the barrier will not prevent a crash from occurring, it works to reduce the severity of the crash by absorbing impact and redirecting the vehicle onto the shoulder. Assumptions based on local experience will help determine whether this treatment benefit will reduce nonrecurrent congestion on the roadway. Factors Influencing Treatment Effectiveness Factors that may influence the effectiveness of EHMBs at reducing nonrecurrent con- gestion include the following: • Number of gawk-inducing incidents. Roadways with frequent crashes, disable- ments, work zones, or other gawk-inducing incidents will receive more significant improvement to operations from EHMBs than roadways with few gawk-inducing incidents. Rubbernecking may not have a significant effect on congestion if traffic flow is relatively light at the time of the incident. Low traffic volumes also reduce the probability of the occurrence of a secondary incident. • Influence area of median barrier. Barriers that only block a driver’s view over a short distance along the road will be less effective than those that block the driver’s view over a longer distance. Influence area is a factor of median design and road- way geometry. • Truck volume. Areas with high truck volumes are more likely to see a reduction in crash severity by reducing the frequency of cross-median crashes involving heavy vehicles.

30 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Cost Factors that affect the cost of implementing EHMBs along a roadway section include the following: • Construction type (new roadway or retrofit of existing roadway) • EHMB material (reinforced concrete, planted shrubs) • Length of roadway over which treatment will be installed • Maintenance required (e.g., caring for vegetation, repair or replacement required when vehicle collides with barrier) Mountable/Traversable Medians Description and Objective Mountable/traversable medians do not physically prevent vehicles from crossing from one direction of travel to the other. Examples of this treatment include the following: • Flush or painted medians • Two-way left-turn lanes (TWLTL) (illustrated in Figure 3.6) • Raised (but traversable) medians The objective of mountable/traversable medians is to provide better access for law enforcement and emergency vehicles responding to an incident, thus reducing the time between when an incident occurs and when emergency responders arrive on the scene. The sooner emergency responders can arrive on the scene of an incident, the sooner the incident can be cleared and normal freeway operations can resume. Figure 3.6. Two-way left-turn lane.

31 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Typical Applications Mountable/traversable medians are primarily used to provide emergency access, as described above, while still providing a visual separation between opposing directions of travel. Various traversable and nontraversable median types are found in every metro politan area around the nation. Design Criteria Following are descriptions of three specific types of traversable medians: • Flush medians. Typical widths for a flush median on an urban street can range from 4 to 13 ft. To accommodate a separate left-turn lane, a flush median should be 13 ft wide. • Flush TWLTL. AASHTO recommends a TWLTL width of between 10 and 16 ft for left-turning vehicles. The usual design widths for flush TWLTLs are 11, 12, or 13 ft. There is some evidence that wide TWLTLs encourage drivers to place their vehicles in an angular rather than parallel turning position and thereby cause encroach ments on adjacent through lanes. • Traversable TWLTL. Where a mountable 2-in.-high curb is used to delineate the edges of a median, this median is designated as a traversable median, and traffic is allowed to turn left across the median. The normal traversable TWLTL median width for new construction is 16 ft. As discussed for the flush TWLTL, wide traversable TWLTL medians also may encourage drivers to store in an angular position. Therefore, wider widths should not be provided. How Treatment Reduces Nonrecurrent Congestion Reduces Incident Duration Mountable/traversable medians reduce the period of time during which disabled, crash-involved, or police vehicles remain in the roadway or on the roadway shoulder by decreasing the response time of emergency personnel. By reducing the lane-blocking time of an incident, the nonrecurrent congestion associated with that incident is re- duced. This reduction in nonrecurrent congestion results in an increased reliability for the roadway segment. Factors Influencing Treatment Effectiveness Factors that may influence the effectiveness of mountable/traversable medians at re- ducing nonrecurrent congestion include the following: • Average response time reduction for the instances when the treatment is used • Percentage of each incident type that will use the treatment to improve response time Cost Factors that affect the cost of installing a mountable or traversable median along a roadway section include the following:

32 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Existing median type (raised curb, TWLTL, none) • Proposed median type (TWLTL, mountable curb, painted median) • Length of section to be improved SHOULDERS Accessible Shoulders Description and Objective An accessible shoulder consists of a wider shoulder or an improvement to the surface of an existing shoulder (e.g., replacing a gravel shoulder with a paved shoulder) such that the shoulder can serve one or both of the following functions: • As a pulloff for disabled or incident-involved vehicles (as illustrated in Figure 3.7 and Figure 3.8) • As a “bypass lane,” allowing emergency responders to go around queued mainline traffic and reach the incident scene more quickly Improving a shoulder to allow mainline traffic to use the shoulder as a travel lane is addressed elsewhere in this chapter (see discussion under Drivable Shoulders). Typical Applications Accessible shoulders may be considered for roadway segments with shoulders that cannot accommodate disabled, incident-involved, or emergency vehicles due to insuf- ficient width or surface type. Figure 3.7. An accessible shoulder in use during a crash investigation. Source: U.S. Fire Administration (2008).

33 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Design Criteria The AASHTO Green Book (2011) states that desirably, a vehicle on the shoulder should clear the edge of the traveled way by at least 0.3 m (1 ft), and preferably by 0.6 m (2 ft). These dimensions have led to the adoption of 3.0 m (10 ft) as the normal shoulder width that is preferred along higher speed, higher-volume facilities. Heavily traveled, high- speed highways and highways carrying large numbers of trucks should have usable shoulders at least 3.0 m (10 ft) wide and preferably 3.6 m (12 ft) wide; however, widths greater than 3.0 m (10 ft) may encourage unauthorized use of the shoulder as a travel lane. . . . [A]lthough it is desirable that a shoulder be wide enough for a vehicle to be driven completely off the traveled way, narrower shoulders are better than none at all. For example, when a vehicle making an emergency stop can pull over onto a narrow shoulder such that it occupies only 0.3 to 1.2 m (1 to 4 ft) of the traveled way, the remaining trav- eled way width can be used by passing vehicles. How Treatment Reduces Nonrecurrent Congestion Reduces Number of Lanes Blocked by an Incident With the installation of an accessible shoulder, crash-involved vehicles and noncrash incidents (such as disabled vehicles) that block travel lanes can be relocated to the shoulder, resulting in reduced lane-blocking time. This decrease in average lane- blocking time reduces the nonrecurrent congestion associated with the incident, thus reducing delay and improving reliability. Figure 3.8. A vehicle fire on the left shoulder of a freeway. Source: U.S. Fire Administration (2008).

34 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Reduces Incident Response Time An accessible shoulder can be used for emergency vehicle access. In this case, emer- gency responders use the shoulder to bypass traffic congestion on the roadway and reach the crash site earlier. This decrease in response time reduces the total incident duration, so lanes are blocked for a shorter period of time, delay is reduced, and reli- ability is improved. Reduces Frequency of Incidents The provision of a shoulder has been shown to be associated with a reduction in inci- dent frequency. The Highway Safety Manual (AASHTO 2010) provided crash modifi- cation factors for provision of a shoulder. Factors Influencing Treatment Effectiveness Factors that may influence the effectiveness of accessible shoulders at reducing nonre- current congestion include the following: • Existing and planned usable shoulder width. An improvement that widens a 2-ft shoulder to a 12-ft shoulder will experience more benefit than an improvement that widens an 8-ft shoulder to a 12-ft shoulder. • Existing and planned shoulder surface type (paved or unpaved). Only paved shoulders will allow emergency vehicles the opportunity to bypass congestion to reach the scene of an incident faster. In addition, reductions in incidents due to shoulder widening are for paved shoulders only. • Frequency of incidents requiring emergency vehicle response in congested condi- tions. The shoulder benefits crashes or other emergencies that require an emer- gency response vehicle that would have a difficult time reaching the incident scene in a timely manner without the use of the shoulder. • Typical emergency response time. The expected reduction in emergency response time when a shoulder is available for use should be compared with typical emer- gency response times. • Frequency of lane-blocking incidents. The proportion of lane-blocking incidents that could be moved to a shoulder, as well as their frequency, should be considered. Cost Factors that affect the cost of implementing an accessible shoulder along a roadway section include the following: • Shoulder material (concrete, asphalt, gravel, or earth) • Width of shoulder (before and after) • Availability and purchase cost of right-of-way for shoulder widening • Topography of roadside (especially in mountainous conditions where considerable cut or fill may be required to provide a shoulder) • Necessary clearing and grubbing of roadside to increase shoulder width

35 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Temporary traffic control costs during construction • Removal or addition of striping • Addition or reinstallation of rumble strips Drivable Shoulders Description and Objective A drivable shoulder can be temporarily used by mainline traffic as a travel lane. Driv- able shoulders can be used to restore lost capacity by routing traffic around a lane- blocking incident, such as a work zone or a crash. Several examples of drivable shoul- ders are illustrated in Figures 3.9 through 3.11. Typical Applications Though the practice of temporarily routing traffic around work zones or incidents is not widespread in the United States, Germany has implemented drivable shoulders (termed shoulder-use lanes or hard shoulder running) to address nonrecurring conges- tion. In Germany, a hard shoulder is required for temporarily routing traffic around scenes of accidents or 1-day work zones. England also uses shoulder-use lanes as a temporary measure for work zones. Design Criteria The Green Book (AASHTO 2011) does not provide design guidance for drivable shoul- ders in the context of converting a shoulder to a travel lane, rerouting traffic around an incident or work zone, or providing emergency vehicle access to an incident. NCHRP Report 254: Shoulder Geometrics and Use Guidelines (Downs and Wallace 1982) provides application and design guidance for converting a shoulder to a travel lane. According to this report, when a shoulder is used to route traffic around Figure 3.9. Drivable shoulder in Germany. Source: Kuhn (2010).

36 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Figure 3.10. Bus-only drivable shoulder on I-35 in Minneapolis, Minnesota. Source: DeCorla-Souza (2007). Figure 3.11. Buses allowed on shoulder in Atlanta, Georgia. a work zone, 9 ft of shoulder width is considered acceptable, and 12 ft of shoulder width is considered optimal. Most agencies will not route traffic around an incident unless the incident is in the lane adjacent to the shoulder because of the difficulty of communicating all the neces- sary lane shifts for other incident locations. How Treatment Reduces Nonrecurrent Congestion Increases Roadway Capacity During an Incident Incidents cause nonrecurrent congestion and delay by blocking one or more lanes for the duration of the incident. If local policy allows, some of the incidents that block the

37 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION rightmost lane of the roadway may be treated using drivable shoulders by diverting traffic around the incident on the right shoulder. Factors Influencing Treatment Effectiveness The following factors may influence the effectiveness of drivable shoulders at reducing nonrecurrent congestion: • Number of days that the shoulder will be used as a lane during each work zone • Percentage of incidents (by magnitude) occurring in outside lane during the analysis period • Percentage of outside-lane incidents (by magnitude) during which treatment is expected to be used • Effective capacity of shoulder • Protocols for opening a shoulder lane during an incident (e.g., severity, location) • Hours of the day and days per year shoulder is used as general-purpose lane Cost Factors that may affect the cost of providing a drivable shoulder along a roadway sec- tion include the following: • Length of roadway section • Width of pavement addition • Type of pavement (asphalt, concrete) • Temporary traffic control required during installation (or whether shoulder wid- ening will be done along with a construction project that is already planned) • Frequency of incidents during which traffic will be rerouted onto the shoulder • Pavement construction costs • Sign installation costs • Ongoing costs to personnel directing traffic around incidents (e.g., law enforcement) Alternating Shoulders Description and Objective Alternating shoulders are used on roadways without sufficient width to provide full shoulders on both sides of the roadway. Instead of providing narrow shoulders on both sides of the roadway, a full width shoulder is provided for one direction of travel for a specified length of roadway, and then provided for the other direction of travel for the next segment of roadway. Alternating shoulders are often used in work zones where limited roadway width is available. In particular, this treatment is commonly used when one side of a divided roadway is shut down for a work zone and traffic is moved to the other side of the divided roadway, resulting in two-way traffic operations on that side. Rather than

38 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION having a full shoulder for one direction of travel and little or no shoulder for the opposing direction of travel, a full shoulder is provided for one direction for some dis- tance, and then the full shoulder is shifted to the opposite direction for some distance. Alternating shoulders may also be implemented as a permanent treatment when one shoulder has been converted to a travel lane to meet increased demand, or where paved shoulders on both sides of an undivided roadway are narrow. Illustrations of two types of permanent alternating shoulder installations are shown in Figure 3.12. Typical Applications Alternating shoulders may be considered when one side of a divided roadway is shut down for a work zone, resulting in two-way traffic operations on the other side. A typical example would involve converting the cross section on that side of the roadway as follows: • From same-direction travel lanes to opposite-direction travel lanes • From a 4-ft inside shoulder to a barrier between travel lanes • From a 10-ft outside shoulder to a 10-ft alternating shoulder Figure 3.13 illustrates a typical cross section on one side of a divided roadway before and after converting it to a cross section with alternating shoulders. Design Criteria The AASHTO Green Book (2011), which provides general guidance for shoulder design, states that 10 ft is a typical shoulder width along freeways and that a width of 12 ft may be desirable on high-speed facilities with heavy truck volumes. In some work zones, the use of 6- to 8-ft shoulders may be warranted, especially if these widths allow the provision of shoulders for both directions of travel. The Green Book also states that “although continuous shoulders are preferable, narrow shoulders and inter- mittent shoulders are superior to no shoulders.” This guidance seems to imply that if shoulders of at least 6 to 8 ft cannot be provided along a roadway segment, intermit- tent shoulders of at least that width would be an acceptable alternative. Figure 3.12. Alternating shoulder examples: (a) median shift and (b) lane shift. (a) (b)

39 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION The Green Book does not provide guidance on the recommended length of each intermittent shoulder. However, lengths of a quarter mile to a half mile have been reported by highway agencies. Intermittent shoulder spacing should not be so long that a disabled vehicle in a section without a shoulder would not be able to be relo- cated to the next available shoulder section. When the alternating shoulder is designed in such a way that the lanes are shifted back and forth, the length of the transition between a shoulder on one side of the road and the other side will be somewhat dependent on travel speed. The faster traf- fic is moving, the longer the transition will need to be. The taper along a shoulder section that is transitioning into a section with no shoulder should be long enough to Figure 3.13. (a) Before and (b) after conversion of a roadway to two-way operation with alternating shoulders. (a) (b)

40 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION accommodate a vehicle trying to reenter the roadway. This is especially true for alter- nating shoulders in the median, where the reentering vehicle will be merging into the fastest lane of traffic. How Treatment Reduces Nonrecurrent Congestion Reduces Frequency of Crashes The provision of a shoulder tends to reduce incident frequency. Zegeer et al. (1988) reported that a reduced crash rate can be calculated on the basis of shoulder width and shoulder type (paved or unpaved). To account for the fact that the wider shoulder is provided on only a portion of the freeway segment, a weighted average is calculated. For example, if 40% of the study segment has a shoulder, the weighted average crash rate is 0.4 times the lower crash rate plus 0.6 times the higher crash rate. Reduces Lane Blocking by Incidents With the installation of a shoulder, crash-involved vehicles that block lanes can be re- located to the shoulder. This results in a decreased lane-blocking time for these crashes. This reduction in average lane-blocking duration reduces the nonrecurrent congestion associated with crashes, thus reducing delay and improving reliability. Factors Influencing Treatment Effectiveness Factors that may influence the effectiveness of providing alternating shoulders include the following: • Existing and planned usable shoulder width (e.g., an improvement that widens a 2-ft shoulder to a 12-ft shoulder will experience more benefit than an improve- ment that widens an 8-ft shoulder to a 12-ft shoulder) • Existing and planned shoulder type (paved or unpaved) • Frequency of lane-blocking incidents, and proportion of those that could be moved to a shoulder • For a given direction of travel, the cumulative length (or percentage of segment) for which a full-width shoulder is available Cost Factors that may affect the cost of implementing an alternating shoulder include the following: • Whether the alternating shoulder will be installed in the median of a divided road or on the outside shoulder • Whether the alternating shoulder is designed as a permanent treatment or a tem- porary treatment during a construction project • The interval spacing that will be used (or how many times the shoulder will switch sides over the length of the treatment implementation) • Need for additional pavement construction

41 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Barrier and impact attenuator costs (moving existing, adding new, or replacing) • Costs associated with removing and adding pavement marking • Temporary traffic control (discussed in more detail below) • Permanent signing In work zones where previously divided two-way traffic is shifted to one direction of the divided roadway, a modular concrete barrier may be required. Impact attenua- tors may also be necessary at the end points of the barrier. If modular concrete barriers are not necessary (perhaps on lower-speed or lower-volume roadways), reflective delin- eators, cones, or barrels may be used to separate traffic and reduce the likelihood that vehicles will travel in lanes designated for the opposite direction of flow. Similarly, permanent designs that involve a median shift may require relocating existing median barriers or adding new barriers along the new median shoulder align- ment. Both temporary and permanent alternating shoulders will require additional guidance signs to inform drivers of the new alignment, and additional signs, such as speed limit reduction signs, may also be needed. Finally, additional law enforcement presence may be beneficial to ensure motorists are using the alternating shoulders properly and obeying all traffic controls. Some costs may be incurred regardless of the treatment installation, such as restriping costs on a project that also includes resurfacing. Careful project planning and staging may help minimize costs associated with this treatment. Portable Incident Screens Description and Objective Portable screening devices placed around an incident (typically along the roadside) obscure motorists’ view of the incident and reduce congestion caused by rubberneck- ing (or “gawking”). The primary objective of this treatment is to reduce rubber- necking caused by the presence of an incident. By reducing rubbernecking, this treat- ment can reduce nonrecurrent congestion and secondary incidents. Typical Applications Portable incident screens are used to block motorists’ view of an incident and incident- response activities, which can be distracting to motorists passing by. Screens may im- prove safety and traffic flow when traffic volumes approach roadway capacity because they discourage gawking (and slowing) by other motorists. Application guidance regarding appropriate traffic, site, and emergency manage- ment conditions include the following. Traffic Conditions • The incident is likely to take more than 3 h to clear. • The incident is likely to cause secondary congestion due to rubbernecking, and it is believed that deployment of the incident screen is likely to reduce this.

42 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Traffic volumes on the opposite roadway from the incident are expected to be relatively high or expected to become high during the anticipated duration of the incident. Site Conditions • It is possible to deploy the incident screen while maintaining at least 3.9 ft (1.2 m) between the incident screen and traffic lanes. • It is possible to maintain an incident-side safety zone of at least 3.9 ft (1.2 m) from the screen at all times while the screen is being set up, is in place, and is being dismantled. (Vehicles and personnel should not enter the safety zone except in an emergency or to maintain the screen. If the safety zone cannot be provided, the screen should not be deployed.) • Weather conditions are appropriate for the use of the screen. In particular, the screen should not be deployed in conditions of snow, ice, poor visibility, or high winds exceeding the maximum wind speed permitted for the stability classification of the screen. Emergency Management Considerations • The police must confirm that they do not consider that use of the screen would disrupt their operations. • The ambulance, fire, and rescue services must agree to the use of the screen in the situation in question. • The emergency services must confirm that it is not expected that a helicopter will land in the vicinity of the incident. • The presence of the incident screen must present no hazard to or interfere with the work of the emergency services (including any operations of the air ambulance service), all other incident-related activity, or any ongoing work activities. Design Criteria The FHWA Manual on Uniform Traffic Control Devices (MUTCD) (2009) provides the following guidance: “Screens should not be mounted where they could adversely restrict road user visibility and sight distance, and adversely affect the operation of vehicles.” One of the most important operational factors in deployment of incident screens is their ability to remain stable in ambient wind conditions. United Kingdom guidelines identify the following stability criteria: • A free-standing incident screen must comprise the following: o A rigid, flexible, or semiflexible sheet material that is restrained at the top, bottom, and both sides by attachment to a rigid (i.e., nonfolding) supporting structure to form a panel.

43 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION o A series of portable bases to support one or more panels, the stability of which may be augmented by means of sandbags. • The size and weight of the screen bases must be such as to ensure the stability of the free-standing incident screen system. • Any sheet material used for either barrier-mount or free-standing incident screen panels is permitted to have a provision for reducing wind loading. This may be through either of the following methods: o Being porous over the entire area of the sheet material (wind-porous material). o Having vents in the sheet material that are effective from both sides. How Treatment Reduces Nonrecurrent Congestion Reduces Number of Gawking Events A portable incident screen reduces rubbernecking within its influence area by obscur- ing drivers’ view of incidents. Without a screen in place, drivers will tend to gawk at incidents in their direction of travel. This gawking usually involves a reduction in speeds as drivers slow down to get a better view of the incident. A portable incident screen can improve reliability by blocking the view of the drivers, such that they are unaware of the incident. By reducing the number of gawking events in the primary direction, portable incident screens can reduce the delay associated with the lower speeds during these events and improve reliability. Reduces Incident Frequency When drivers gawk at incidents, their attention is diverted from the task of operating their vehicle. As a result, crashes can occur that are a direct result of gawking at an incident. These crashes are termed secondary crashes. To the extent that gawking at incidents causes secondary crashes, a portable inci- dent screen will eliminate these secondary crashes. By eliminating these secondary crashes, delay is reduced and reliability is improved. Factors Influencing Treatment Effectiveness Factors that may influence the effectiveness of portable incident screens at reducing nonrecurrent congestion include the following: • Expected number of incidents on the roadway • Proportion of incidents that are gawk inducing (i.e., that cause rubbernecking) • Percentage of gawk-inducing incidents that use screens • Response time of screen crew to incident scene • Treatment deployment time Roadways with high numbers of incidents and with a high proportion of gawk- inducing incidents will receive the most benefit from portable incident screens. Faster response and set-up times of screen crews will also result in a larger benefit for this treatment.

44 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Cost Factors that may affect the cost of portable incident screens include the following: • Level of wind resistance needed • Type of screen (e.g., fabric screen with metal tube support structure, chain link fence with opaque covering, or other) • Screen purchase cost • Total length of screen needed • Screen maintenance and storage costs • Cost for multipanel connections • Cost for stabilizing hardware (e.g., sandbags) • Cost for vehicle or trailer for transporting screens to the site • Other transportation-related costs (e.g., maintenance and fuel) • Personnel required for deployment Vehicle Turnouts Description and Objective Vehicle turnouts are short sections of shoulder provided on routes that have either no shoulder or very narrow shoulders. Vehicle turnouts allow a disabled or crash- involved vehicle to pull completely out of the travel way. Vehicle turnouts are typically at least 8 ft wide and are most often found on rural, two-lane highways with less than an 8-ft paved shoulder. Typical Applications Typical applications of vehicle turnouts include the following: • Rural two-lane roads with little or no passing opportunity • Scenic routes where travelers may want to slow or stop their vehicle to enjoy the scenery • Locations without safe locations for law enforcement officers to monitor and enforce traffic violations • Urban or suburban areas without paved shoulders wide enough to accommodate emergency stops • Along freeways or on freeway ramps where space is needed for maintenance vehicles Design Criteria Design elements for turnouts include turnout width, turnout length, taper length, pavement marking, and signing. These characteristics depend on traffic characteristics and roadway characteristics. Roadways with higher design speeds require longer taper

45 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION lengths to allow a vehicle to safely and smoothly leave the traveled way and merge back onto it. Turnouts are often provided in rural locations when queues of traffic typically form behind slow-moving vehicles, causing impeded vehicles to make risky passing maneuvers that may result in crashes. In urban and suburban areas, turnouts are often provided in locations that have experienced a high number of incidents resulting from vehicles blocking all or part of a travel lane. They may also be located along freeways and ramps to accommodate maintenance vehicles during roadway maintenance activi- ties. Turnouts are sometimes provided as a low-cost alternative to providing or widen- ing a shoulder. How Treatment Reduces Nonrecurrent Congestion Reduces Number of Lanes Blocked by an Incident With the installation of a vehicle turnout, crash-involved vehicles that block lanes can be relocated to the turnout. This results in a decreased lane-blocking time for these crashes. This reduction in average lane-blocking duration reduces the amount of nonrecurrent congestion associated with crashes, thus reducing delay and improving reliability. Factors Influencing Treatment Effectiveness Factors that may influence the effectiveness of vehicle turnouts in reducing non- recurrent congestion include the following: • Existing and planned usable turnout width (e.g., an improvement that widens a 2-ft shoulder to a 12-ft shoulder will experience more benefit than an improve- ment that widens an 8-ft shoulder to a 12-ft shoulder • Existing and planned shoulder type (paved or unpaved) • Frequency of lane-blocking incidents • Proportion of lane-blocking incidents that could be moved to a turnout • Cumulative length of turnout (or shoulder) (e.g., if there are two turnouts of 300 ft each, this value would be 600 ft) • Shoulder material and depth (i.e., whether the shoulder is stable enough to be used as a travel lane during a work zone) Cost Several factors may affect the cost of installing vehicle turnouts along a roadway section: • Width of the shoulder to be added • Shoulder material (asphalt, concrete, gravel, or earth) • Depth of shoulder material • Availability of right-of-way

46 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Topography of roadside (especially in mountainous conditions where considerable cut or fill may be required) • Percentage of roadway segment that will have shoulder (e.g., 100% would be a continuous shoulder) Bus Turnouts Description and Objective A bus turnout (or bus pullout) is a designated paved area to the side of the roadway for buses to pick up and drop off passengers. The bus turnout allows vehicles on the road- way to continue without being obstructed by a stopped or idling bus. Bus turnouts are often found on suburban arterial roads and include tapers or acceleration–deceleration lanes, a paved bus bay area, and a passenger waiting area. A bus turnout reduces disruptions to traffic flow due to bus stops along major roadways. It improves passenger safety during boarding and deboarding and can reduce the potential for rear-end crashes. Bus turnouts also provide a safe place for passengers to wait that is offset from the main travel lanes. Typical Applications Bus turnouts are typically constructed along high-volume, high-speed suburban cor- ridors in growing areas where sufficient right-of-way is available. Transit agencies with considerable experience with bus turnouts are located in states such as Florida, Washington, and California. However, bus turnouts exist in various forms all over the United States. Design Criteria On streets with operating speeds of 40 mph or greater and where a bus blocking a travel lane causes unacceptable delay or presents a potential safety concern, a bus turnout should be considered. The bus turnout should be at least 12 ft wide (Darnell & Associ- ates 2006). How Treatment Reduces Nonrecurrent Congestion Reduces Frequency of Lane-Blocking Stops If no bus turnout is provided, buses will stop in the travel lane while passengers board and deboard the bus. Traffic flow is interrupted during these lane-blocking events, and congestion occurs. Bus turnouts can reduce the frequency of these events and the asso- ciated congestion, thus improving reliability. Reduces Frequency of Incidents Bus turnouts help reduce the potential conflicts between buses and general traffic. Reduc ing these conflicts can reduce the frequency of incidents and crashes (particu- larly rear-end crashes). By reducing the frequency of incidents and crashes, this treat- ment reduces the congestion associated with these incidents and improves reliability.

47 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Factors Influencing Treatment Effectiveness Bus turnouts are effective in maintaining uninterrupted traffic flow along the main thoroughfare. They also potentially improve safety by removing stopped vehicles from the roadway. However, they sometimes create delay for bus riders due to the need for buses to merge back into the flow of traffic. Some states have enacted yield-to-bus laws and developed various placards and signs, but driver adherence to these laws is sometimes limited. The merge can also create potential safety issues. TCRP Report 19: Guidelines for the Location and Design of Bus Stops reports that bus drivers will not use a bus bay when traffic volumes exceed 1,000 vehicles per hour per lane (Fitzpatrick et al. 1996). Drivers explain that the heavy volumes make it extremely difficult to maneuver a bus out of a midblock or nearside bay and that the bus must wait an unacceptable period of time to reenter the travel lane. Consideration should be given to these concerns when contemplating the design of a bay on a high- volume road. Using acceleration lanes, signal priority, or farside (versus nearside or midblock) placements are potential solutions. Cost Factors that may affect the cost of installing a bus turnout along a roadway section include the following: • New pavement • Signs • Pavement markings • Lighting • Additional right-of-way • New curb and gutter CRASH INVESTIGATION SITES Description and Objective A crash investigation site (CIS) is a paved area provided near highways to allow the re- location of crash-involved vehicles from the crash site to a safer area out of the way of traffic where crash investigations can be conducted. In addition to providing a location for crash investigations, these sites can be used to make cell phone calls, change a tire, or even as a rest area. Each of these uses can make mainline travel safer by reducing the number of emergency stops made in or near the roadway. CISs reduce nonrecurrent congestion by minimizing the time during which vehicles remain in the roadway or on the roadway shoulder after an incident. Some typical installations are shown in Figures 3.14 through 3.16. The first figure shows a CIS within a freeway interchange in Chicago, Illinois, the second figure shows an off- system CIS in Minnesota, and the third figure shows a roadside CIS in Atlanta, Georgia.

48 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Typical Applications CISs are typically used on high-volume urban freeways, where crash investigations on the roadway shoulder have the greatest potential to create congestion. Design Criteria It is important that a CIS be clearly identified through signing and other markings. The more accessible that a CIS is, the more likely it is that motorists will make use of the site. Therefore, locating CISs near the freeway and making them easily accessible is recommended. Providing sufficient lighting and a paved area greater than 1,000 ft2 are also recommended to encourage CIS use (Dudek et al. 1988). CISs are typically used in the following types of location: • A gravel or paved area separated from the roadway and accessible from the main- line (see Figure 3.14) • A widened shoulder area or purpose-built parking area on a freeway off-ramp • A widened shoulder area on the arterial crossroad near a freeway off-ramp terminal • A widened shoulder area on a side street off the arterial crossroad in the vicinity of a freeway off-ramp terminal Police officers generally prefer CISs that are directly accessible from the mainline. When possible, it is preferable that police be consulted during the design and place- ment of CISs in order to identify the most ideal locations where both officers and motorists will be encouraged to use the sites. Figure 3.14. Aerial view of a crash investigation site (on the right side of the photo) within a freeway inter- change in Chicago, Illinois. Source: © Google Earth 2013.

49 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION How Treatment Reduces Nonrecurrent Congestion Reduces Lane-Blocking Time of Incidents By providing a preestablished relocation spot, this treatment can reduce the time dur- ing which disabled, crash-involved, or police vehicles remain in the roadway or on the roadway shoulder. This reduction in lane-blocking time reduces the congestion associ- ated with these incidents and increases the reliability of the roadway. Figure 3.15. Roadside crash investi- gation site in Minnesota. Figure 3.16. Crash investigation site in Atlanta, Georgia.

50 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Factors Influencing Treatment Effectiveness Several factors may influence the effectiveness of a CIS at reducing nonrecurrent congestion: • Expected number of crashes per year (by severity) • Proportion of crash-involved vehicles that are physically movable (by type) • Proportion of physically movable vehicles that are drivable • Presence or absence of local agency policy to move crash-involved vehicles to a nearby CIS, when appropriate • Number of CISs on the analysis segment • Location and accessibility (mainline, ramp, crossroad) • Presence of ancillary signing to identify and encourage the use of CISs Cost Factors that may affect the cost of constructing a CIS along a roadway section include the following: • Number of CISs to be installed along the roadway section • The total square footage of each • The material used for construction of the CIS (concrete, asphalt, gravel) • Right-of-way cost and availability • Need to install signs identifying the CIS and directing motorists into the site • Pavement marking RIGHT-OF-WAY EDGE Emergency Access Between Interchanges Description and Objective Gated emergency access points can be provided between standard interchanges to con- nect a freeway facility with the local street system for the purpose of providing quicker access to remote locations not directly served by an interchange for fire, medical, and other emergency vehicles. These types of access points can also be used for mainte- nance access to remote utility facilities and, as part of right-of-way considerations, to provide land access to remote and otherwise inaccessible properties. As this treatment is not intended to provide freeway access to the public, locked gates are typically installed at the freeway right-of-way line to restrict access. FHWA requires special approval to provide locked gate access to or from roadways in the Interstate highway system, and many state agencies have followed suit in requiring permission to provide locked gate access to or from freeway facilities. Some agencies, such as Caltrans (the California DOT), have developed policies prohibiting these types of access points.

51 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION The primary objective of locked gate access, as illustrated in Figure 3.17, is to allow emergency and utility vehicles to access the local street network or a specific site from the freeway in remote locations where ordinary access is not provided via free- way ramp interchanges. In the reverse direction, these access points could also prove beneficial if an emergency services agency or utility company would have an unusually circuitous route to access the freeway. Typical Applications Where interchange spacing is relatively long, emergency responders may have to travel a long distance past an incident site to reach an interchange, exit, and return to the incident site. Gated emergency access points allow emergency responders to access the incident site more directly and quickly, and the greater the interchange spacing, the greater the amount of time saved. Gated emergency access points can be used to reroute traffic via a U-turn maneuver in response to a major lane-blocking incident. When a major incident blocks traffic for a significant amount of time, traffic may back up behind the incident and cause signifi- cant delay to motorists. If an emergency access gate is opened to traffic, motorists can exit the freeway system and make their way to their destinations via alternate routes, thus dissipating the freeway queue and reducing delay. Design Criteria The Federal Aid Policy Guide (FHWA 1998) provides specific guidance for obtaining permission to use locked gate emergency access: • “Locked gate access points on the Interstate system are used primarily to provide access for fire, medical and other emergency vehicles to reduce travel time, for maintenance activities at remote utility facilities and as part of the right-of-way consideration, to provide land access in remote locations. • “Any request for locked gate access should be reviewed to ensure that vehicles can enter the Interstate safely, appropriate sight distance is available to and from the access, and the access is located such that the intended function is served (distance to nearest interchange and/or median crossover). Each new locked gate access ap- proval needs to incorporate the following conditions: o The gate shall be locked at all times except when opened for passage of the authorized vehicles. The distribution of keys for the lock should be limited. o The access roadway will be constructed of an inconspicuous natural material to discourage unauthorized use. o The purpose of the access should be specified.” Limited information about locked gate access points, outside of federal and state regulations and policies, is available in the literature.

52 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION How Treatment Reduces Nonrecurrent Congestion Reduces Incident Duration A gated emergency access point between standard freeway interchanges reduces the time during which disabled, crash-involved, or police vehicles remain in the roadway or on the roadway shoulder by decreasing the response time of emergency personnel. By reducing the lane-blocking time of an incident, the nonrecurrent congestion associ- ated with that incident is reduced. This reduction in nonrecurrent congestion results in an increased reliability for the roadway segment. Factors Influencing Treatment Effectiveness Factors that may influence the effectiveness of emergency access between interchanges on nonrecurrent congestion include the following: • Average response time reduction for the instances when the treatment is used • Percentage of each incident type that will use the treatment to improve response time Figure 3.17. Emergency access between interchanges. Source: © Google 2013.

53 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Cost Factors that may affect the cost of installing emergency access between interchanges include the following: • Type of control to be used (manually operated gate, automated gate, temporary barricades) • Pavement surface to be used (gravel, asphalt, concrete) • Length of pavement required to connect to local road network • Width of pavement connection • Signs needed to identify and discourage unauthorized use of gated emergency ac- cess points ARTERIALS AND RAMPS Ramp Widening Description and Objective This treatment consists of widening a freeway ramp by the construction of additional pavement. The benefits of this treatment discussed in this section relate specifically to the increased ramp capacity associated with ramp widening. The objective of ramp widening is to increase the capacity of a ramp by providing additional lanes or additional maneuvering space within the existing lanes, as illus- trated in Figure 3.18. This capacity increase is intended to eliminate (or preemptively Figure 3.18. A two-lane off-ramp in Minnesota.

54 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION avoid) the situation in which a ramp queue backs up onto the freeway mainline. Such queues can be detrimental to mainline operations and can result in significant delays and reduced reliability. In addition, this treatment can reduce nonrecurrent congestion by eliminating some of the crashes that occur on or near the ramp. Additional ramp width can pro- vide additional maneuvering space that can sometimes be used by drivers to avoid a collision. By eliminating crashes, this treatment can eliminate the congestion associ- ated with these crashes. Typical Applications Ramp widening may be considered at any ramp–arterial intersection where long queues form. FHWA’s Ramp Management and Control Handbook (Jacobson et al. 2006) provides the following reasons for widening a ramp: • Entrance ramps o Need for additional storage capacity (e.g., metered ramps where traffic fre- quently backs up into the adjacent arterial may be a candidate for widening) o Need for enforcement zones where personnel can be stationed safely and where ramp meter operations are clearly visible o Need for adequate room to perform maintenance activities (e.g., removing debris, trimming nearby vegetation, or repairing infrastructure) o Need for designated lanes for special classes of vehicles, such as HOVs (the additional capacity in these situations will promote use of transit and carpool- ing and vanpooling by providing benefits in terms of reduced delay for these vehicles • Exit ramps o Need for more storage at the ramp terminal traffic signal to keep queues from backing onto the freeway o Need to separate traffic movements at the traffic signal to provide efficient or safe signal operations o Need for additional turn lanes to efficiently handle high traffic volumes Design Criteria The MUTCD (FHWA 2009) and the Green Book (AASHTO 2011) offer informa- tion on various design considerations for any ramp or arterial intersection. Widen- ing entrance or exit ramps is often implemented in conjunction with adjustments to traffic signal timing to prevent queues from forming on the arterial or freeway. On exit ramps, ramp widening is often implemented with pavement markings to separate dif- ferent traffic movements. Design considerations for ramp widening include the following: • Storage capacity of the existing ramp and storage capacity needed to eliminate queuing on the freeway or arterial

55 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Required lane width for buses or other special vehicles, if added lane is to be an HOV lane or restricted to certain vehicles • Available right-of-way • Potential use of existing shoulder • Space for right-of-way maintenance to occur outside of lanes • Space for law enforcement outside of lanes • Ramp meters, if in use or if planned for future use • Auxiliary lanes on the arterial or freeway to accommodate additional lanes on the ramp Whenever construction is needed to widen a ramp, practitioners may find it ben- eficial to complete additional work, if needed, while the ramp is being widened. Such work may include fixing geometric deficiencies, repairing the roadway surface, and posting additional signs. In these situations, it may be more cost-effective to complete additional work if the ramp is already closed or if the resources are readily available. This practice reduces the level of effort required to close the ramp and to set up work zone–related equipment (e.g., signs, barriers, cones). If the ramp must be closed, com- pleting additional work may also reduce the number of times the ramp must be closed, which consequently reduces the impact on motorists. Ramp widening can have a positive effect on roadway safety by providing more maneuvering space, which can allow some crashes to be avoided. The safety effective- ness of a ramp-widening project will depend on several factors, including the following: • Safety concerns before the widening project (e.g., crash frequency near the ramp, queues around the ramp) • Allocation of space after the widening (e.g., additional lanes, HOV lane, enforce- ment zone, wider shoulders) • Traffic control changes in conjunction with the widening (e.g., signal timing changes at interchange, addition of meters) How Treatment Reduces Nonrecurrent Congestion Reduces Frequency of Queue Spillback Events Ramp widening can reduce the occurrence of off-ramp queues backing up onto the mainline of the freeway by increasing the capacity of the ramp. Because a queue effec- tively blocks a freeway lane until it dissipates, such events can have a significant effect on freeway reliability. By reducing the frequency of these lane-blocking events, this treatment can reduce the delay associated with these events and improve the reliability of the freeway segment. Factors Influencing Treatment Effectiveness Several factors may influence the effectiveness of ramp widening at reducing nonrecur- rent congestion:

56 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Frequency of off-ramp queues backing up onto the freeway mainline • Frequency of treatment use • Percentage capacity increase to off-ramp movements associated with ramp widening • Frequency of ramp-related crashes that could have been avoided if more maneuveri ng space were available Off-ramps with high demand-to-capacity (d/c) ratios will tend to benefit most from this treatment, particularly where the available storage space on the ramp is lim- ited, because a high d/c ratio creates a situation in which queues may frequently back up onto the mainline. The mainline demand volume is also important, because the effect of a queue blocking a freeway lane will be significantly stronger where mainline d/c ratios are high. Cost Factors that may affect the cost of ramp widening include the following: • Available right-of-way • Amount of proposed widening • Ramp grade • Length of proposed widening • Associated improvements or additions of auxiliary lanes • Associated changes to traffic control • Temporary traffic control required during construction • Type of pavement to be constructed (concrete, asphalt) • Changes to pavement markings • New signing required • Installation of additional signal heads • Adjustments to signal timing Ramp Closure Description and Objective A ramp closure is a method of managing traffic patterns on and surrounding freeway ramps. Closures may be implemented with the use of automatic gates, barriers, or gates that need to be moved manually or by physically removing a ramp. There are three general types or classifications of ramp closures: (1) permanent, (2) time of day or scheduled, and (3) temporary. Permanent and time-of-day (or scheduled) ramp clo- sures are applicable to recurrent congestion, and temporary ramp closures are appli- cable to nonrecurrent congestion. Only temporary ramp closures are considered here.

57 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Temporary ramp closures may be done in response to one of the following sources of nonrecurrent congestion: (1) major incidents, (2) work zones, (3) special events, or (4) inclement weather conditions. Temporarily closing a ramp will decrease the mainline demand, leading to a reduction in mainline congestion and improved reli- ability. This decrease in mainline demand necessarily results in an increased demand on alternate routes, which can cause increased congestion and reduced reliability on these routes. To mitigate this problem, improvements to the alternate routes should be considered (see Improvements to Detour Routes in this chapter). Figure 3.19 illustrates an example of a gate used to close a ramp. Typical Applications Temporary ramp closures may be done in response to one of the following sources of nonrecurrent congestion: (1) major incidents, (2) work zones, (3) special events, or (4) inclement weather conditions. Design Criteria Design considerations for temporary ramp closures include ramp layout, traffic con- trol equipment, traffic control layout, and signal timing. These elements of the design are determined by traffic conditions, ramp geometry, length of closure, and reason for closure. Some of these design considerations are briefly discussed below. The ramp closure layout depends on a variety of factors, including the length of the closure and the reason for closure. Guidance can be found in the MUTCD (FHWA 2009). Figure 3.19. An automated gate used to close access in response to severe weather conditions. Source: Jacobson et al. (2006).

58 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Barricades are often used for temporary closures because they are a low-cost option for infrequent closures. However, as setting up and removing barricades can be labor intensive, they are less desirable when closures are expected to be implemented more frequently. Semipermanent barriers such as flexible pylons or water-filled bar- rels may be used for planned full ramp closures. In locations where ramp closure is expected to occur periodically (such as for snow and ice, fog, planned special events, or peak period ramp closures), automatic ramp gates may be used. Gates may be con- trolled manually by staff in the field or remotely from a traffic management center, and they may swing vertically or horizontally. Ramp design may have to accommodate the path of horizontally swinging gates. The various types of equipment used to control the gates have various degrees of flexibility and various levels of required maintenance. Drivers should be provided advanced warning that a ramp is closed. Signs, flags, and pavement markings can all be used to indicate to the driver that the ramp is not accessible. Changeable or dynamic message boards should be used in advance of the ramp where available. Static signing may also be used. At ramp approaches with dedi- cated lanes, those lanes should be closed with cones, signing, or pavement marking in advance of the closed ramp. Even for ramp closures that are short in duration, the disruption to traffic flow may be significant, and the consequences to those who use the ramp may be severe. Outreach is especially important when ramps will be closed during peak times. For planned activities, such as special events and construction activities, travelers should be made aware of the closure well in advance and should be provided with informa- tion about alternate routes. This can be accomplished through electronic and print media, such as newsletters, website postings, e-mail distribution lists, and 511 systems. How Treatment Reduces Nonrecurrent Congestion Reduces Mainline Demand Ramp closures improve traffic operations on the mainline by shutting off access to the mainline at one or more ramp locations. Decreasing access reduces mainline demand, which decreases mainline congestion and improves reliability. Reduces Frequency of Queue Spillback Events The occurrence of off-ramp queues backing up onto the mainline of the freeway is re- duced by closing the off-ramp during special events. Such events can have a significant effect on freeway reliability, because the queue effectively blocks a freeway lane until it dissipates. By reducing the frequency of these lane-blocking events, this treatment can reduce the delay associated with these events and improve the reliability of the freeway segment. Factors Influencing Treatment Effectiveness Several factors may influence the effectiveness of ramp closure at reducing non recurrent congestion:

59 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Duration of the mainline event. Ramp closures will have a more prominent effect when a mainline work zone or other incident is expected to reduce the roadway capacity for a significant period of time. • Frequency of treatment use. Ramp closures that are used multiple times per year (e.g., for recurring special events like baseball games) will tend to see a greater benefit from this treatment than infrequently used ramp closures. • Capacity of alternate routes. If alternate routes are already badly congested, this treatment will be less effective (from a global system perspective) because the con- gestion will only be shifted from the mainline to other routes. Conversely, if alter- nate routes are operating well below capacity, ramp closures will be much more beneficial because traffic can divert to these other routes without significant delay increase to the roadway system in general. Cost Factors that may affect the cost of closing a ramp include the following: • Frequency of closure • Barrier or gate type (temporary barricades, permanent railroad crossing–type gates, permanent gates that require manual operation) • Personnel and time required to deploy closure Ramp Terminal Traffic Control Description and Objective Ramp terminal traffic control involves increasing the traffic capacity at the termination of a freeway off-ramp. These capacity improvements may involve the following: • Installation of signals • Improved signal timings • Installation of a roundabout • Installation of turn lanes • Changes to signing, striping, or signals used at the ramp terminal to control the movement of traffic • Manual traffic control by law enforcement (most commonly used during a special event) The objective of ramp terminal treatments is to increase the capacity at the ter- minal of a ramp to eliminate (or preemptively avoid) the situation in which a ramp queue backs up onto the freeway mainline. Such queues can be detrimental to mainline operations and can result in significant delays and reduced reliability. Figure 3.20 illus- trates a ramp terminal intersection. Figure 3.21 illustrates a signalized roundabout at a ramp terminal; the signal is triggered when a detector on the ramp detects a queue on the ramp.

60 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Figure 3.21. (a) Aerial image of Maryland roundabout and (b) photo showing the signalized arterial leg of the roundabout. Source: © Google 2013. (a) (b) Typical Applications Coordinating ramp terminal signal timing with other signals along the arterial corri- dor can reduce delay and increase traffic flow, resulting in fewer conflicts and crashes near the ramp terminal. Peak period spillback onto the freeway or arterial, which can also lead to crashes, can be reduced or eliminated by phasing adjustments, which can help clear peak period ramp queues. Properly assigned turning lanes on exit ramps and extra storage can help prevent queues from spilling back onto the freeway. Advanced signing for lane assignments and directional information can assist drivers in making lane placement decisions early and can prevent weaving and unexpected movements that lead to crashes near the ramp terminals. Clear pavement markings can assist in this, as well. Figure 3.20. Ramp terminal intersection. Source: Hughes et al. (2010).

61 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Traffic control provided by law enforcement is a common treatment at inter- changes near arenas, stadiums, and concert halls when high volumes of traffic are arriving or leaving at the same time. Although police presence is not directly design related, certain design elements may provide assistance to police officers or help make their directions more clear. Some examples may include permanent variable message signs, changeable lane-control signs, clear striping, median refuges, appropriate place- ment of signal boxes, and others. Police presence may also be used at a ramp terminal to direct traffic in cases of incidents that occur on or near a ramp. One example of an innovative ramp terminal treatment is found on Maryland Highway MD-100 near Baltimore, Maryland. At this location, a roundabout was used at the ramp terminal. Peak hour traffic flowing from the arterial toward the freeway entrance ramp was quite heavy. This steady stream of traffic prevented vehicles on the exit ramp from entering the roundabout, and therefore caused those vehicles to queue back onto the freeway. To address this problem, the Maryland State Highway Administration installed a signal above the arterial leg of the roundabout that coor- dinates with an advanced detection loop on the ramp. When vehicle queues reach a determined interval on the ramp, the signal shows a red indication to the arterial traffic. This provides a gap for ramp traffic to enter the roundabout. After the queue dissipates, the signal indication changes to allow the arterial traffic to continue onto the roundabout. Design Criteria Ramp terminal improvements may be implemented wherever issues are occurring at or near a ramp. Specific criteria for implemented treatments have not been defined in the literature; typically, the decision to implement one or more of the treatments out- lined below is based on engineering judgment in response to observed problems such as excessive delay, spillback queuing onto the freeway or arterial, increase in crashes at or near ramps, or inadequate vehicle storage. The MUTCD (FHWA 2009) provides guidance and standards for various applications of traffic signals, signing, and striping. Improved signal timing and clear signing and striping have documented safety effects; however, these treatments are not typically evaluated for their specific impact at ramp terminals. It is expected that these treatments, particularly when they address ramp queue spillback onto the freeway, reduce conflicts that can lead to crashes. How Treatment Reduces Nonrecurrent Congestion Reduces Number of Queue Spillback Events Ramp terminal traffic control reduces the occurrence of off-ramp queues backing up onto the mainline of the freeway. Such events can have a significant effect on freeway reliability, because the queue effectively blocks a freeway lane until it dissipates. By reducing the frequency of these lane-blocking events, this treatment can reduce the delay associated with these events and improve the reliability of the freeway segment. Factors Influencing Treatment Effectiveness Several factors may influence the effectiveness of ramp terminal traffic control at re- ducing nonrecurrent congestion:

62 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Frequency of off-ramp queues backing up onto the freeway mainline • Frequency of treatment use (applicable specifically to treatments that require on- going activity, such as manual control by law enforcement) • Capacity increase expected from a particular ramp treatment (e.g., the addition of a turn lane will increase capacity significantly more than a small adjustment to signal timings) Off-ramps with high d/c ratios will tend to benefit most from this treatment, par- ticularly when the available storage space on the ramp is limited, because a high d/c ratio creates a situation in which queues may frequently back up onto the mainline. The mainline demand volume is also important, because the effect of a queue blocking a freeway lane will be more significant when mainline d/c ratios are high. Cost Factors that may affect the cost of implementing improved ramp terminal traffic con- trol include the following: • Type of proposed change (e.g., signal installation, signal retiming, turn lane con- struction, roundabout installation) • Personnel needed (e.g., police officer to direct traffic) • Construction costs o Pavement construction o Signal installation o Signal retiming o Changes to pavement markings o Roundabout installation • Ongoing costs o Personnel deployment to direct traffic o Resources to monitor ramp and determine when treatment should be deployed (e.g., use of a traffic management center) Ramp Turn Restrictions Description and Objective Ramp turn restrictions can be implemented at the ramp–arterial intersection to better manage traffic operations. Ramp turn restrictions to limit access to a ramp may be implemented to intentionally divert traffic away from a ramp either because (1) the ramp does not have enough storage capacity for turning vehicles, or (2) queues that form on the arterial exceed the storage limits of the turn lane and, because of this, traffic flow on the mainline is impeded.

63 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Ramp turn restrictions limit access to an arterial in order to increase capacity at the ramp terminal. This capacity increase is intended to eliminate (or preemptively avoid) the situation in which a ramp queue backs up onto the freeway mainline. Such queues can be detrimental to mainline operations and can result in significant delays and reduced reliability. Typical Applications Ramp turn restrictions may be considered at any ramp–arterial intersection where long queues form. Turn restrictions may be implemented for specific time frames (e.g., weekdays from 7:00 to 9:00 a.m.), during specific events when demand is particularly high (such as sporting events or concerts), or permanently. Design Criteria Ramp turn restrictions are implemented along the arterial street network at the ramp location or on the ramps near the intersection with the arterial. Ramp turn restric- tions are often deployed using temporary signing and barriers and enforced on site by enforcement personnel. However, at locations where turn restrictions may be used on a more regular basis (e.g., near stadiums or arenas), consideration may be given to attaching dynamic message signs to traffic control devices at the ramp–arterial inter- section. For example, at signalized intersections near special event locations, prefitted dynamic message signs can be installed to implement ramp turn restrictions on de- mand. Cameras may also be installed along sections of roadway or at intersections to allow personnel to better manage operations, particularly surrounding special events. A key consideration in the planning and design of ramp turn restrictions is to maintain good flow on the arterial and manage the queues that may form either on the ramp or on the arterial turn lane. Consideration should be given to where traffic will reroute in response to these restrictions and whether the roadways in these areas can accommodate increased traffic demands. The MUTCD (FHWA 2009) and Green Book (AASHTO 2011) offer information on various design considerations for any ramp or arterial intersection. Each highway agency may also have its own guidelines and policies regarding temporary traffic con- trol for nonrecurrent congestion situations. The primary purpose of implementing a ramp turn restriction is to improve the operational effectiveness of the ramp–arterial intersection. However, ramp turn restric- tions may have safety benefits, as well, particularly if ramp queues have been extending onto the highway. The following safety-related issues should be given consideration: • Are drivers given sufficient warning about the ramp turn restriction? • Is secondary congestion being created as a result of the ramp turn restriction, and does this create a safety issue? • Does the ramp turn restriction affect pedestrian and bicycle maneuvers at the intersection?

64 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION How Treatment Reduces Nonrecurrent Congestion Reduces Frequency of Queue Spillback Events Ramp turn restrictions reduce the occurrence of off-ramp queues backing up onto the mainline of the freeway. Such events can have a significant effect on freeway reliability, because the queue effectively blocks a freeway lane until it dissipates. By reducing the frequency of these lane-blocking events, this treatment can reduce the delay associated with these events and improve the reliability of the freeway segment. Factors Influencing Treatment Effectiveness Several factors may influence the effectiveness of ramp turn restrictions at reducing nonrecurrent congestion: • Frequency of off-ramp queues backing up onto the freeway mainline • Frequency of treatment use • Percentage capacity increase to off-ramp movements associated with turn restrictions Off-ramps with high d/c ratios will tend to benefit the most from this treatment, particularly where the available storage space on the ramp is limited, because storage limitation creates a situation in which queues may frequently back up onto the main- line. The mainline demand volume is also important, because the effect of a queue blocking a freeway lane will be significantly stronger where mainline d/c ratios are high. Cost Factors that may affect the cost of ramp turn restrictions include the following: • Duration of restriction (all hours or select hours only) • Permanent signing required • Use of dynamic signing on ramp • Use of dynamic signing on signal mast arm • Need to change pavement markings • Frequency of use of manual ramp restrictions • Personnel required for manual restrictions • Materials required for manual restrictions (such as barriers and traffic barrels) • Construction costs (e.g., permanent signing, dynamic signs, changes to pavement markings) • Ongoing costs (e.g., personnel needed to manually restrict turns at ramp and equip- ment such as traffic cones and vehicles needed for manual traffic direction)

65 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION DETOURS Improvements to Detour Routes Description and Objective Detour routes are designated roadways set up to carry traffic along a secondary route to bypass a congested location on the primary route. Detour routes typically connect to the primary route at locations in close proximity to the point of congestion both upstream and downstream. The roadways that serve as detours (typically arterials and minor streets) must serve not only the displaced traffic, but also their normal traffic volumes. Thus, improvements to the corridor are often advantageous, if not necessary, to provide adequate service to motorists using the route while the detour is in effect. The objective of this treatment is to provide acceptable levels of service to mainline traffic that will be negatively affected by a work zone, incident, or other major event. By increasing the capacity of a detour route, the traffic demand on the mainline can be reduced, thus easing congestion and improving reliability. Typical Applications Most detour route improvements on streets and arterials center on reallocating avail- able roadway space to meet the new traffic demands. This includes restriping or tem- porarily designating lanes to allow vehicles to use shoulders or on-street parking space to increase the route’s capacity. For freeways and limited-access highways designated as detour routes, additional improvements involve eliminating normal lane restrictions that might have been present. As an example, after the I-35W Mississippi Bridge collapse in the Minneapolis– St. Paul, Minnesota, metropolitan area, Trunk Highway 280 was designated as the detour route to service the predominant traffic flows heading to and from the down- town area. To accommodate the new demand, additional lanes were created by using shoulders and narrowing existing lanes. The other major improvement was the closing of all at-grade intersections along the detour stretch to create, in effect, a limited-access freeway environment. Design Criteria FHWA’s Alternate Route Handbook (Dunn Engineering Associates 2006) is an avail- able resource for assessing and implementing effective detour routes. Any improve- ments to the route that involve changes to the geometry or traffic control of the road- way should be done in consultation with such manuals as the AASHTO Green Book (2011), the MUTCD (FHWA 2009), and agency standards for work zone design and implementation. A variety of treatments may be used to improve a detour route, but the primary objective is to improve the detour route so that it can provide an acceptable level of service to the additional traffic it will be required to handle. Improvements may include the following: • Providing additional lanes by using existing pavement surface for traffic through the use of shoulders or on-street parking areas

66 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Providing additional surface area with new pavement to increase the number of usable lanes • Restriping or resigning routes to accommodate additional volumes • Minor widening at intersections • Retiming signalized intersections to give additional green time to directions of travel with increased volumes • Accommodating trucks and other large vehicles that may not have used the detour route previously (e.g., by increasing weight limits of bridges or increasing turning radii) • Improving the road surface, limiting access, improving sight distance, and other improvements that may allow a higher speed limit • Reconstructing isolated bottlenecks • Using high-quality pavement markings and stripes for unfamiliar drivers to follow • Using upgraded size and retroreflectivity of warning signs due to increased traffic load • Adding stop and yield signs or even traffic signals if existing intersection controls are inadequate for increased traffic loads • Adding temporary lighting to reassure drivers that they are following correct de- tour route • Adding an asphalt overlay before detour implementation to accommodate in- creased traffic load if detour is located on a poor pavement surface • Using a partial detour diversion such as keeping semitruck traffic (left lane) on the highway while passenger vehicles (right lane) exit the highway and enter the detour (the detour would lead those exiting users back onto the highway at a later mile marker) • Implementing a truck diversion (opposite of previous bullet) and allowing the highway to carry the remaining traffic load (however, nighttime detour of truck traffic may be objectionable to residential neighborhoods) • Providing advance notice, especially during nighttime detours, to allow com muters and other drivers who frequent the route to plan an alternative route Detour routes that are expected to be used frequently or that may not be capable of handling Interstate traffic volumes may be improved proactively, before an event requiring the use of the route as a detour is ever needed. Computer simulation can be a useful planning tool in determining the effects that a certain improvement may have on a detour route. During unplanned events or emergencies, traffic must be rerouted immediately, leaving little time for analysis, planning, or improving the detour routes. Proactive detour planning may help an agency identify routes that are appropriate for use as detours before an incident occurs, and improvements can be made to these routes if

67 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION it is determined that they are likely to be used frequently (near high-crash locations) or potentially for long periods of time (near locations susceptible to severe damage, such as bridges near fault lines). When unplanned events requiring a detour occur where a detour has not been planned, and the detour is expected to be long-term, it may be appropriate to improve the detour route after it is in use. Examples of major events that fall into this category would be the closure of the Santa Monica Freeway after the Northridge earthquake of 1994, and more recently the closure of I-35W in Minneapolis–St. Paul after the bridge collapse across the Mississippi River in 2007. How Treatment Reduces Nonrecurrent Congestion Reduces Mainline Demand Detour route improvements improve traffic operations by increasing the capacity of detour routes. When a detour route has higher a higher capacity, some of the mainline traffic demand can be diverted to the detour route. This diversion results in a decreased demand on the mainline, which decreases mainline congestion and improves reliability. Factors Influencing Treatment Effectiveness Several factors may influence the effectiveness of improvements to detour routes at reducing nonrecurrent congestion: • Duration of the mainline event. This treatment will have a more prominent effect when a mainline work zone or other incident is expected to reduce the roadway capacity for a significant period of time. • Directness of the detour. Detour routes that divert traffic farther out of the way will tend to be used less. • Capacity effectiveness of the improvement. Adding lanes, for example, will have a greater effect on improving mainline reliability than simply improving the pave- ment surface of the detour route. Cost Factors that may affect the cost of improvements to a detour route include the following: • Length of roadway to be improved • Type of improvement (construction of new pavement, restriping, traffic signal adjustments) • Material to be used for improvement (concrete, asphalt)

68 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Figure 3.22. Runaway truck ramp. Source: Washington State Depart- ment of Transportation (2009). TRUCK INCIDENT DESIGN CONSIDERATIONS Runaway Truck Ramps Description and Objective A runaway truck ramp (or truck escape ramp) is typically a long sand- or gravel-filled lane (sometimes a paved lane) adjacent to a roadway that is designed to slow or stop heavy vehicles that have lost control or are unable to stop due to brake failure on a downgrade. The objective of a runaway truck ramp, illustrated in Figure 3.22, is to reduce crashes involving heavy vehicles on or near downgrades by providing a path for out- of-control trucks to come to a safe stop away from other traffic. Crashes involving heavy vehicles can cause significant nonrecurrent congestion because they are often quite severe, involving several vehicles and blocking multiple lanes of travel. Runaway truck ramps reduce nonrecurrent congestion by preventing such heavy-vehicle crashes. Typical Applications Runaway truck ramps have been in use for over 40 years in the United States, as well as internationally. They are typically found near the bottom of steep downgrades or along extended downgrades, especially where a heavy vehicle might be required to slow or stop, such as before a horizontal alignment change (curve) or an intersection. Most often, runaway ramps are found along rural Interstates in mountainous regions, where prolonged braking can cause truck brakes to overheat and fail. However, escape ramps can also be found in suburbs and some urban areas where a runaway truck would have a higher likelihood of crashing with other vehicles and causing injuries or fatalities.

69 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION The AASHTO Green Book (2011) provides guidance on where runaway truck ramps should be located. It suggests that on existing facilities, a truck ramp should be provided as soon as need is established. This need could be determined by operational problems on downgrades reported by truck drivers, law enforcement, or the general public. It could also be determined by characteristics of the roadway, such as align- ment, gradient, length of grade, and vehicle speed, all of which influence the potential for out-of-control vehicles. In addition, evidence of highway segments that may benefit from a truck ramp, such as damaged guard rails and gouged pavement, may be found in a field study. These considerations should be combined with engineering judgment to assess the need for truck ramps. In some cases, the potential damage that could be caused by a runaway truck might be enough to justify building a ramp even if no previ- ous problems have been documented. Design of runaway truck ramps should take into account various site character- istics to most effectively reduce out-of-control truck crashes. Site characteristics that should be considered include the following: • Topography. Roadways in mountainous regions tend to have greater need for runaway truck ramps than rolling or level topography. • Grade. Both the length of grade and percentage grade are important considerations. • Speed. The design speed, operating speeds, and posted speed limit can be impor- tant factors in estimating the potential speed of an out-of-control truck. • Crash rates. Roadway segments with a high number of recorded crashes involving out-of-control trucks can be good candidate locations for a runaway truck ramp. The grade severity rating system, a technique developed in the early 1980s by FHWA for analyzing operations on a grade, can be used to determine expected brake temperatures along a downgrade. Ramp locations can be determined by identifying the points along the grade where expected brake temperatures exceed the temperatures at which brakes may fail. Ramps should be built in advance of road alignments that may not be able to be safely negotiated by an out-of-control truck and in advance of populated areas. Escape ramps should be clearly signed in advance of the ramp, and ramp exits should be on the right side of the travel way. Ramps should be tangent to the roadway to minimize the control required by the driver to navigate the truck from the lanes onto the ramp. Design Criteria The AASHTO Green Book (2011) classifies ramp types in three general categories: gravity ramps, which slow trucks by relying primarily on gravitational forces; sandpile ramps, which slow and stop vehicles by significantly increasing rolling resistance; and arrester beds, which are long gravel ramps that gradually slow trucks by increasing rolling resistance. Arrester beds can be descending, horizontal, or ascending in grade; in either case, grade affects the ramp length required to slow and stop the truck. Gravity ramps are typically paved or composed of densely compacted aggregate. The steep ascending grade prevents forward motion of the truck but cannot prevent it from rolling backwards and jackknifing.

70 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Sandpile ramps consist of loose, dry sand that quickly decelerates trucks entering the ramp. Weather can affect the rolling resistance provided by the sand in this type of runaway truck ramp. The severe deceleration provided by sandpile ramps makes them a less desirable option than arrester beds, but it also allows for a shorter ramp because trucks are slowed to a stop more quickly. Typically, sandpile escape ramps are less than 400 ft long and can be used in locations where there is inadequate space for other types of ramps. Arrester-bed escape ramps use loose aggregate to increase rolling resistance and slow the truck. A descending grade arrester-bed ramp will be longer than the other types of ramps because gravity will work against slowing the vehicle. These ramps should have a clear path back to the highway so that trucks that are not completely stopped on the ramp can reenter traffic at a much slower speed. Ascending grade arrester beds are the most common, using both gravity and rolling resistance to slow vehicles. The Green Book provides design considerations for truck escape ramps that are based on the assumption that an out-of-control truck will rarely, if ever, exceed a speed of 80 to 90 mph. Some design considerations follow: • The ramp must be sufficient in length to dissipate the kinetic energy of the out-of- control vehicle. • The ramp should be wide enough (preferably 30 to 40 ft wide) to accommodate more than one vehicle. • The aggregate depth in the arrester bed should be at least 3 ft deep, which is tapered from a 3-in. depth at the start of the bed to the full depth within 100 to 200 ft of the bed length. • Arrester beds should include a means of drainage to minimize freezing and contamination. • The departure angle of the ramp from the roadway should be equal to or less than 5°; adequate site distance should be provided in advance of the ramp; and the driver should be able to see the entire length of the ramp. • The ramp should be delineated and visible at night; illumination is desirable. • A service road should be provided along the side of the escape ramp to allow ser- vice and maintenance vehicles to access the ramp without becoming trapped in it. • Anchors should be located adjacent to the arrester bed at 150- to 300-ft intervals to assist wreckers in returning a captured vehicle to the roadway. If the only practical ramp location may not provide enough distance for a vehi- cle to stop, or when the consequences of a vehicle overrunning the end of the ramp are severe, including an impact attenuator at the end of the ramp may be desirable. Impact attenuators are protective systems that dissipate the energy of out-of-control vehicles in order to decelerate the vehicle more smoothly. Examples of impact attenu- ators include crash cushions (steel or wood members of the cushion sheer at impact to dissipate energy) and inertial barriers (plastic containers are filled with sand or water, exploding on impact to dissipate energy). To be considered as a ramp-end treatment

71 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION for a runaway truck ramp, the advantages of the impact attenuators should outweigh the disadvantages. How Treatment Reduces Nonrecurrent Congestion Reduces Frequency of Incidents The FHWA Desktop Reference for Crash Reduction Factors (Bahar et al. 2007) in- dicates that the installation of truck escape ramps has a crash reduction factor of 18% for all crashes, 75% for run-off-road crashes, and 33% for rear-end crashes. In an evaluation of several safety projects, Caltrans found that 75% of runaway truck crashes were eliminated with the use of truck escape ramps. Runaway truck ramps can reduce the annual number of runaway truck crashes. This reduces the number of hours during the year that lanes are blocked due to truck crashes, which reduces congestion and improves reliability. Factors Influencing Treatment Effectiveness Factors that influence the effectiveness of a runaway truck ramp include the following: • Frequency of out-of-control or brake failure–related truck crashes along the roadway. The more crashes experienced and expected, the bigger the treatment’s impact will be. • The location and number of truck ramps used along the roadway. Truck ramps are not helpful to a truck that experiences brake failure beyond the ramp, so plac- ing ramps where brake failure is most likely to occur, or periodically along the roadway, will maximize the chances of an out-of-control truck having a nearby, downstream ramp to use. • Traffic volumes. Truck crashes are more likely to involve multiple vehicles when traffic is heavy. Higher volumes also mean that a truck incident will lead to more congestion. Eliminating truck crashes in high-volume areas will provide more con- gestion and reliability benefit than in areas with low volumes. Cost Factors that may affect the cost of installing a runaway truck ramp for a roadway sec- tion include the following: • Dimensions of the escape ramp (length, width, depth) • Material used for the escape ramp • Availability of right-of-way for ramp location (especially in urban or suburban areas) • Topography of ramp location (especially in mountainous conditions where consid- erable cut or fill may be required) Itemized Cost List To construct a runaway truck ramp, the following actions may be needed:

72 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Construction of a gravel arrester-bed ramp • Construction of a gravel gravity ramp • Construction of a sand arrester-bed ramp • Clearing and grubbing • Grading • Installation of impact attenuators • Installation of new signs CONSTRUCTION Reduced Construction Duration Description and Objective Reduced construction duration is a treatment category that encompasses innovative techniques that can be implemented to reduce the duration of a work zone or other construction project. Such techniques may include total road closures, night work, and the use of innovative construction materials. Figure 3.23 presents an example of a construction project causing extensive freeway congestion. Typical Applications Innovative techniques to reduce construction duration include implemented manage- ment techniques, contracting types, and material usage. These techniques are often used in combination. Simple strategies, such as notifying the public in advance of project work to allow motorists to find alternative routes, can alleviate congestion resulting from the work zone. Project websites can be used as a communication tool in urban areas, notify- ing users of lane closures, access restrictions, and alternative routes to reach local businesses. In designing detours, highway agencies should consider safety, geometrics, truck-turning needs, shoulder widths, and pavement condition. The alternative route should be able to withstand any high-volume traffic that results from the detours. Installing temporary concrete barriers to provide local access and to separate the work zone from the traffic zone can help minimize the inconvenience to local stakeholders. Design Criteria Among the innovative techniques discussed above to reduce construction duration, only full road closure is truly a directly design-related treatment to reduce non recurrent congestion. The other techniques involve better construction management and im- proved construction materials. These techniques also have the potential to reduce non- recurrent congestion, but they do not accomplish this through physical changes to the roadway.

73 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION How Treatment Reduces Nonrecurrent Congestion Reduces Duration of Work Zone: Nonpeak Construction Reducing the duration of a work zone will decrease the lane-blocking time (and there- fore the duration of the diminished capacity) associated with that work zone. In addi- tion, if the timing of the work zone is planned such that lane-blocking time during high-demand periods is decreased, operations will improve. Reduces Duration of Work Zone: Full-Closure Construction An FHWA study (2003) found that full road closure is a successful tool that reduces the impact of work zones. Of six projects, construction time was reduced by an aver- age of 76% compared with traditional part-width construction. Reduces Duration of Work Zone: Fast-Track Construction In research conducted by Konchar (1997), construction speed for design–build projects was, on average, at least 12% faster than design–bid–build projects and 7% faster than construction management at risk. The data used reviewed 351 building projects from the United States. Konchar also found that construction management at-risk projects were at least 5.8% faster than design–bid–build, unit costs were at least 6.1% less, overall project delivery speed was at least 33.5% faster, cost growth was at least 5.2% less, schedule growth was at least 11.4% less, and the quality of work was equal or better. Factors Influencing Treatment Effectiveness Factors that may influence the effectiveness of reduced construction duration at reduc- ing nonrecurrent congestion include the following: Figure 3.23. Construction project causing freeway congestion. Source: Knauer et al. (2006).

74 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Duration of each work zone • Construction solutions in regards to timing, sequencing, and management of the project • Percentage decrease of roadway capacity for each hour of the day while the work zone is in effect Cost Factors that may affect the cost of implementing a reduced construction duration plan include the following: • Type of change to the construction plan (e.g., using more expensive materials that can be placed faster, providing bonuses to contractors for finishing early, working at night only) • Traffic management plan • Design time and fees associated with creating a new construction plan • Contractor bonus for each hour completed ahead of schedule • Fast-forming materials • Additional materials or equipment required Improved Work Site Access and Circulation Description and Objective In many construction or maintenance operations, vehicles carrying materials must access the work site from the traveled roadway. Poorly marked or controlled-access points can result in substantial delays when work vehicles enter and leave the work site. Improved access treatments allow work vehicles to enter and leave the work site more quickly and safely. The objective of providing improved work site access and circulation is to mini- mize the traffic congestion associated with operations. The negative effect of a work zone on traffic operations is mitigated by minimizing the impact of vehicles entering and leaving the work site. Typical Applications Improved work site access and circulation would have application in any work zone where work vehicles are required to enter or leave the work site directly from the traveled way of a higher-volume roadway. Common examples of the need for such worksite access are paving operations (where trucks bring paving materials to the job site) or trenching sites (where trucks haul excavated materials from the job site). Design Criteria Adequate acceleration and deceleration areas are needed for trucks to safely enter and leave work spaces. These access points should also be clearly signed or delineated to discourage motorists from inadvertently following trucks into the work area.

75 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Temporary traffic-control zone design should consider work vehicle access to the work site. Graham and Burch (2006) describe the steps in assuring improved access for trucks as follows: “Truck drivers should be briefed on how to access the project site, the path to follow to deliver materials, where to stop for staging, and how the spotter will instruct them once they are near the work operations.” MUTCD (FHWA 2009) specifies that the path of work vehicles should be delin- eated and that, where possible, the paths of work vehicles should be separated from workers on foot. How Treatment Reduces Nonrecurrent Congestion Mitigates Capacity Reduction Caused by Work Zone In most cases, a work zone will reduce the total capacity of the roadway for the dura- tion of the work zone activity. One of the ways that a work zone diminishes capacity is by large or slow-moving vehicles accessing the work zone from the mainline. When one of these vehicles slows down to safely navigate into the site, the traffic behind the slow-moving vehicle must also slow down. This decrease in speed can cause a dramatic reduction in capacity during the event, and if vehicle demand is heavy enough, the sys- tem can reach a d/c ratio greater than one. This may result in the formation of a queue, and traffic operations may remain poor for a long period of time. Improved work site access and circulation can mitigate this problem by separating the slow-moving vehicles from the main traffic stream. The slow-moving vehicles are provided an area to safely navigate into the work site without blocking traffic or caus- ing significant slowdowns to the traffic behind them. The resulting congestion is thus reduced or eliminated, and the reliability of the roadway improves. Factors Affecting Treatment Effectiveness Improved work site access and circulation will be most effective on roadways with high traffic demands, particularly when d/c ratios are high. This treatment will also tend to be most effective when mainline speeds are high, because the speed reductions required for a large vehicle to access the work site will be more dramatic in relation to the average travel speeds of mainline traffic. Cost Factors that may affect the cost of improving work zone access or circulation include the following: • Materials required for improvement (gravel, traffic cones, traffic barrels) • Additional construction required for improvement (new pavement, restriping) ANIMAL–VEHICLE COLLISION DESIGN CONSIDERATIONS Description and Objective Animal–vehicle collision design considerations include a wide range of design-related features intended to prevent or reduce animal–vehicle collisions. Animal–vehicle col- lisions are also referred to as wildlife–vehicle collisions (WVCs) because the animals

76 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Figure 3.25. Boulders in the right-of-way (alternative to wildlife fencing). Source: Huijser et al. (2008). involved in the crashes are usually wildlife with habitats close to the roadway areas. Although there are a large number of treatments found in the literature, the focus of this section is on design-related treatments as they pertain to nonrecurrent congestion. The most common objective for the implementation of these design treatments is to reduce the potential for WVCs by preventing animal crossings at undesirable loca- tions and providing means for animals to cross safely at selected desirable locations. Figures 3.24 through 3.26 illustrate wildlife on the roadway and design treatments to address animal–vehicle collisions, including boulders and animal crossings. Typical Applications Animal–vehicle collision design may be considered at locations where there is great- est potential for WVCs. Various data sources can be used to identify these locations, including the roadkill data from the local police department and crash data from the insurance industry. Common examples of specific WVC mitigation measures include the following: • Wildlife fencing • Boulders in the right-of-way Figure 3.24. A bull elk on the roadway. Source: Gray (2009).

77 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Figure 3.26. Long tunnels and long bridges physically separate animals from vehicular traffic. Source: Huijser et al. (2008). • Long tunnels and bridges over the landscape • Wildlife underpasses and overpasses Wildlife fencing is one of the most commonly applied measures to separate wildlife from motorists (Romin and Bissonette 1996). Wildlife fences in North America typi- cally consist of wire-mesh fence material 2.0 to 2.4 m (6.5 to 8 ft) high. Several types of fence material are used, but page wire or cyclone fence material is most common. Wooden or metal fence posts are typically used; the latter are particularly important when fencing over rock substrates. Large boulders can be placed in the right-of-way to deter animals from cross- ing highways at undesirable locations. Large boulders can be difficult for animals, especially ungulates, to traverse, which makes them a desirable alternative to wildlife fencing. The boulders should be placed outside of the clear zone. Long tunnels and long bridges over the landscape can be used to separate animals from traveling vehicles. For this discussion, long tunnels (or landscape bridges) are defined as tunnels that are at least several hundreds of meters long, sometimes many kilometers. Long tunnels and bridges are primarily constructed because of the nature of the terrain (e.g., through a mountain, across a floodplain), but in some cases they are constructed to avoid areas that are ecologically sensitive and where no alternatives are available. If the nature of the terrain permits, animals can move freely through long tunnels or under long bridges, and because the animals are physically separated from traffic, WVCs are eliminated. Although long tunnels and long bridges may be among the most effective mitigation measures to reduce animal–vehicle collisions, they are rarely specifically designed to reduce WVCs. Wildlife underpasses and overpasses are used to provide habitat linkages for vari- ous species and to allow animals to cross roads safely by separating them from vehicu- lar traffic. Wildlife fencing is sometimes used to funnel animals to these overpasses or underpasses.

78 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Design Criteria Wildlife fencing is one the most widely used design treatments to prevent or reduce WVCs. One of the most undesirable effects of wildlife fencing is that it creates a bar- rier between natural habitats of different species and can potentially have harmful ecological consequences. It is therefore necessary to provide gaps in the fencing with animal detection and warning signs, or provide for overpasses and underpasses. Wild- life fencing (and other “absolute barriers”) should always be accompanied with escape opportunities for animals that may end up in between the fences. These escape oppor- tunities can be provided using one-way gates or jump-outs. Jump-outs use a sloping mound of soil placed against a backing material approximately 1.5 m (5 ft) in height and constructed on the right-of-way side of the fence to form an “escape ramp” for the animal. The highway fence (2.4 m [8 ft]) is lowered at the ramp site and forms an integral part of the jump-out that allows deer or other species to jump to the safe side of the fence. The vertical drop-off on the back side of escape ramps is designed to be of sufficient height to preclude deer from gaining access to the right-of-way from the nonhighway side of the fence. How Treatment Reduces Nonrecurrent Congestion Reduces Frequency of Incidents A WVC treatment can reduce the number of WVC crashes that occur. By eliminating crashes, this treatment reduces the lane-blocking time associated with these crashes. This results in a decrease in nonrecurrent congestion for the roadway and improved reliability. Factors Influencing Treatment Effectiveness Several factors may influence the effectiveness of animal–vehicle collision design considerations: • Type of WVC treatment • Annual number of WVC crashes • Average duration of a WVC incident • The species involved in the WVCs, including migration patterns, mating seasons, and population Several methods can be employed for reducing WVCs, including the following: • Building a barrier to prevent wildlife from entering the roadway • Building an alternative route for wildlife to avoid the roadway, such as an overpass or underpass • Controlling wildlife population to minimize vehicle exposure to wildlife • Making drivers aware of approaching wildlife through warning systems • Making drivers aware of the likelihood of wildlife presence through static or dynamic signing

79 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Each method will provide a different level of WVC reduction. Treatments that keep most wildlife off the roadway will result in a very large reduction in WVCs; treatments that attempt to make drivers aware of the potential for WVCs may have a smaller impact. However, the former treatment types tend to be significantly more expensive that the latter types. Treatments will have the greatest safety and operational benefits in areas where WVC rates are high and where the crashes that do occur are severe and result in sig- nificant lane-blocking time. Although the safety benefits of WVC treatments will be realized at all locations with these types of crashes, the congestion-reducing benefits will be seen on roadways with higher volumes or where passing an incident may be difficult (e.g., mountainous two-lane roads). These conditions are likely to result in nonrecurrent congestion when a WVC occurs. Cost To estimate the installation cost of a WVC reduction measure, the treatment type to be installed must be determined. Each treatment type will have its own set of cost con- siderations. Several common treatment types with their respective cost considerations are listed below: • Animal overpasses and underpasses o Bridge or culvert construction o Right-of-way acquisition o Traffic control associated with construction of the project o Landscaping to rebuild wildlife habitat near the project site o Maintenance of the habitat • Fencing systems o Fence material (e.g., barbed wire, woven wire, orange plastic) o Grading o Right-of-way acquisition Animal detection systems will have their own costs, depending on the type. Low- cost treatments include vegetation removal to make animals near the roadway more visible to motorists and signs to warn motorists of the likelihood of animal presence. WEATHER Snow Fences Description and Objective A snow fence is a structure or vegetative planting used to trap and control blowing and drifting snow before it reaches the roadway. There are two types of snow fences: snow fences that trap snow upwind of the area to be protected (collector-type snow fences) and snow fences that deflect snow around the protected area (deflector-type snow fences).

80 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Winter weather can impair motorists’ visibility, cause loss of vehicle control, reduce sight distance at curves and intersections, obscure signs, reduce effective road- way width, and render safety barriers ineffective. Snow fences can mitigate the nega- tive impacts of winter weather by improving visibility and providing drier pavement surface conditions on the travel way. This mitigation allows this treatment to reduce the number of snow-related crashes and the congestion associated with these crashes. Typical Applications Snow fences may be designed in various ways and constructed with different materi- als. The traditional wood-slatted fence with horizontal rails works reasonably well, as do lightweight plastic fences with wooden frames. In either case, fences with openings work best. Studies indicate that snow fences with about 50% porosity are most effec- tive in trapping snow. Living snow fences refer to vegetative plantings used to control blowing snow. Plant materials may include trees, shrubs, grass, or agricultural crops such as corn or sunflowers left standing throughout the winter. For example, shrubs planted at the top of a cut can be used in place of taller barriers placed farther upwind. Design Criteria Blowing snow problems are best identified either through discussions with mainte- nance crews or through analyses of historical crash data. The following information should be considered in the analysis of a problem related to blowing snow: • Type of problem (snowdrift, poor visibility, slush, ice) • Effect (crashes, excessive snow removal costs, pavement repair costs) • Source of blowing snow (within right-of-way, adjacent open field, frozen lake) • Cause of problem (cross-section geometry, horizontal or vertical alignment, delinea tion, safety barrier, roadside vegetation or structure, snow removal prac- tices, traffic) The following questions should be answered to help justify and prioritize the prob- lem and identify appropriate mitigation measures: • Is the problem drift encroachment, poor visibility, road icing, or a combination of these factors? • If the problem is snow deposition, what is the safety hazard (e.g., restricted sight distance, poor visibility caused by snow blowing off the drift at windshield level, loss of vehicle control)? • What is the crash history at the site? • Does a drift block roadside drainage or otherwise contribute to water infiltration? Is there any evident pavement damage? • What impact does blowing snow have on crew requirements, duty cycle, and overtime? • What is the year-to-year variability in problem occurrence and severity?

81 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • What benefits would be derived by solving the problem? Would it improve safety for the traveling public or maintenance crews? Reduce overtime? Free equipment for use at other locations? The Minnesota Department of Transportation and the University of Minnesota have developed an interactive internet site to help design a snow fence for a prob- lem location: http://climate.umn.edu/snow_fence/Components/Design/locationb.asp. Tools on this site allow the user to determine the required height, setback, and overlap of snow fence systems for any location in Minnesota, and they can be used to design snow fences for places outside the state by finding a Minnesota location with similar snowfall and snow relocation coefficients. The website is an excellent tutorial of the guidelines for designing snow fences. How Treatment Reduces Nonrecurrent Congestion Reduces Frequency of Incidents Snow fences can reduce the number of snow-related crashes that occur. By eliminating crashes, this treatment reduces the lane-blocking time associated with these crashes. This reduction results in a decrease in nonrecurrent congestion for the roadway and improved reliability. Factors Influencing Treatment Effectiveness Several factors may influence the effectiveness of snow fences at reducing nonrecurrent congestion: • Annual number of snow-related crashes • Average duration of a snow-related crash • Effectiveness of a snow fence at eliminating snow-related crashes Snow fences will be most effective on roadways that have a significant number of snow-related crashes each year and long average lane-blocking durations. A 34-year study of I80 in Wyoming found that snow fences reduced crashes in blowing snow conditions by approximately 75% (Tabler and Meena 2006). The study evaluated the effectiveness of snow fences in reducing crashes and road closures on a 50-mi section of I-80 in southeastern Wyoming. When the highway was first opened to traffic in 1970, no snow fences were in place. However, due to substantial snow- drifting problems, nearly 73% of the roadway was protected by snow fences between 1971 and 1990. In addition to the resulting benefit of crash reduction, reduced traffic delay is another benefit of the snow fence protection: the present snow fence system reduces road closure time by an average of 8.3 days each year. Cost Factors that may affect the cost of installing snow fences along a roadway section include the following: • Type of fence (wood slat, polyethylene, polymer, vegetation) • Fence height

82 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Length of fence • Leasing agreements or land purchase for fences placed on private property Snow fences can significantly reduce the cost of snow removal. Blowing and drift- ing snow can add substantially to the cost of snow and ice removal for maintenance crews. Snow fences can also increase the pavement life of the roadway surface by reducing potential drainage issues and reducing exposure of the road surface to win- ter maintenance equipment. Snow drifts directly contribute to pavement damage by blocking ditches, drains, and culverts and by serving as a source of water infiltration under the pavement. Winter maintenance equipment can also damage the roadway surface. Blowing Sand Mitigation Description and Objective Blowing sand can have significant negative effects on highway operations. Sand can pile up on the roadway and create dangerous conditions for drivers by causing vehicles to turn unexpectedly when crossing uneven mounds of sand. In addition, drivers can be adversely affected by the reduced visibility caused by blowing sand. These unsafe conditions can lead to vehicle crashes. The objective of these treatments is to reduce the amount of blowing sand on or around a roadway, thus increasing visibility and maneuverability on the highway and reducing the frequency of sand-related crashes. Major treatment types include the following: • Structures that trap sand and dirt o Berms o Fences • Treatments that help hold soil in place during high winds o Plantings o Irrigation systems • Blowing sand warning systems The first two treatment types listed above reduce the amount of sand blowing across or coming to rest on the roadway. The third type does not reduce the amount of blowing sand, but rather is intended to make motorists aware of adverse conditions and encourage them to slow down, be more cautious, or choose an alternative route. All three treatment types can effectively reduce the frequency of blowing sand–related incidents. Blowing sand can also interfere with the ability of emergency responders to reach a crash site and provide services. On June 16, 2011, television station KESQ reported on a five-vehicle collision on Highway 86 and Avenue 52 in Coachella, California. Emer- gency personnel reported difficulty in responding to this incident due to poor visibility caused by blowing sand. The highway was closed to traffic in both directions the night

83 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION after the incident. Types of treatments that reduce the amount of blowing sand on the roadway could be effective in improving emergency personnel response time. Figure 3.27 illustrates how blowing sand can substantially reduce visibility. Typical Applications Many areas in the southwest United States are subject to blowing sand and dust. Some of the major causes of blowing sand include droughts, high winds, soil erosion, and any other process that results in minimal soil cover. In many cases, undisturbed desert areas are not a source of blowing sand and dust. Rather, it is the disturbing of the natural conditions that leads to problems with blowing sand and dust. Therefore, in areas that experience blowing sand problems, care should be taken with any construc- tion or roadwork activity that will disturb the natural soil conditions. By planning to stabilize the soil over the course of an entire construction project, blowing sand can be eliminated directly at the source. Some typical methods for stabilizing disturbed soil include the following: • Covering the disturbed soil with wind-resistant material • Planting small vegetation and grasses (avoid planting when winds are high [typi- cally spring]) • Building wind breaks • Building wind barriers Wind barriers are structures that provide total wind blockage. Examples of wind barriers include the following: Figure 3.27. Example of how blowing sand can reduce visibility. Source: Goodwin (2003).

84 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Berms made from wood chips or soil • Solid fences • Stacks of hay bales • Walls of concrete blocks or other structural materials Wind breaks provide partial wind blockage, as they are less solid and continuous than wind barriers. Examples of wind breaks include the following: • Porous fences (such as standard orange construction fence) • Rows of large vegetation (trees and shrubs) Design Criteria Some of the most common practices for protecting a facility from blowing sand can be summarized in the following categories: • Direct shielding o Wind barriers o Wind breaks • Stabilization of nearby planted areas o Soil stabilizers (e.g., salts, surfactants, wood byproducts) o Straw crimping o Straw blankets o Irrigation to provide dust control and to optimize plant growth In the Large Area Land Managers Guide to Controlling Windblown Sand and Dust, the Dustbusters Research Group (2011) recommends the following multistep strategy for controlling dust: 1. Protect existing vegetation and structures that serve as wind barriers as long as possible before the necessary clearing or demolition. 2. Construct wood chip and earthen berms at critical points along property boundaries. These berms will stop encroaching sand and provide protected areas for possible development of new vegetative wind breaks. 3. Plant and irrigate permanent tree and shrub wind breaks in the protected down- wind area of berms as part of the overall site landscaping plan. 4. Place a thin wood chip layer on loose blowing sand areas within the property. 5. Stabilize large areas of disturbed soil with temporary vegetation or furrows, es- pecially where land development activities will not begin immediately on previ- ously cleared land. Temporary vegetation such as mustard, barley, or cereal grains will require irrigation for quick and effective germination during cool weather conditions. 6. Construct wood chip and earthen berms at critical points within the property to

85 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION stop the blowing sand and to provide protected areas for possible development of new vegetative wind breaks. 7. Reduce the length of unobstructed terrain over which the wind flows, including straight stretches of unpaved roads that run parallel to the direction of prevailing high winds. 8. Develop a plan and work schedule that minimize the time during which each par- cel of a new development remains in a disturbed condition. Blowing sand problems are best identified either through discussions with local maintenance crews or through analyses of historical crash data. The following infor- mation should be considered in the analysis of a problem related to blowing sand or dirt: • Type of problem (visibility reduction or sand drifting onto the roadway that either reduces friction or causes an irregular surface texture that can lead to loss of con- trol of the vehicle) • Effect of problem (crashes, excessive sand removal costs, pavement repair costs) • Source of blowing sand or dirt • Cause of problem (nearby construction projects, abandoned farmland, recent drought) How Treatment Reduces Nonrecurrent Congestion Reduces Frequency of Incidents Blowing sand mitigation can reduce the number of blowing sand–related crashes that occur. By eliminating crashes, this treatment reduces the lane-blocking time associated with these crashes, which results in a decrease in nonrecurrent congestion for the road- way and improved reliability. Reduces Incident Response Time Berms or fences can reduce the amount of blowing sand on the roadway. By reducing the amount of blowing sand, emergency responders may be able to identify and reach an incident site more quickly. This results in faster response and clearance times, thus reducing congestion and improving reliability. Factors Influencing Treatment Effectiveness Several factors may influence the effectiveness of blowing sand mitigation at reducing nonrecurrent congestion: • Annual number of blowing sand–related crashes (caused by either diminished vis- ibility or sand drifts on the roadway) • Average duration of a blowing sand–related crash • Effectiveness of treatment at eliminating sand-related crashes (different treatments will have different expected levels of effectiveness at preventing and trapping blowing sand)

86 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION This treatment will be most effective on roadways that have a significant number of blowing sand–related crashes each year and long average lane-blocking durations. The vehicle demand volume is also an important consideration, as the delay associated with a lane-blocking event on a roadway with a high d/c ratio will be greater than that of a roadway with a lower demand volume. Cost The costs for installing this treatment will vary depending on which specific mitigation measure is selected and the length of roadway to be protected. For example, installing a construction fence would generally be very inexpensive. However, the installation of an irrigation system to encourage vegetation growth and increase soil moisture content could be quite expensive or complicated, especially if the treatment is needed through- out areas not within the roadway right-of-way. Building berms or barriers might have moderate expenses, even if the barrier material is inexpensive, after construction costs have been accounted for. Each treatment will also have associated maintenance costs. Barriers will collect sand and dirt on the upwind side of the structure that will have to be occasionally removed for the barrier to maintain effectiveness. Vegetation may have to be fertilized, watered, or periodically replanted. In addition, new plantings may not be effective at retaining the soil or blocking blowing sand and dirt until a few years of growth have been achieved. Each treatment will also have a different life expectancy and will require the consideration of replacement costs. Anti-Icing Systems Description and Objective Anti-icing involves the proactive application of chemicals to a roadway surface before a winter storm. Anti-icing helps prevent snow and ice from bonding to the pavement (in contrast to deicing, which involves the reactive application of chemicals to a road- way surface during or after a storm, when snow and ice may have already formed). The objective of anti-icing systems is to reduce the number of snow- and ice-related crashes that occur on a roadway by reducing or eliminating the time during which the pavement surface is covered in ice. Figure 3.28 presents a schematic of a bridge anti- icing system in Minnesota. Typical Applications Anti-icing systems may be fully automated, semiautomated, or manually activated. Fully automated systems rely on sensors to determine the need for application of anti- icing chemicals and automatically activate anti-icing measures. Semiautomated sys- tems can be activated by someone from a remote location in response to a sensor indication. Manually activated systems do not include roadway sensors and must be activated directly at the site. Automated anti-icing systems are composed of roadway sensors and electrical and mechanical systems that disperse anti-icing chemicals onto the roadway surface to help prevent the formation of ice. The most common automated anti-icing systems use a series of spray nozzles connected to a chemical storage tank. Using a pump system,

87 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION the chemicals are dispersed along a roadway segment through nozzles, which can be embedded into the pavement or placed along the edges of the roadway. Anti-icing systems are commonly applied on bridge decks, which tend to freeze more quickly than other parts of the roadway, and horizontal curves, where traction is critical. Application criteria for where to apply anti-icing systems are generally not available, and the decision to use anti-icing is left to the discretion of the highway agency. Factors to consider include past ice-related accident history, benefit–cost analysis, and the potential harmful effects of the chemicals on the roadway structures and environment. Design Criteria Anti-icing technology systems have been installed in California, Kentucky, and Min- nesota. Several systems combine road weather information system technology with the automated anti-icing technology. These systems have not been widely implemented because of their initial and operating costs. How Treatment Reduces Nonrecurrent Congestion Reduces Frequency of Incidents Anti-icing systems can reduce the number of ice-related crashes that occur. By elimi- nating crashes, this treatment reduces the lane-blocking time associated with these crashes, which results in a decrease in nonrecurrent congestion for the roadway and improved reliability. Factors Influencing Treatment Effectiveness Several factors may influence the effectiveness of anti-icing systems at reducing nonre- current congestion: Figure 3.28. Bridge anti-icing system in Minnesota. Source: Goodwin (2003).

88 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Annual number of ice-related crashes • Number of hours per year when ice covers the roadway (if no anti-icing system were used) • Number of hours per year during which the melting capacity of the system is ex- ceeded, making the system ineffective Several studies have shown reductions in the frequency of wintertime accidents from 25% to 100% due to the installation of anti-icing technologies. Cost Factors that may affect the cost of installing an anti-icing system along a roadway sec- tion include the following: • Type of system to be installed (automated, semiautomated, manually activated) • Area of roadway to be covered by treatment (considering both length of segment or bridge and pavement width) • Ongoing material costs (based on expected frequency of use of the system) o Deicer fluid o Electricity o Personnel to operate system • Maintenance of the system

Next: 4 CATALOG OFSECONDARY TREATMENTS »
Design Guide for Addressing Nonrecurrent Congestion Get This Book
×
 Design Guide for Addressing Nonrecurrent Congestion
Buy Paperback | $68.00
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

TRB’s second Strategic Highway Research Program (SHRP 2) Report S2-L07-RR-2: Design Guide for Addressing Nonrecurrent Congestion catalogs highway design treatments that can be used to reduce nonrecurrent congestion and improve the reliability of urban and rural freeways.

The draft design guide is accompanied by a report titled Identification and Evaluation of the Cost-Effectiveness of Highway Design Features to Reduce Nonrecurrent Congestion.

SHRP 2 Reliability Project L07 also produced an Analysis Tool for Design Treatments to Address Nonrecurrent Congestion: Annotated Graphical User’s Guide Version 2. The guide is intended to assist users of the Microsoft-based Excel tool designed to analyze the effects of highway geometric design treatments on nonrecurrent congestion using a reliability framework.

The tool is designed to analyze a generally homogenous segment of a freeway (typically between successive interchanges). The tool allows the user to input data regarding site geometry, traffic demand, incident history, weather, special events, and work zones. Based on these data, the tool calculates base reliability conditions. The user can then analyze the effectiveness of a variety of treatments by providing fairly simple input data regarding the treatment effects and cost parameters. As outputs, the tool predicts cumulative travel time index curves for each hour of the day, from which other reliability variables are computed and displayed. The tool also calculates cost-effectiveness by assigning monetary values.

Subsequent to the analysis tool's release, SHRP 2 Reliability Project L07 produced an Microsoft-based Excel demand generator as a supplement to the analysis tool.

Analysis and Demand Generator Tools Disclaimer: The analysis tool is offered as is, without warranty or promise of support of any kind either expressed or implied. Under no circumstance will the National Academy of Sciences or the Transportation Research Board (collectively "TRB") be liable for any loss or damage caused by the installation or operation of this product. TRB makes no representation or warranty of any kind, expressed or implied, in fact or in law, including without limitation, the warranty of merchantability or the warranty of fitness for a particular purpose, and shall not in any case be liable for any consequential or special damages.

READ FREE ONLINE

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!