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OCR for page 23
HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 1.0 SPEEDING COUNTERMEASURES: USING ROADWAY DESIGN AND TRAFFIC CONTROL ELEMENTS TO ADDRESS SPEEDING PROBLEMS Introduction Using roadway design and traffic control elements to address speeding problems refers to guidelines and best practices for selecting and using geometric design features and traffic signals to support safe speed decisions by drivers. Much of the information in this guideline, as well as its companion guidelines (Speeding Countermeasures: Setting Appropriate Speed Limits on page 17-10 and Speeding Countermeasures: Communicating Appropriate Speed Limits on page 17-12), is adapted from Neuman et al. (1). As part of NCHRP Report 500: Guidance for Implementation of the AASHTO Strategic Highway Safety Plan, the study by Neuman et al. (1) was developed to address two key problems involved in excessive or inappropriate speeds: (1) driver behavior (i.e., deliberately driving at an inappropriate or unsafe speed) and (2) driver response to the roadway environment (i.e., inadvertently driving at an inappropriate or unsafe speed, failure to change speed in a proper or timely manner, or failure to perceive the speed environment). Both these problems result in an increased risk of a crash or conflict. Design Guidelines The design guidelines below should be used to select and use geometric design features and traffic signals to support safe speed decisions by drivers. Additional guideline information is provided in the discussion section; however, the original source of these recommendations--Neuman et al. (1)--should be consulted for more specific design guidance. Objective General Strategy Design Guideline Design features such as curve radius, tangent length, length of spirals, vertical grades and curves, Use consistent combinations of available sight distance, and cross- geometric elements to control section features should be designed speeds. consistently across locations, in a manner that meets driver expectancies. Clearance intervals should account for expected approach speeds and Ensure that roadway design and should reflect operating speeds, traffic control elements support Provide adequate change + intersection width, vehicle lengths, appropriate and safe speeds clearance intervals at signalized and driver characteristics such as intersections. reaction time and braking. See Tutorial 4 for the equation developed by ITE (2) for determining clearance intervals. Implement protected-only signal Provide protected left turns. phasing for left turns at high-speed signalized intersections. Install lighting at high-speed Provide improved visibility. sections of the roadway, especially intersections. Based Primarily on Based Equally on Expert Judgment Based Primarily on Expert Judgment and Empirical Data Empirical Data 17-14

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HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 1.0 Discussion As discussed in Neuman et al. (1), while drivers ultimately select their own speeds, they receive, process, and use a number of cues from the immediate driving environment when doing so. Key elements of the driving environment that can effectively communicate safe speeds are roadway design and the use and operation of traffic control devices. Design consistency is a key principle in roadway design. Using consistent combinations of geometric elements leads to roadway elements that meet driver expectancies and can result in consistent speeds and fewer unexpected speed changes. For example, large differences and sudden changes in horizontal alignment, available sight distance, curve radii, etc. should be avoided, as these can increase driver workload, misperceptions, errors, and--ultimately--the likelihood of crashes. Clearance intervals provide safe transitions in right-of-way (ROW) assignment between crossing or conflicting flows of traffic. One way to accomplish safe transitions is an all-red interval, which should be designed to account for expected approach speeds to reduce the likelihood of collisions resulting from red light running. Clearance intervals that are too short can result in drivers not being able to stop in time for the red light; drivers can also stop too quickly, increasing the risk of rear-end collisions from following vehicles. Clearance intervals that are too long can lead to driver impatience, or red light running, especially in drivers familiar with the intersection. Whether the concern is red light running or increased risk of collisions, both outcomes are exacerbated by speeding. On high-speed roadways, especially in high traffic volume situations, there may be inadequate gaps for left-turning vehicles. Protected-only left-turn signals have a phase designated specifically for left-turning vehicles. Other factors that may warrant the use of protected-only left-turn phases include delay, visibility, distance of the intersection, and safety at the intersection (e.g., crash history) (1). The benefits of protected-only left turns include increasing left-turn capacity and reducing intersection delays for vehicles turning left (3). The use of protected left- turn phases also improves safety by removing conflicts during a left-turn movement. This improved safety can be especially important on roadways where high operating speeds can contribute to the crash severity and may play a role in the difficulty a driver has with identifying and selecting a safe gap (1). However, the use of protected-only left-turn signals will usually increase the cycle length, which also increases delay. For additional discussion and guidance on the type of left-turn phase to use in a given situation, see Pline (4). On high-speed roads, drivers have less time to detect visual information because vehicles are traveling faster. This problem is compounded at night when the visual contrast of some roadway elements is reduced and drivers require more time to detect visual information (drivers at higher speeds will also travel farther during this elongated detection period and consequently have less time to react to hazards). While increasing lighting on its own will not prevent speeding, it will make potential hazards or other important information easier for drivers to see, particularly during nighttime and adverse weather conditions. Design Issue This guideline, and its companion guidelines ("Speeding Countermeasures: Setting Appropriate Speed Limits" and "Speeding Countermeasures: Communicating Appropriate Speed Limits"), only include those countermeasures provided by ITE (2) that are directed at roadway design. Neuman et al. (1) should be consulted for a more detailed discussion of these countermeasures, as well as countermeasures intended (1) to heighten driver awareness of speeding-related safety issues and (2) to improve the efficiency and effectiveness of speed enforcement efforts. Cross References Speeding Countermeasures: Setting Appropriate Speed Limits, 17-10 Speeding Countermeasures: Communicating Appropriate Speed Limits, 17-12 Design Consistency in Rural Driving, 16-8 Key References 1. Neuman, T.R., Slack, K.L., Hardy, K.K., Bond, V.L., Potts, I., and Lerner, N. (2009). NCHRP Report 500: Guidance for Implementation of the AASHTO Strategic Highway Safety Plan, Volume 23: A Guide for Reducing Speeding-Related Crashes. Washington, DC: Transportation Research Board. 2. ITE (1994). Determining Vehicle Signal Change and Clearance Intervals. Washington, DC. 3. Brehmer, C.L., Kacir, K.C., Noyce, D.A., and Manser, M.P. (2003). NCHRP Report 493: Evaluation of Traffic Signal Displays for Protected/Permissive Left-Turn Control. Washington, DC: Transportation Research Board. 4. Pline, J.L. (1996). NCHRP Synthesis of Highway Practice 225: Left-Turn Treatments at Intersections. Washington, DC: Transportation Research Board. 17-15