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HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 1.0 SPEEDING COUNTERMEASURES: SETTING APPROPRIATE SPEED LIMITS Introduction Setting appropriate speed limits refers to guidelines and best practices for determining appropriate speed limits that take into account the unique traffic, design, and environmental aspects of the roadway. Much of the information in this guideline, as well as its companion guidelines ("Speeding Countermeasures: Communicating Appropriate Speed Limits" on page 17-12 and "Speeding Countermeasures: Using Roadway Design and Traffic Control Elements to Address Speeding Problems," on page 17-14), are 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 help set appropriate speed limits. 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 Consider the: Design speed of a major portion of the road, Vehicle operating speed, measured as a range of Set speed limits that account 85th percentile speeds taken from spot speed for roadway design, as well surveys of free-flowing vehicles on the roadway, as traffic and environmental Safety experience of the roadway, in the form of conditions crash frequencies and outcomes, and Enforcement experience; i.e., law enforcement's allowance for driving above the posted speed limit as well as the level of enforcement. Set appropriate While the efficacy of VSLs is uncertain (see also speed limits Milliken et al. (2)), they can be used for: Implement variable speed Predictable events, such as during school hours limits (VSLs) and construction activities, and Unpredictable events, such as poor visibility due to fog or snow, and traffic incidents. In high-speed areas, consider posting a lower speed Implement differential limit for heavy trucks in order to reduce the severity speed limits for heavy of collisions involving trucks. vehicles (high-speed Note: Not all researchers agree that differential speed areas only) limits for trucks should be used, see the following discussion section. Based Primarily on Based Equally on Expert Judgment Based Primarily on Expert Judgment and Empirical Data Empirical Data 17-10
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HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 1.0 Discussion As discussed in Neuman et al. (1), speed limits that appear inconsistent, fail to reflect the immediate roadway environment, or are inconsistent with driver expectancies may be ignored by drivers. This situation, in turn, can contribute to a lack of respect for and compliance with speed limits. The posted speed limit provides drivers with not just a legal limit, but also the maximum speed that highway engineers and road designers consider to be safe and appropriate. As noted by Milliken et al. (2), well-conceived speed limits also provide the basis for enforcement by law enforcement officers and the court system. For the set speed limits that account for roadway design, as well as traffic and environmental conditions strategy, practicality and enforcement are key considerations. Setting the speed limit at the 85th percentile speed is expected to result in compliance by most drivers; however, unique design, traffic, or environmental characteristics of the roadway can also affect actual driving speeds. Such characteristics include proximity to schools or hospitals, an unusually high percentage of trucks in the traffic flow, unusually heavy pedestrian volumes, or a concentration of elderly pedestrians. Variable speed limits (VSLs) are generally communicated through CMSs or other traffic control devices. A critical issue with VSLs is determining where they should be used, when the speed limits should be changed, and what the "other" speed limits should be; cameras or other detection equipment can be used to make these determinations (1). Visible and regular enforcement is also required to ensure compliance with the speed limits. The use of differential speed limits for heavy trucks is an option for locations associated with a high incidence of truck crashes; however, the research is mixed with respect to the efficacy of doing so. The logic underlying the use of having a lower posted speed limit for trucks than for passenger vehicles is "that trucks have much longer stopping distances than do light vehicles and have other speed-related risks such as rollover at lower speeds and vulnerability to loss of control in cross winds" (3). The counterargument is that differential speed limits for trucks vs. cars increases the overall variability in vehicle speeds (at a given location at a given time), resulting in a greater potential for conflicts and crashes between trucks and cars. In a review of safety outcomes associated with heavy vehicles, Harwood, Potts, Torbic, and Glauz (4) found that the use of differential speed limits does not seem to reduce crashes, but may vary the distribution of crash types. Design Issues This guideline, and its companion guidelines ("Speeding Countermeasures: Communicating Appropriate Speed Limits" on page 17-12 and "Speeding Countermeasures: Using Roadway Design and Traffic Control Elements to Address Speeding Problems" on page 17-14), only include those countermeasures provided by Milliken et al. (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: Communicating Appropriate Speed Limits, 17-12 Speeding Countermeasures: Using Roadway Design and Traffic Control Elements to Address Speeding Problems, 17-14 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. Milliken, J.G., Council, F.M., Gainer, T.W., Garber, N.J., Gebbie, K.M., Hall, J.W., et al. (1998). Special Report 254: Managing Speed: Review of Current Practice for Setting and Enforcing Speed Limits. Washington, DC: Transportation Research Board. 3. Knipling, R.R., Waller, P., Peck, R.C., Pfefer, R., Neuman, T.R., Slack, K.L., et al. (2004). NCHRP Report 500: Guidance for Implementation of the AASHTO Strategic Highway Safety Plan, Volume 13: A Guide for Reducing Collisions Involving Heavy Trucks. Washington, DC: Transportation Research Board. 4. Harwood, D.W., Potts, I.B., Torbic, D.J., and Glauz, W.D. (2003). CTBSSP Synthesis of Safety Practice 3: Highway/Heavy Vehicle Interaction. Washington, DC: Transportation Research Board. 17-11