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HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 1.0 SPEEDING COUNTERMEASURES: COMMUNICATING APPROPRIATE SPEED LIMITS Introduction Communicating appropriate speed limits refers to guidelines and best practices for communicating posted speed limits to 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: 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 communicate 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 Locate speed limit signs where drivers expect them to be, such as following a major intersection. Use advance notice signs (e.g., "Reduced Speed Ahead") to alert the driver to an upcoming speed Improve speed limit change. signage Consider context: where other traffic signs and/or commercial signs are abundant, use larger speed signs, increase the number of speed signs, or Communicate remove unnecessary signs. appropriate speeds through the use of Use in locations where speeding has been observed Implement active speed or poses a safety risk, such as school zones, sharp traffic control warning signs horizontal curves, or locations with a history of devices speed-related crashes. Use in-pavement measures May include transverse lines, peripheral transverse to communicate the need to lines, chevron lines, and rumble strips. reduce speeds Use CMSs to present information relevant to traffic Implement changeable conditions, work zones, weather and road surface message signs (high-speed conditions, detour/directional information, crashes areas only) and incidents, and appropriate speed limits. Based Primarily on Based Equally on Expert Judgment Based Primarily on Expert Judgment and Empirical Data Empirical Data 17-12
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HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 1.0 Discussion As discussed in Neuman et al. (1), information about speed limits--in the form of signs or markers--should be clearly communicated to drivers, at appropriate locations on the roadway. 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. The placement and visibility of speed signs are key to properly communicating speed limits. Improving speed limit signage is especially important in areas where signs are frequently obscured by other signage, vegetation, or adverse weather conditions. Also, having a high percentage of older drivers on a particular section of the roadway is often a good reason to address signage location and visibility. Providing conspicuous and redundant information about unexpected posted speed changes, such as those greater than 10 mi/h, can also increase driver awareness of a speed change. This information can be provided by using "Speed Reduction Ahead" signs in advance of the change, placing signs on both sides of the roadway, and using signs with salient features (e.g., fluorescent flags) (1). Additional supplementary signs spaced every 60 s of travel (or more frequently in urban areas with increased access to the road) can also promote driver awareness of the speed limit. Active speed warning signs improve drivers' awareness of both their current speed and the posted speed limit in order to deter speeding behaviors. In Bloch (2), a beforeafter evaluation was conducted to assess the benefits of using a speed warning sign. The study found that mean speed was reduced at the sign location, but that intermittent enforcement was required to significantly reduce speeds downstream from the sign. The sign was effective in reducing excessive speeds (i.e., speeds 10 mi/h above the posted speed). In-pavement measures and other perceptual measures can be used to encourage drivers to adhere to speed limits (1). Pavement marking--such as transverse lines, peripheral transverse lines, and chevron lines--gives the illusion that the driver is driving faster than his/her actual speed and can be used as a means to decrease excessive speeds by reducing the driver's comfort level at higher speeds (1). These approaches can be used along any roadway segment where speed may be a problem, as well as locations where speed reductions are necessary, such as intersection approaches, work zones, toll plazas, and ramps. Rumble strips (e.g., continuous shoulder rumble strips, centerline rumble strips, or transverse rumble strips) may also be used to reduce vehicle speeds or to prevent crashes where speed is a causal factor (1). In this role, rumble strips are used as a traffic calming device in, for example, high- pedestrian areas such as parks, schools, hospitals, and residential areas. Rumble strips are also discussed in "Shoulder Rumble Strips" on page 16-6. CMSs can also be used to display information on appropriate speeds relative to current conditions. See Chapter 19 for more details on when and how to use CMSs. Design Issues This guideline, and its companion guidelines ("Speeding Countermeasures: Setting Appropriate Speed Limits" on page 17-10 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. (3) 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: Using Roadway Design and Traffic Control Elements to Address Speeding Problems, 17-14 Rumble Strips, 16-6 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. Bloch, S.A. (1998). A comparative study of the speed reduction effects of photo-radar and speed display boards. Transportation Research Record, 1640, 2736. 3. 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. 17-13