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SUMMARY SPEED REDUCTION TECHNIQUES FOR RURAL HIGH-TO-LOW SPEED TRANSITIONS North Americaâs rural landscape is dotted with isolated settlements, villages, and small towns that are typically located on rural roads where the general speed limit is 55 to 60 mph (90 to 100 km/h). Motorists are expected to slow down as they pass through these settlement areas, reducing their operating speed to 30 or 40 mph (50 or 65 km/h) in sec- tions of road known as transition zones. In designating and designing transition zones, the ultimate goal is for motorists to reduce their speed in the transition zone to the point where they reach the lower speed limit at the start of the settled area. However, several factors thwart the goal of using transition zones to reduce driver speeds in settlement areas, includ- ing speed adaptation (the tendency for motorists to underestimate their travel speeds after having driven at a higher speed for a prolonged period) and the generally abrupt change from the rural road to the town or village streetscape. As a result, state departments of transportation (DOTs) and other agencies are interested in developing more effective speed reduction techniques for transition zones. At present, there are no national North American design guidelines for rural speed transitions, except for the general guidance provided on reduced speed ahead signing in the 2003 Manual on Uniform Traffic Control Devices (MUTCD). However, most existing transition zone design guidelines from the literature reviewed for this study are generally consistent in providing the following information: ⢠More extensive and aggressive measures tend to produce greater reductions in speed and crash occurrence than less extensive and passive measures. ⢠There needs to be a distinct relationship between a settlement speed limit and a change in the roadway character. ⢠No one particular measure is appropriate for all situations. Each settlement must be assessed and treated based on its own characteristics and merits. ⢠To maintain a speed reduction downstream of the transition zone, it is necessary to provide additional measures through the village. Otherwise, speeds may rebound to previous levels as soon as 820 ft (250 m) from the start of the lower speed zone. There is clearly a need for better and more information concerning rural high-to-low speed transitions. This synthesis report is a preliminary step in that direction. This synthesis is a state-of-the-practice report concerning effective and ineffective rural high-to-low speed transition treatments that have been tried by state DOTs and some over- seas agencies. The scope of this research was limited to engineering measures that are used to transition motorists from high-to-low speed areas, and does not include the broader topics of speed management or the more specialized techniques and methods required for areas such as work zones, toll plazas, and school zones. Literature in the subject of rural speed transition zones is not extensive. It is sometimes difficult to identify measures that are specific to transition zones, as researchers do not always distinguish between speed changing measures and speed maintaining measures. In general, research has shown that although engineering measures are effective at reduc-
2 ing speeds and crashes, and public acceptance of rural traffic calming measures is high, the effects of transition zone treatments are not sustained beyond the urban-rural thresh- old without additional downstream measures. The reported crash reduction factors have been quite significant, although methodological shortcomings with some of the studies may lead researchers to overestimate their effectiveness. With respect to the design of transition zones, there needs to be greater attention to treating the transition zone as the length of road rather than a specific point of speed change. Current information on high-to-low speed transitions was compiled through a literature review and a 42-question survey that was distributed to United States and Canadian traffic engineering personnel. Together, the practitioner survey and the literature review provide a comprehensive snapshot of the state of the practice concerning rural high-to-low speed tran- sitions. Thirty-six U.S. state DOTs responded to the survey, a 72% response rate. The survey of practice revealed that most jurisdictions do not have a set or standard approach to transi- tion zone treatment. Those that do are typically applying traffic control devices according to the state MUTCD, which involves advance signing of a lower speed limit and/or posting of a stepped-down or intermediate speed limit to mitigate an abrupt change in speeds. The most frequently mentioned conditions that would prompt consideration of enhanced transi- tion zone measures are a poor crash record, public opinion, access density, and a significant drop in the posted speed limit. Most state and provincial respondents agree that vertical deflections (e.g., speed humps and raised intersections) and removal of traffic control devices are inappropriate measures for speed transition zones. Between 40% and 50% of state/pro- vincial respondents have never tried any geometric design, surface treatment, or roadside measures outside of what might be considered the standard approach. Although practitioners generally recognize that traffic signing alone is an ineffective method of managing speed, respondents are reluctant to experiment with more aggressive and physical measures. At present, there is an interesting bifurcation between the North American and European approaches to speed management and road safety in small towns, settlements, and villages. Most effectiveness studies on transition zone treatments and traffic calming in rural areas have been conducted in Europe, and several European countries are currently experiment- ing with minimizing and removing traffic control and adding design features that physically separate road user types. These approaches attempt to create a measure of uncertainty in the driving environment that reduces operating speeds and requires motorists to pay closer attention to the driving task. This approach has not been specifically linked to transition zones, but it is in stark contrast to the North American approach to speed management, which has been to add measures. The results of these experiments are very preliminary, but should be monitored. The lessons learned from foreign testing might be used as a starting point for a North American effort into this important area of research.
