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84 In the United States, design guidance for high- to low-speed transition zones for rural highways is in its infancy. This research and other recent reports/documents, such as Speed Reduction Tech- niques for Rural High-to-Low Speed Transitions (Forbes, 2011); Determining Effective Roadway Design Treatments for Transitioning from Rural Areas to Urban Areas on State Highways (Dixon et al., 2008), Evaluation of Gateway and Low Cost Traffic Calming Treatments for Major Routes in Small Rural Communities (Hallmark et al., 2007), and Main Street . . . When a Highway Runs Through It: A Handbook for Oregon Communities (ODOT, 1999), are steps toward establishing national guidelines for rural high- to low-speed transition zones. Clearly, more work needs to be done to achieve such a goal. One of the areas in which more work needs to be accomplished is the obtaining of more accurate and reliable information on the effectiveness of transition zone treatments on reducing speeds and improving safety. The primary findings from this research that contribute to the body of knowledge on the effectiveness of transition zone treatments include the following: ⢠Roundabouts and TPMs do not necessarily decrease mean speeds from upstream to down- stream of the transition zone any more than does no treatment, but they do increase speed- limit compliance. ⢠Roundabouts and TPMs increase the rate of compliance of vehicles traveling at or below the speed limit at the end of a transition zone by 15 and 20 percent, respectively, compared to no treatment. ⢠Roundabouts increase the rate of compliance of vehicles traveling at or below the speed limit +5 mph at the end of a transition zone by 11 percent, compared to no treatment. ⢠The findings support previous research (Forbes, 2011), indicating the need to provide addi- tional measures through the community to maintain a speed reduction downstream of the transition zone through the community. ⢠Based upon the crash analysis for this research, there is no evidence to suggest that the instal- lation of a roundabout, TPMs, or welcome signs in a transition zone either improves or negatively impacts safety based upon an analysis of a combination of roadway segment and intersection crashes over an extended length of roadway beginning at the upstream end of the transition zone to 0.25 mi downstream of end of the transition zone. Given the limited crash dataset for this research, the most reliable safety information available for the three treatments analyzed (roundabout, TPMs, and welcome sign) is for roundabouts, which can be found in the HSM and research by Rodegerdts et al. (2007, 2010). Still, more work needs to be done on the effectiveness of transition zone treatments, and this can only be accomplished by more agencies conducting evaluations of treatments using the most scientifically valid methodologies. S e c t i o n 5 Conclusions
conclusions 85 Finally, another recommendation for establishing national guidelines for rural high- to low-speed transition zones is the following. While the AASHTO Green Book (AASHTO, 2011) does not address transition zones, a paragraph could be added to the next edition in Chap- ters 6 and 7, explaining the transition zone related issues and the need to consider further design guidance for transition zones. In each chapter, the new text could refer the reader to this report for more details. Incorporation of detailed design guidance for rural high- to low- speed transition zones in the Green Book does not seem appropriate. Guidance on the design of transition zones is almost of the same nature as design guidance on traffic calming, and detailed guidance on traffic calming is not provided in the Green Book; therefore, reference in the Green Book to an external document seems most appropriate. It is also proposed that the next edition of the Roadside Design Guide include a general discussion of roadside issues related to transition zones.
