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1 This report presents the results of a study undertaken to develop improved design guid- ance for high-speed to low-speed transition zones on rural highways. The primary steps of the research included a literature review and state-of-practice review on speed reduction treatments utilized in transition zones (both domestically and internationally) and obser- vational field studies of several key treatments that have been implemented in the United States. Several important or fundamental findings from the observation field studies are as follows: ⢠Roundabouts and transverse pavement markings (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 within 5 mph of the speed limit 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 additional measures through the community to maintain any speed reduction downstream of the transition zone. This report also provides design guidance for selecting geometric design, traffic control device, pavement surface, and roadside treatments for transitioning from high- to low-speed roadways on rural highways. The design guidance covers a wide range of issues to be consid- ered in the design of high- to low-speed transition zones, including the following: ⢠Definitions and site characteristics to define the geographical limits or boundaries of the transition zone study area. ⢠A methodology for assessing whether a high- to low-speed transition zone has speed-limit compliance or safety issues to support the need for and the selection of an appropriate treatment to address the issue(s). ⢠Guiding principles and design concepts to be considered in the design of a transition zone. ⢠A catalog of potential transition zone treatments with a description and illustration of the treatments and information on effectiveness, cost, contraindications, and installation location. ⢠The importance of evaluating the effectiveness of transition zone treatments after implementation. ⢠Legal/liability issues to be considered when evaluating and designing transition zones. In the United States, the development of national design guidelines for rural high- to low-speed transition zones would be valuable. This document and other recent reports and documents are steps toward achieving this goal, but more work needs to be done. Several suggested steps for building upon the guidelines in this report are as follows. First, more S u m m a r y Design Guidance for High-Speed to Low-Speed Transition Zones for Rural Highways
2 Design Guidance for High-Speed to Low-Speed Transition Zones for rural Highways accurate and reliable information needs to be collected on the effectiveness of transition zone treatments on reducing speeds and improving safety (and potentially other benefits). This can only be accomplished by more agencies conducting evaluations of treatments using the most scientifically valid methodologies. Second, the AASHTO Green Book does not address transition zones. In the next edition of the Green Book, it is proposed that a paragraph be added to Chapters 6 and 7, explain- ing 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 other research, such as 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 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 issues related to transition zones.
