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3 1.1 Background The American Association of State Highway and Transportation Officialsâ (AASHTO) A Policy on Geometric Design of Highways and Streets (AASHTO, 2011), commonly known as the Green Book, makes a distinction between design criteria for high-speed facilities and low-speed facili- ties. The boundary between high-speed design and low-speed design is in the range of 45 to 50 mph. The lower limit for high-speed design is 50 mph, and the upper limit for low-speed design is 45 mph. Where high-speed facilities meet low-speed facilities, there is a transition zone where drivers in one direction are expected to reduce their speed to one suitable for the environment they are entering. An example of this is where a high-speed rural two-lane highway (e.g., with a posted speed limit of 55 mph) enters an area where lower speeds are expected of drivers, such as a community or other developed area (e.g., with a posted speed limit of 35 mph). Through the community, higher speeds are not appropriate for a number of potential reasons that include turning maneuvers at intersections and driveways, higher development density, on-street parking, higher pedestrian and bicycle activity levels, and use of curb and gutter cross sections. AASHTOâs current policy provides a sufficient level of detail for designing roads in both the high-speed environment and the low-speed environment; however, the Green Book lacks sufficient guidance on appropriate design of the transition zones between the two facility types. Such design guidance is needed to increase safety through the transition zone and entire low-speed environment (i.e., the community) and to encourage motorists to reduce their travel speeds to a level consistent with the low-speed environment. A transition zone is defined to be a section of road that is continuous with and connects a road section with a high posted speed limit to a road section with a lower posted speed limit (Forbes, 2011). The transition zone should not be considered as a specific point along a roadway where a speed change is to occur; rather, it extends over a length of roadway. Designs that encour- age gradual speed reductions over the length of a transition zone are preferred to designs that bring about sudden reductions in speed at the downstream end. Drivers need well-designed transition zones with explicit traffic control devices and road- way design features that convey the need to reduce speeds and that encourage gradual, smooth reductions in speed as they transition from high- to low-speed facilities. The treatments for reducing vehicle speeds at transition zones include effective use of geometric design features. But in achieving this goal, a broader scope of guidelines is needed that considers the potential effects of traffic engineering and traffic control, land use and adjacent development, streetscape, community context, aesthetics, and multimodal travel demands. The design of high- to low-speed transition zones is closely related to the basic concepts of traffic calming. Traffic calming has been used extensively in the United States in urban and sub- urban areas but has been used to a lesser degree in rural areas or at rural/suburban or rural/urban S e c t i o n 1 Introduction
4 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways boundaries. The use of traffic calming in rural areas internationally is more advanced than in the United States. Several countries including Denmark, Ireland, the Netherlands, Sweden, and the United Kingdom have developed guidelines for the use of traffic calming treatments in rural towns along national routes, particularly in the transition zone into a community. In developing guidance for the design of high- to low-speed transition zones for rural highways, the principles of traffic calming play a central role and much can be learned from global experiences. Recently, the National Cooperative Highway Research Program (NCHRP) published NCHRP Synthesis 412: Speed Reduction Techniques for Rural High-to-Low Speed Transitions (Forbes, 2011). The synthesis is a state-of-the-practice report on effective and ineffective rural high- to low-speed transition treatments that have been tried by state departments of transportation (DOTs) in the United States and some that have been tried in other countries. This research builds upon the results published in NCHRP Synthesis 412. 1.2 Research Objective and Scope The objective of this research is to develop design guidance for selecting effective geomet- ric, streetscape, and traffic engineering treatments for transitioning from high- to low-speed roadways, particularly rural highways entering communities. The design guidance identifies specific treatments for use in encouraging drivers to reduce their speeds and, where possible, quantifies the effectiveness of those techniques. In developing the design guidance, consid- eration is given to transition zone-specific factors such as land use; community context; aesthetics; and the accommodation of trucks, parking, pedestrians, bicyclists, and public transportation services. The scope of the research includes the development of design guidance for high- to low-speed transition zones on rural two-lane highways and rural multilane divided and undivided high- ways (i.e., non-freeways). The research focuses on that portion of the highway near the change from a high-speed to a low-speed environment (i.e., the transition zone), but consideration is also given to vehicles as they continue through the low-speed environment (i.e., through the community). Furthermore, this research focuses on engineering treatments that encourage driv- ers to reduce their speeds through transition zones. Other speed management components such as driver education and enforcement programs can be employed to reduce speeds and improve safety through transition zones, but these other speed management components and programs are not addressed in detail in this research. 1.3 Overview of Research Methodology In Phase I of the research, the literature was reviewed to identify geometric, streetscape, and traffic engineering treatments that have been implemented, or could potentially be implemented, in a transition zone and that have been evaluated to assess their effectiveness in either reducing speeds, decreasing crashes, or both. In particular, the review focused on previous evaluations that provided quantitative assessments. As part of this effort, draft material for NCHRP Synthesis 412 was reviewed in addition to other domestic and international resources. Also in Phase I, a survey of state and county highway agencies was conducted to create an inven- tory or catalog of transition zone treatments implemented across the United States. Results of the literature review and survey were used to develop a prioritized list of practical, implementable transition zone treatments, and for which more reliable information on their effectiveness in reducing speeds through transition zones was desired.
introduction 5 In Phase II of the research, field studies were performed to evaluate the effectiveness of several treatments in reducing speeds through transition zones and through the community. Crash data were also obtained to assess the safety experience near the study locations. Combining results from the field studies in Phase II with the information gathered in Phase I, the research team developed design guidance for high- to low-speed transition zones on rural highways. The design guidance addresses issues associated with defining the geographical limits of a transition zone and assessing the need for some type of improvement to increase compliance with desired speeds through the community. The design guidance also addresses issues to con- sider when selecting potential treatments for implementation and provides a catalog of transi- tion zone treatments, including effectiveness assessments. Finally, the design guidance describes a methodology for evaluating the effectiveness of treatments following implementation and possible litigation issues that agencies should consider during the planning and design of transi- tion zones projects. 1.4 Outline of Report This section outlines the entire research effort, with the remainder of the document organized as follows: ⢠Section 2 summarizes findings of the literature review, including a review of international guidelines. ⢠Section 3 describes the field studies conducted to evaluate the effectiveness of several transi- tion zone treatments in reducing speeds and presents the analysis results. ⢠Section 4 presents design guidance for selecting geometric, traffic control device, pavement surface, and roadside transition zone treatments for transitioning from high- to low-speed rural highways. ⢠Section 5 provides final conclusions and recommendations for next steps. ⢠Section 6 presents a list of references cited in the report. For practitioners who wish to focus on the primary product of this research rather than the details of the research methodology, Section 4 on design guidance will be of most interest.
