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5 chapter one Introduction BacKground Cross section (including relevant roadside elements), Intersections (including channelization, roundabouts, The national policy for geometric design in the United States and recent innovative intersection designs), is the AASHTO A Policy on Geometric Design of Highways Interchanges, and Streets (commonly referred to as the Green Book). This Design consistency, document has been updated numerous times, with revisions Access management, and based on new information from research findings. As of the Pedestrian and bicycle issues. writing of this report, the Green Book had most recently been revised in 2001, which included a substantial reformatting of the document from previous editions, and in 2004, to incorpo- Study Objective rate additional guidance based on recently completed research. The objective of this study was to identify and summarize During the decade since 2000, a great deal of geometric roadway geometric design literature completed and published design-related research was conducted on a wide variety of from 2000 through early 2011, particularly research that topics and issues. Results from the research were produced identified impacts on safety and operations. To identify such to inform the profession not only on design techniques and information the study used two approaches: a review of the processes, but also on the safety and operational effects of relevant literature contained in national databases, and a those designs and how they influence other activities, such request to state design and traffic engineers for additional as maintenance. As a result, there was a significant addi- information on studies conducted within their jurisdictions. tion to the body of research related to safety and operational impacts of roadway geometric design decisions during the The national literature review represented the vast major- decade. The ability of practitioners to apply this knowledge ity of the effort for this synthesis study. TRB's Transportation can be limited because of the sheer volume of information Research Information System (TRIS), the Transport online that exists and the rapid pace in which it is produced and pub- database, and the TRB online publications catalog were all lished. In addition, geometric design research results are scat- used to identify potential sources from papers and reports tered across a variety of different tools and publications, some published during this period. Other sources of information of which are not easily accessible to designers and policy included responses to the request for information from state makers. To avoid the significant time investments that would departments of transportation (DOTs), input from the Syn- otherwise be required to find, critique, and implement the thesis Study Topic Panel, and the author's personal knowledge of recently completed research. The information collection research results into practice, a synthesis of recent research was from state DOT design and traffic engineers was requested needed. A standard for such a synthesis was established with by members of the AASHTO Subcommittees on Design and the publication in 2001 of NCHRP Synthesis 299 (Fitzpatrick Traffic Engineering. and Wooldridge 2001). NCHRP Synthesis Project 20-05 funded a study to develop an updated synthesis to build on This document is a synthesis of research, not of current or the previous work of NCHRP Synthesis 299; the result of the implementable practice. Therefore, the study did not employ a study is this document. survey or questionnaire on current practices, as is typical for such projects. An example of such a survey, documenting recent This synthesis study reviewed and summarized the geomet- state DOT practices on design guidance and standards for ric design research published between 2000 and early 2011, non-freeway resurfacing, restoration, and rehabilitation (3R) particularly research with improved safety and operations projects, can be found in NCHRP Synthesis 417 (McGee 2011). implications. The following topics were addressed in the review: Design speed, Organization of Report Additional design controls and criteria (e.g., vehicles, consistency, and driver characteristics), This synthesis report consists of the introduction, five chap- Horizontal alignment, ters that summarize the findings from the literature, and a Vertical alignment, concluding chapter with suggestions for future research. This