Recent Roadway Geometric Design Research for Improved Safety and Operations (2012)

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

6 introductory chapter provides an overview of the project, including background information. Chapters two through six provide summaries and relevant tables and figures of the findings from the literature. These chapters are organized to match the presentation of material in the Green Book: • Chapter two—Design Controls and Criteria • Chapter three—Elements of Design • Chapter four—Cross-Section Elements • Chapter five—Intersections • Chapter six—Interchanges The final chapter of the report is a summary of key find- ings from the literature, along with potential issues to be con- sidered for future research. It is important to note that the recommendations included in the findings from the literature throughout this report are those of the authors cited. Before any revisions to AASHTO’s Green Book were to be made on the basis of these recom- mendations, they would need to be considered on the basis of the rigor of the research and logic that underlie them. No endorsement of these recommendations is implied by their inclusion in the findings from the literature. relatIonShIp to other documentS Results from the research synthesized in this document rec- ommended a number of changes to the AASHTO Green Book, the Manual on Uniform Traffic Control Devices (MUTCD), and other guidance documents. The research also produced two FHWA Informational Guides on roundabouts and con- tributed to other guides on access management, pedestrian and bicycle accommodation, designing for older drivers, the Highway Capacity Manual (HCM), and the Highway Safety Manual. Discussion of the relationship between this synthesis report and other documents, along with relevant cross refer- ences, is also provided. It should be emphasized that this synthesis report does not contain the applicable policies or design tools; it is con- fined to summarizing the literature. References to appropriate research reports, policies, and guidance documents are provided throughout the document, compiled into a comprehensive list of references, which follows the Conclusions (chapter seven). Additional, relevant sources are provided in a Bibliography following the References; these sources are not specifically included in the body of the report, but may provide readers additional information on issues and topics relevant to those contained in the report.