Design Guidance for Intersection Auxiliary Lanes (2014)

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5 Background Research Problem Statement There were approximately 5.8 million traffic crashes in 2008, with 55% of them occurring at intersections. Auxiliary turn lanes have been clearly identified as an effective counter- measure to address these crashes, as documented by the siz- able crash modification factors for turn lanes in the AASHTO Highway Safety Manual (HSM) (1). Auxiliary lanes can also be used to increase capacity and improve traffic operations at an intersection. The AASHTO Policy on Geometric Design of Highways and Streets (commonly known as the Green Book) (2) contains lim- ited criteria for geometric design of auxiliary lanes at intersec- tions. Additional support for these criteria and expansion of the material to cover additional designs are needed to realize fully the safety and operational benefits of auxiliary lanes at intersec- tions and to improve the consistency of application for these fundamental road features. Research Objectives The objective of this research was to recommend improve- ments to the guidance provided in the AASHTO Green Book for auxiliary lanes at intersections, thereby leading to improved safety and operations. Research Approach The research consisted of six tasks. Each task listed is followed by the objectives of that task: • Task 1. Conduct Review of Literature and Ongoing Research: identify the state of design practice for auxiliary lanes at intersections by gathering and synthesizing informa- tion on existing (customary and innovative) practices and research. • Task 2. Identify Issues: determine the issues that merit fur- ther study to validate, enhance, and expand current Green Book guidance. Task 2 activities included a review of exist- ing state design manuals, interviews with practitioners in state DOTs, and an initial review of Chapter 9 of the Green Book to describe items that are candidates for further study and potential revision. • Task 3. Submit Interim Report and Research Plan: submit the interim report along with the research plan. • Task 4. Perform Approved Research Plan: implement the revised field data collection and analysis plan. • Task 5. Develop Recommended Changes to Green Book and Other Guidance Documents: develop recommended changes to the Green Book and other guidance documents, as appropriate. • Task 6. Prepare Final Report: prepare, in accordance with NCHRP guidelines, this final report. Report Organization This final report contains the following chapters and appendices: • Chapter 1: Background. This chapter provides an over- view of the research problem and the approaches used in the research. It also presents the objectives of the research project. • Chapter 2: Review of Literature and State Design Guid- ance. This chapter presents the findings from a review of relevant literature and a review of design manuals on state DOT websites. • Chapter 3: State of the Practice. As part of Task 2 efforts, the research team conducted a state-of-the-practice review of current design considerations. To accomplish that objec- tive, the research team contacted representatives from a selection of state DOTs to inquire about current practices and potential guidance needs, based on their professional experience and the policies of their respective departments. This chapter describes that review and its findings. C H A P T E R 1

6• Chapter 4: Typical Designs. As part of the question- naire sent to state DOTs in Task 2, respondents were asked to identify locations with installations that would be considered best-practice sites. These best-practice sites were to demonstrate preferred design treatments for five design categories: island design, deceleration lane design, double left-turn lane design, triple left-turn lane design, and double right-turn lane design. A single site considered representative of the design treatment was identified for each of the five design categories. This representative site was examined in detail through a case study approach; those case studies are provided in this chapter. • Chapter 5: Double Left-Turn Lane Field Study. The goal of the Task 4 double left-turn lane study was to determine the effects of geometric characteristics on operations, as mea- sured using saturation flow rate (SFR) and lane utilization, for double left-turn lanes. The geometric variables that were the focus of this study were receiving leg width, left-turn lane width, and downstream friction location (type and distance). Chapter 5 describes the activities and findings associated with that study. • Chapter 6: Deceleration Field Study. The objectives of the Task 4 deceleration study were to determine the effects of taper length and posted speed limit on approach speeds and deceleration rates of left-turning vehicles, as compared to those described in the Green Book. This chapter con- tains the details of that study. • Chapter 7: Conclusions, Recommendations, and Sug- gested Research. This chapter lists the research team’s con- clusions from the information obtained in Tasks 1 through 5. It also lists researchers’ recommendations for applying the findings and suggestions for future research. • Appendix A: Recommended Revisions to AASHTO Green Book. Appendix A contains the results of the research team’s review of the Green Book in conjunction with the findings and conclusions from the research in Tasks 1 through 4. The appendix also contains researchers’ recommendations for revisions to consider including in the next edition of the Green Book.