Development of Left-Turn Lane Warrants for Unsignalized Intersections (2013)

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1 CHAPTER 1 INTRODUCTION BACKGROUND Left-turn movements at intersections, including driveways—especially movements that are made from lanes that are shared with through traffic—cause delays and adversely impact safety. Although updated warrants have been developed in some jurisdictions for when to provide left- turn lanes, many agencies still use research from the mid-1960s. The research by M. Harmelink in Canada that was published in 1967 focused on “Volume Warrants for Left-Turn Storage Lanes at Unsignalized Grade Intersections” (1). As indicated in recent research, some values used in the methodology are no longer valid. In addition, current conditions require a broader assessment of when to provide left-turn accommodations. Explicit consideration of safety and operations (e.g., delay) is needed. Technical warrants are an important element of the decision-making process. The addition of a left-turn lane can improve the operations and safety at an intersection. Functions of left-turn lanes include: • Reduce the number of conflicts and crashes; • Separate through, turning, and/or queuing traffic; • Decrease delay and increase capacity; • Provide more operational flexibility and impacts; and • Provide an area for left-turning vehicles to decelerate outside of the through traffic lane. Factors considered when making the decision to install a left-turn lane can include: • Type/function of roadway, • Number of lanes, • Prevailing speeds, • Traffic control/operations, • Turn and other volumes, • Roadway(s) alignment, and • Safety (conflict, crash numbers, and crash types/causes). A recent Federal Highway Administration (FHWA) study (2) found that the addition of a left- turn lane can result in reductions of crashes from 7 to 48 percent (see Table 1). Other studies (3) have demonstrated the benefits of delay reductions with the installation of a left-turn lane on two-lane highways. Guidelines as to when to include a left turn in intersection design are plentiful. Some are based on minimizing conflicts in terms of the occurrence of a through vehicle arriving behind a turning vehicle, some are based on decreasing the amount of delay to through vehicles, and others are based on consideration of safety. Because of the quantity of methods, questions are asked regarding which method to use. For example, are certain techniques better for a rural versus an urban setting? Do the evaluations differ for number of lanes and for type of intersection?

2 Table 1. Expected percent reduction for adding turn lane identified in FHWA study (2). Treatment/Area/Intersection Type Expected Percent Reduction of Total Crashes One Approach Both Approaches Stop Signal Stop Signal Add Left-Turn Lane Rural 3-leg 4-leg 44 28 15 18 NA 48 NA 33 Urban 3-leg 4-leg 33 27 7 10 NA 47 NA 19 Add Right- Turn Lane Rural/Urban All 14 4 26 8 NA = value not identified in research OBJECTIVES The objectives for this research were to: • Develop an objective and clear process for the selection of left-turn accommodations at unsignalized intersections and • Provide guidance on the design of these accommodations. APPROACH The research for the project was conducted within tasks split into two phases. Phase I focused on reviewing the literature, conducting interviews, identifying performance measures, identifying sources for data, and developing the Phase II work plan. In Phase II data were collected at 30 sites to be used to calibrate a simulation model and to update the assumptions in the Harmelink approach. An economic analysis through a benefit-cost approach was conducted to identify warrants. The benefit-cost ratio included consideration of crash savings, delay savings, and construction costs. The simulation model was used to determine expected delay reduction when a left-turn lane is added. Crash savings and construction costs were identified as part of this project. Economic analysis provides a useful method for combining traffic operations and safety benefits of left-turn lanes to identify situations in which left-turn lanes are and are not justified. A Design Guide (4) was developed as part of this research project. It contains the recommended left-turn lane warrants along with design and traffic control treatment discussions for left-turn lanes. ORGANIZATION OF THIS REPORT The research for this project is presented in the following chapters and appendices: • Chapter 1: Introduction. This chapter provides an overview of the research problem and the approaches used in the research. It also presents the objectives of the research project. • Chapter 2: Literature Review—Installation Guidelines. A review of the literature was performed using many sources including research reports, state and federal design manuals, and handbooks. Details are provided on those methods that appear to have distinctive results.

3 • Chapter 3: Literature Review—Left-Turn Lane Design. This chapter contains information on the design of left-turn lanes, reviewing information from state design manuals and national reference documents, as well as recent research. • Chapter 4: Driver Behavior Study. The methodology used and results from the field study at 30 intersections are documented in this chapter. • Chapter 5: Delay, Crash, and Construction Cost Studies. A benefit-cost approach was used to determine when a left-turn lane installation is justified. The approach uses the benefits from crash reductions (change in number of crashes) and delay reduction (improvements in roadway capacity from removing the slow-moving or stopped left-turn vehicles) and compares them to the cost of constructing a left-turn lane. • Chapter 6: Comparison of Procedures. The findings from the research are used to generate suggested warrants using the benefit-cost ratio and the approach developed by Harmelink. • Chapter 7: Summary and Conclusions. The final chapter of the report provides a summary of the research along with the research team’s conclusions. • Appendix A: Revised Text on Left-Turn Lane Warrants for the AASHTO Green Book. This appendix presents the suggested revisions to the AASHTO Green Book based on the research. • Appendix B: Revised Text on Left-Turn Lane Warrants for the TRB Access Management Manual. This appendix presents the suggested revisions to the Transportation Research Board (TRB) Access Management Manual based on the research. • Appendix C: State Warrants/Guidelines for Left-Turn Lanes. A summary of the warrants and guidelines for left-turn lanes from states is presented in this appendix. • Appendix D: Interview Questions. This appendix lists the interview questions. The 25 questions in the interview were structured into planning, design, legal/policy/finance, and potential future applications. • Appendix E: Interview Findings. Results from the interviews conducted of representatives from state departments of transportation, county governments, city governments, and consultants are documented in this appendix. • Appendix F: Legal Review. The legal review conducted as part of this research addressed the following question: When a government seeks to fulfill a broad public objective such as safety and, in this project, left-turn accommodation, who should bear the costs—the developer who would be adding traffic to the roadway network or the general public?