The objective of this research was to recommend improvements to the guidance provided in the AASHTO Green Book for auxiliary lanes at intersections, leading to improved safety and operations.
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 a significant countermeasure to address these crashes, as documented by the significant crash modification factors for turn lanes in the AASHTO Highway Safety Manual (HSM). Auxiliary lanes can also be used to increase capacity at an intersection. The design components of a traditional auxiliary turn lane consist of the length needed to store an appropriate number of waiting vehicles, a vehicle deceleration area, and the taper needed to develop the full lane width. Offset and indirect turn lanes and other types of auxiliary lanes (e.g., acceleration lanes) have similar components but they are developed in a manner different from traditional turn-lane designs. The guidance and practice used throughout the United States for auxiliary lane designs and application vary by intersection location (e.g., rural or urban), traffic control (e.g., stop-control or signal-control), and lane type (e.g., right- or left-turn). The AASHTO Policy on Geometric Design of Highways and Streets (Green Book) contains limited criteria for geometric design of auxiliary lanes at intersections. Additional support for these criteria and expansion of the material to cover additional designs are needed to fully realize the safety and operational benefits of auxiliary lanes at intersections.
Task 1. Review the pertinent literature on the geometric design of auxiliary lanes at intersections and the safety and operational impacts of their design and installation. Identify ongoing relevant research projects and obtain any interim materials, if available. From the literature, compile recommendations for revisions to the Green Book material on the design of auxiliary lanes at intersections. Identify literature that provides the basis for the current Green Book material that should be added as references.
Task 2. Identify issues that merit further study to validate, enhance, and expand current Green Book guidance (e.g., differences between urban and rural designs, accommodations for site constraints, superelevation on auxiliary lanes, indirect left-turn designs for divided highways, offset designs, storage lengths, and pedestrian and bicycle accommodation). Develop typical designs related to these issues; describe the likely safety, operational, and other impacts of those designs; and determine methods that could be used to quantify those impacts. Prioritize issues and related designs for inclusion in the research plan for Task 4.
Task 3. Within 6 months of the contract start date, submit an interim report summarizing the work done in preceding tasks and presenting a research plan for Task 4. For each design to be studied, the research plan should identify the method that will be used to quantify the impacts, the range of design values, the traffic volumes and characteristics, and any human factor elements of concern. The Task 4 research plan should include options totalling approximately 120% of the amount originally budgeted for that task; the excess is intended to provide the project oversight panel with discretion.
Task 4. Carry out the research plan, as approved by the panel at the interim meeting.
Task 5. Based on the results of Tasks 1, 2, and 4, prepare a recommended rewrite of the Green Book section on Auxiliary Lanes within the Intersections Chapter, including appropriate exhibits. The rewrite should provide practical guidance for designers, consistent in approach with the rest of the Green Book, and include supporting references, as appropriate. The rewrite should be in an appropriate format for AASHTO’s further use.
Task 6. Prepare a final report providing the results of the research, including the Task 5 Green Book section rewrite as a stand-along appendix.