Design Guidance for Intersection Auxiliary Lanes

Abstract

A large proportion of crashes occur at intersections and auxiliary turn lanes are a key countermeasure for addressing such crashes. Auxiliary lanes can also be used to increase capacity and improve operations 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. 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). AASHTO’s A Policy on Geometric Design of Highways and Streets (the Green Book) contains limited criteria for geometric design of auxiliary lanes at intersections. This report expands on that guidance, particularly regarding bypass lanes, channelized right-turn lanes, deceleration and taper length, design and capacity of multiple left-turn lanes, and alternative intersection designs. In National Cooperative Highway Research Program (NCHRP) Project 03-102, the Texas A&M Transportation Institute reviewed existing literature and ongoing research projects and identified issues meriting further study to validate, enhance, and expand current Green Book guidance. Field studies were conducted to assess the operation of double left-turn lanes and deceleration lanes. The research team then developed practical guidance for designers on auxiliary lanes, including recommendations for improving the Green Book. This report will be useful in updating agency design manuals and to those designing intersections.