Computing the Minimal U-Turn Offset for an Unsignalized Superstreet. Issue 1 (2017) - Xu, Liu
Despite the extensive implementations of unsignalized superstreets in practice, reliable methodologies for their geometric design remain unavailable. In recognition of the critical role of the U-turn offset length design, this study proposed an applicable tool to help traffic professionals determine the minimum required offset length for safety needs. The proposed model offers a convenient tool to evaluate a candidate design on the basis of traffic dynamics and driver gap acceptance behavior. In addition, this model can be used to assess whether installation of signals is needed to accommodate growing traffic. With the use of the surrogate safety assessment model as an unbiased tool, this study evaluated the safety performance of one superstreet with varied U-turn offset lengths (i.e., 700, 1, 100, and 1, 500 ft). Comparisons of the results of different designs indicated that the safety performance of the superstreet would not be compromised if the U-turn offset was reduced from 1, 500 to 1, 100 ft. However, more severe crashes were observed when the U-turn offset length was 700 ft. Further extensive analyses on different levels of demand also confirmed the need to properly design the offset length in an unsignalized superstreet.