Operating Speed Model for Low-Speed Rural Two-Lane Highways

Operating speed, with its deviations along tangent and curve segments of highways, is one of the most common measures to evaluate the consistency of rural two-lane highways. Several models, including the Design Consistency Module (DCM) of FHWA's Interactive Highway Safety Design Model, estimate operating speeds on higher-speed rural two-lane highways. The DCM includes operating speed models for evaluating design consistency on rural two-lane highways. Through 2009 the DCM was limited to higher-speed highways (posted speeds of 45 mph and higher). The goal of the research presented in this paper was to extend the capabilities of the DCM by developing an operating speed model for lower-speed rural two-lane highways (i.e., those with posted speeds of 25 to 40 mph) using data collected by FHWA for that purpose. To determine the operating speed profile along a highway, models to estimate the 85th percentile speeds on tangents and curves were developed, and existing DCM models for acceleration and deceleration rates for transitions between preferred speeds on horizontal design elements were expanded to apply to lower-speed highways. This paper documents the model development process and provides a case study to illustrate application of the model.