Analytical Methods for Deriving Work Zone Capacities from Field Data

Abstract

Work zone capacity is a key variable in scheduling construction activity on roadways. Understanding the effect of different definitions of capacity on the values computed from field data will result in better planning and scheduling of lane closures. Capacity values obtained using three different definitions (and methods) of capacity: (1) maximum sustained flow, (2) rescaled cumulative flow curves, and (3) 85th percentile flow were compared using field data from short-term work zones in Missouri. The queue discharge flow (QDF) values were found to be the most conservative estimates of capacity. The 85th percentile flows were lower than the 15-min sustained flow values in all but one location. The prequeue flow (PQF) values, indicative of near-constant flow prior to breakdown, did not occur in any of the four work zones. The four-site average capacities were 1,149, 1,267 and 1,301 vphpl for QDF, 85th percentile flow, and 15-min sustained flow, respectively. These capacity values were close to the 1,240 vphpl Highway Capacity Manual (HCM)-based capacity value used in Missouri, but lower than most values found in the nationwide survey.