Effect of Site and Population Characteristics on Freeway Bottleneck Capacity

Abstract

In an attempt to explain variations in prequeue flow (PQF) and queue discharge flow (QDF), one- and two-stage regression models that link high-volume PQF and QDF at freeway bottlenecks to site geometric, vehicle population, and driver population characteristics were developed and evaluated. One-stage models linked site characteristics directly to PQF and QDF; two-stage models used the average time gap in the critical lane and the ratio of critical lane flow to flow per lane as intervening variables (where time gap is the time separation between the rear of the lead vehicle and the front of the following one and critical lane is the lane with the highest flow). Explanatory variables included number of lanes, vertical alignment, proportion of heavy vehicles, and population characteristics derived from census data. Models were identified by stepwise regression analysis and evaluated by comparing predicted PQF and QDF values with the results of Highway Capacity Manual (HCM) capacity calculations and average values of PQF and QDF measured at the sites used to develop the models and at a second group of sites used for model verification. In most cases, the initial models developed by the study performed similarly–better than the HCM (which tends to overestimate PQF and QDF)–but in no case did calculated PQF and QDF values correlate with measured flows for the verification sites. After one site with apparently anomalous data was eliminated, the only significant explanatory variable was the number of lanes. Although the number of lanes explains some variation in PQF and QDF among bottlenecks, a great deal more remains unexplained.