Bottleneck and Queuing Analysis

This paper discusses the link performance functions used in travel demand models with a focus on the strengths and weaknesses of the most commonly used volume–delay functions. These include the Bureau of Public Roads function, the conical delay function, Highway Capacity Manual procedures, and the Akcelik function. Improvements to the volume–delay functions used in travel demand models are of particular importance in light of the increased emphasis on reliable speed outputs to support air quality initiatives, improved accessibility measures for various submodels, and the desire to evaluate a broader range of policy issues. One of the key challenges that analysts face in the development of locally calibrated volume–delay functions is how best to represent the regime in which the volume—or, more aptly stated, the demand—exceeds capacity, a regime that cannot be directly observed, even though it is required for highway assignment. This paper explores the use of freeway detector data along with bottleneck and queue analysis as a relatively straightforward approach for estimating demand beyond capacity for fitting locally calibrated volume–delay functions. The results of this study show that bottleneck analysis and queue length estimation are effective means of accomplishing this goal, providing a valuable tool for improving models with locally collected data.