Measuring and Estimating Congestion Using Travel Time-Based Procedures

Procedures are presented for measuring and estimating roadway congestion levels. An assessment of users, uses, and audiences indicates a need for congestion measures that are understood by nontechnical audiences, yet are rigorous enough for technical analyses. Travel time-based measurements are deemed most useful for this wide range of needs. Data collected for an NCHRP project were used to identify the number of travel time observations and roadway segments required for reliable estimates of congestion through direct data collection. The data and previous congestion studies were used to develop surrogate procedures that can estimate congestion statistics with readily available traffic count and roadway inventory data. The surrogate estimation procedures were developed to assist agencies when direct data collection is not practical or feasible. Both of these processes—direct measurement and estimation of travel time-related quantities—are important for quantifying congestion.