NCHRP Report 398, Quantifying Congestion, has two volumes. Volume 1, Final Report, reviews the state of the practice in congestion measurement, describes why a jurisdiction should measure congestion, describes how a congestion measurement program should be organized, and discusses how to interpret the measures of congestion. It will be useful to those responsible for developing a congestion management or measurement program. Volume 2, User's Guide, describes how to measure congestion in the field (including determining the number of samples that should be collected), presents methods for estimating congestion when field measurement is not possible or practical, and describes different ways to present congestion measures so that they are understandable to policy makers and the public. It will be useful to those responsible for measuring or estimating congestion or presenting their findings to the public and policy makers. Each document includes a summary of the information in the companion document.

In recent years, congestion on streets and highways has grown to critical dimensions in many areas of the United States. This congestion has become a major problem and has many detrimental effects including lost time, higher fuel consumption, more vehicle emissions, increased accident risk, and greater transportation costs. The concept of congestion as a serious problem has been embraced by the media, the public, policy makers, and transportation professionals. However, there is no consistent definition of congestion in terms of a single measure or set of measures that considers severity, duration, and spatial extent. Quantification of congestion on individual facilities or for individual trips, measurement of the rate of change of congestion within an area, and comparison of congestion severity, extent, duration, and variability between areas are very difficult. Accurate measures of congestion are needed for analytical purposes, such as system evaluation and improvement prioritization, and for use by policy makers and the public.

The researchers for this project reviewed definitions of congestion, evaluated different measures of congestion, developed methods to obtain the recommended measures, and prepared a final report and user's guide. The report and user's guide were extensively tested and validated by the researchers' close interaction with various jurisdictions to ensure that the report and user's guide would be useful to practitioners.

The two documents present a cost-effective procedure for accurately and consistently measuring congestion of one or several modes on a roadway. The procedure provides methods to evaluate and compare congestion on corridor, subarea, and regional bases and is sensitive to both recurring and incident congestion. The procedure generates measures that are useful and understandable to policy makers and the public. While directly applicable to roadways, the procedure produces measures that can be calculated for other modes allowing easy comparisons to be made for multimodal systems.