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Highway studies have determined that once traffic volumes exceed the capacity of the roadway, the system can rapidly "break down" to the point where all traffic slows markedly, and the capacity and throughput of the roadway drops precipitously. This study examines traffic volumes, speeds and delays at various freeway traffic counters in the Washington, DC region to specifically evaluate congested versus uncongested travel. The observations identify the specific "tipping point(s)" at which free-flow traffic "breaks down", and conversely estimate the volume of traffic that would have to be reduced in peak periods to keep traffic free-flowing. The study also examined survey data to estimate the number and percent of trips that people take in peak hours on our freeways that are discretionary trips. Finally, the study briefly reviewed empirical findings on experiences with congestion pricing in the US and abroad.

This report will be of interest to policymakers who are concerned about highway congestion and approaches that can be used to reduce congestion. It will also be of interest to transportation planners and engineers who are interested in empirical verification of traffic engineering principles.

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