An empirical test of hypercongestion in highway bottlenecks

Abstract

There is a widely-held view that as demand for travel goes up, this decreases not only speed but also the capacity of the road system, a phenomenon known as hypercongestion. We revisit this idea in the context of highway bottlenecks. We propose an empirical test using an event study design to measure changes in highway capacity at the onset of queue formation. We apply this test to three highway bottlenecks in California for which detailed data on traffic flows and vehicles speeds are available. We find no evidence of a reduction in highway capacity at any of the three sites during periods of high demand. Across sites and specifications we have sufficient statistical power to rule out even small reductions in highway capacity. This lack of evidence of hypercongestion stands in sharp contrast to most previous studies and informs core models in urban and transportation economics.