Public transit accounts for only 1% of U.S. passenger miles traveled but nevertheless attracts strong public support. Using a simple choice model, we predict that transit riders are likely to be individuals who commute along routes with the most severe roadway delays. These individuals’ choices thus have very high marginal impacts on congestion. We test this prediction with data from a sudden strike in 2003 by Los Angeles transit workers. Estimating a regression discontinuity design, we find that average highway delay increases 47% when transit service ceases. This effect is consistent with our model’s predictions and many times larger than earlier estimates, which have generally concluded that public transit provides minimal congestion relief. We find that the net benefits of transit systems appear to be much larger than previously believed.

Key findings

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Using a simple choice model, we show that transit provision should have much larger impacts on traffic congestion than predicted by models that do not incorporate within-city heterogeneity in driving delays.

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Back-of-the-envelope calculations suggest that the congestion relief externality of a peak-hour transit passenger mile ranges from $1.20 to $4.10.

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Both the model calibration and the regression discontinuity estimates apply specifically to the Los Angeles metropolitan area. The exact magnitudes of the effects are thus unlikely to generalize to other U.S. urban areas. Nevertheless, there are good reasons to believe that the qualitative effects of transit on congestion are similar in other large cities.

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