The Impacts of Public Transit on Traffic

Average highway congestion delays increased 47 percent when public transit service was not available.

While some have questioned the benefits of mass transit systems, which are used by only a small fraction of commuters, research by Michael Anderson suggests that transit riders likely would otherwise commute along already heavily congested roadways, and that congestion along those roadways would increase if mass transit were scaled back. In Subways, Strikes, and Slowdowns: The Impacts of Public Transit on Traffic (NBER Working Paper No. 18757), he studies traffic-congestion data from before, during, and after an October-November 2003 transit-worker strike in Los Angeles. He estimates that average highway congestion delays increased 47 percent when public transit service was not available.

Public transit receives 23 percent of all federal highway dollars but represents only 1 percent of all U.S. passenger miles traveled. Federal, state, and local government subsidies for public transit exceed $40 billion a year, and cover 100 percent of capital costs. There has been relatively little research, however, on the effects of mass transit availability on the peak hour congestion experienced by commuters on different roadways within the same metropolitan area. This is because the counterfactual -- the absence of mass transit access in cities with mass transit systems -- is not often observed.

Not surprisingly, Anderson finds that while average roadway delays increased during peak hours of the transit strike, the effects were largest on freeways running parallel with transit lines with heavy ridership. His estimates suggest a total "congestion relief benefit" of the Los Angeles public-transit system of between $1.2 billion and $4.1 billion per year.

--Jay Fitzgerald