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A new study finds that transit does cut down on traffic — but only on roads that parallel heavy transit corridors.

The idea that public transportation relieves road congestion is both logical and popular, but the evidence for it is decidedly mixed. The "fundamental law of road congestion," for instance, suggests that transit fails to relieve traffic because latent demand for road space is so great. (The only hope of reducing congestion, according to this work, is establishing road fares.) So while there are any number of reasons to support the public funding of public transportation — sustainability, for instance, or social welfare — economists don't all find congestion relief to be one.

That debate just got a lot more interesting with some new work (via Paul Krugman) by Berkeley scholar Michael Anderson, who argues that "the net benefits of transit systems appear to be much larger than previously believed." By analyzing the impact of a Los Angeles transit strike in 2003, Anderson found that congestion did decrease considerably — but only on roads that paralleled heavy transit corridors. He suggests that previous research has focused too much on general metro traffic and not enough on the specific roads that transit is most likely to influence:

The intuition is straightforward: Transit is most attractive to commuters who face the worst congestion, so a disproportionate number of transit riders are commuters who would otherwise have to drive on the most congested roads at the most congested times. Since drivers on heavily congested roads have a much higher marginal impact on congestion than drivers on the average road, transit has a large impact on reducing traffic congestion.

In October of 2003, Los Angeles transit workers went on strike for 35 days, shutting down major bus and rail lines. Anderson looked at hourly traffic speed data on major L.A. freeways during this time. The data showed that, during peak periods, the average delay increased 47 percent on these roads. The effects were largest on freeways that paralleled transit — and statistically insignificant on roads in neighboring counties.

A few examples. On U.S. 101, for instance, which parallels the major Red Line subway, the average delay increased 90 percent during the strike, according to Anderson. For freeways that didn't parallel transit corridors, the average delay increased just 29 percent. The delay on 101 during morning peak, in particular, increased 123 percent; the morning delay on freeways that didn't parallel transit only went up 56 percent.

Anderson then extrapolates his findings to show the economic benefit of public transit to the city. Using some back-of-the-envelope calculations, he says the congestion relief provided by the Los Angeles system ranges between $1.2 billion and $4.1 billion per year. In other words, the high cost of constructing transit systems come with considerable economic gains. Anderson concludes:

Contrary to the conclusions in the existing transportation and urban economics literature, the congestion relief benefits alone may justify transit infrastructure investments.

The study is certain to stir the academic pot, and it will be interesting to see how transport economists respond. (Writing about the paper on his blog, Krugman likes the report's conclusions, but doesn't evaluate its calculations.) In any event, Anderson has made a valuable contribution to the research by emphasizing the fact that when it comes to transit's impact on congestion, not all roads are creating equal.

Top image: Konstantin Sutyagin /Shutterstock

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