Politicians and officials frequently claim that mass transit reduces traffic congestion.
Former Nashville Mayor Megan Barry, for example, claimed that a light rail expansion plan was the “number one way” to reduce the region’s traffic congestion. And William Millar, retired president of the American Public Transportation Association, claimed that “expanding transit is the key to reducing traffic congestion.”
Yet work by academics and practitioners finds transit does not have a measurable effect on reducing traffic congestion.
Professor Brian Taylor of the University of California, Los Angeles, found in a landmark study that the best ways to reduce congestion were through significantly enlarging roadway capacity, restricting roadway usage and pricing roadways. Expanding transit did very little.
David Fields, former president of the American Planning Association’s (APA) transportation division, wrote an essay in Planning Magazine pointing out that transit does not reduce traffic congestion and urging planners to quit claiming that it does.
There are a number of reasons mass transit does not reduce traffic congestion. In many cities, there are far too few commuters using transit to make a significant dent. Transit can also induce new trips, encouraging folks who would have stayed at home to use the service. These new riders did not cause roadway congestion. But the biggest reason transit does not reduce congestion is induced demand.
Induced demand is the phenomena noted by Robert Cervero, a recently retired scholar from the University of California, Berkeley, who found drivers have unmet trip demands. Drivers in most metro areas would make additional trips or switch their trips from off-peak hours to rush hour if not for traffic congestion. For example, a family in the Washington, DC, area who would eat at a restaurant on a Thursday night chooses to eat at home instead because of traffic congestion. If additional capacity is added to roadways, those trips might take place. Thus adding non-priced capacity is not an effective way at reducing congestion.
There are advantages to adding non-priced lanes. New capacity allows these new trips. The family eating out would increase economic activity and, together with other families and trips, would lead to new restaurants or other business growth. However, those new lanes don’t reduce congestion and will themselves likely become congested within two-to-five years. The best example of this phenomenon is I-405 in Los Angeles. As part of the highway widening, a new non-priced lane was added to the facility. The new lane became congested just weeks after it opened.
Induced demand for roadways also affects transit’s ability to reduce congestion. Having drivers switch to transit does not reduce congestion because other drivers will simply take their place. That new resident in the mixed-use development, who switches from driving to taking the train, will be replaced by a commuter who takes I-405 to work. The Fortune 500 employee, who adjusts his work schedule to leave at 7 pm instead of 6 pm to avoid traffic, will be replaced at 6 pm by the family going out to eat if traffic is reduced.
Transit serves a number of purposes. Most importantly, it offers a commuting option, particularly to low-income families, disabled individuals, and elderly workers who could not drive. It offers a choice to those who want to use their travel time to work productively by checking e-mails or read while on the bus or train. In addition, transit can lead to increased economic activity by allowing additional folks to commute in a given geographic area. But transit does not reduce congestion. Voters should beware of any politician selling congestion relief as a primary transit benefit. And transit advocates need to be honest about the many things transit can do—but also the things it cannot accomplish.