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Transit and Congestion, an Indirect Connection

11:01 AM EDT on October 2, 2009

Yesterday, Freakonomics linked to a new piece of research [PDF] on congestion that I'd been musing over for a few days. Let me quote the abstract here (paragraph break and emphasis mine):

We investigate the relationship between interstate highways and highway vehicle kilometers traveled (vkt) in us cities. We find that vkt increases proportionately to highways and identify three important sources for this extra vkt: an increase in driving by current residents; an increase in transportation intensive production activity; and an inflow of new residents.

The provision of public transportation has no impact on vkt. We also estimate the aggregate city level demand for vkt and find it to be very elastic. We conclude that an increased provision of roads or public transit is unlikely to relieve congestion and that the current provision of roads exceeds the optimum given the absence of congestion pricing.

The first inclination of most urbanists, when confronted with something like this, is probably to bristle and conclude that the authors are nuts. Transit can't relieve congestion? What would happen to New York or Washington if transit systems were shut down for a day? There would be chaos!

191334.jpgPhoto by derang0.

There would indeed be chaos. But that doesn't mean that the authors are wrong, and it's important to understand why.

When a transit system is built, two things happen to area roadways. Some subset of drivers will find that commuting by transit is faster, or cheaper, or more convenient (or all three) than driving and will switch to transit. If that were where things ended, then transit construction would indeed reduce congestion.

But there is a knock-on effect. When drivers switch to transit, roads become less congested, and driving therefore becomes more attractive. Drivers who were previously commuting at off-peak times to avoid congestion will switch to peak times, and drivers who were otherwise adapting to congestion by working at home some days or taking longer, alternative routes will adjust their behavior as well.

The end result will be ... a return to road congestion. This may not happen immediately. When new capacity of any sort -- roads or rails -- is built, there may be a time period during which traffic flows more freely, but ultimately settlement and transportation patterns will adjust until roads are again congested.

This happens because roads are under-priced (and often free). If drivers don't have to pay to use scarce road space, and don't have to pay to cover the cost of congestion their driving imposes on others, then drivers will use the road until it is congested. Because the government isn't using price to ration demand (as is done with most consumer goods), demand will rise until the cost of lost time rations demand, and pushes drivers to take other routes or modes.

I don't think there's any point in denying this. Instead, there are key points that urbanists should take to heart.

One is that roads are overbuilt. Years of trying to fight congestion, ineffectually, by building new capacity have generated a road network that is far too large.

Another is that road congestion can only be addressed through imposed rationing, and the most efficient way of doing this rationing is through variable tolling. This, of course, should raise a lot of new and desperately needed revenue.

The third point is that just because transit can't permanently reduce congestion doesn't mean that there aren't good reasons to build transit. Indeed, the authors of the above study conclude that it will often make sense to do so.

What kinds of reasons? Well, just because the road network is overbuilt doesn't mean that all transportation demand is adequately met. In many areas -- particularly in center cities and job concentrations -- new capacity is needed, and transit offers an effective way to move a lot of people through dense areas.

Another reason is that in the busiest areas, congestion tolls are likely to be fairly high, and transit can offer an affordable alternative to driving to low- and middle-income travelers.

And finally, transit can help address one of the key contributors to transportation demand -- poor land use. Transit networks offer a framework around which denser, mixed-use neighborhoods can be built, allowing more people to walk or bike to work or reduce their travel needs. Infrastructure is expensive to build and maintain, and ideally we'd like to grow in a more intensive way that reduces that needed infrastructure for a given amount of land.

Urbanists ought to trumpet these results. Drivers are more likely to accept congestion tolls if it is made clear to them that such tolls are the only way to reduce road congestion. And tolling will create both the demand and the revenue for new transit capacity. The relationship between capacity and congestion is one we'd all do well to understand.

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