Building new roads increases traffic

Does not help congestion

Jay Blazek Crossley, Jun 06, 11.

Building new roads does not reduce congestion for metropolitan regions and can actually increase traffic, according to an upcoming report to be published in the American Economic Review, as explained in Streetsblog Capital Hill. 

The report also notes that transit is not an appropriate tool for the auto-based problem of combating congestion, while congestion pricing is the “main candidate tool to curb traffic congestion.” The researchers looked mostly at building new roads and transit, but are conducting research now on the potential of the fourth major available tool for reducing congestion: smart growth.

Professors Gilles Duranton and Matthew Turner analyzed travel data from hundreds of metro areas in the US, resulting in what they call the most comprehensive dataset ever assembled on the traffic impacts of road construction. They write:

For interstate highways in metropolitan areas we find that VKT [vehicle kilometers traveled] increases one for one with interstate highways, confirming the “fundamental law of highway congestion” suggested by Anthony Downs (1962; 1992). We also uncover suggestive evidence that this law may extend beyond interstate highways to a broad class of major urban roads, a “fundamental law of road congestion”. These results suggest that increased provision of interstate highways and major urban roads is unlikely to relieve congestion of these roads.

Duranton and Turner say building more roads results in more driving for a number of reasons: People drive more when there are more roads to drive on, commercial driving and trucking increases with the number of roads, and, to a lesser extent, people migrate to areas with lots of roads. Given that new capacity just increases driving, they find that “a new lane kilometer of roadway diverts little traffic from other roads.”

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The implications for this research are significant, especially as Congress considers whether to integrate performance measures into federal transportation spending decisions. These findings make a strong case that Congress should not allocate too many scarce resources to road expansion when that’s not a real solution for congestion.

Duranton and Turner say that metropolitan areas tend to get new roads regardless of whether or not the prevailing level of traffic warrants expansion. They urge the establishment of transportation policies based on their findings and the data they compiled, rather than the “claims of advocacy groups”:

Unfortunately, there is currently little empirical basis for accepting or rejecting the claims by the American Road and Transportation Builders Association that “adding highway capacity is key to helping to reduce traffic congestion”, or of the American Public Transit Association that without new investment in public transit, highways will become so congested that they “will no longer work”. Our results do not support either of these claims.

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“The menu of policy responses to congestion is not really that long,” Turner said in our interview. “You’ve got building more roads, building more transit, and congestion pricing, and if you’d like you can put smart growth on there. We looked at two of those really carefully and found that they didn’t perform as advertised. So if you’re thinking about these things purely as responses to congestion, it doesn’t look like they work. There is some evidence that congestion taxes work. So if you were going to pick one of these things to go for, that would be it.”

They’re working on research now to investigate the impacts of smart growth on traffic.

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