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More Roads, More Traffic

Building roads in urban areas doesn't alleviate traffic, a study finds, because every mile built leads to a corresponding increase in vehicle-miles driven. Building roads in urban areas doesn't alleviate traffic.

Researchers used Federal Highway Administration data from 1983, 1993 and 2003 for 228 metropolitan areas, a national survey of transportation habits, and other sources. The finding held for interstate and major urban roads.

Traffic density on the typical interstate roughly doubled from 1983 to 2003, despite new miles having been added. All else being equal, the study found, a 10% rise in interstate mile-lanes built in 2000, in a given city, led to a 10% increase in annual vehicle-miles, by the end of the decade. New commercial traffic and additional trips by current residents made up the bulk of that increase, followed by driving by new residents attracted by the development. The amount of public transportation didn't affect driving levels.

"The Fundamental Law of Road Congestion: Evidence from U.S. Cities," Gilles Duranton and Matthew A. Turner, American Economic Review (forthcoming)