Factors Contributing to the Growth in Driving

Population increases are invariably cited as a root cause of traffic congestion. Yet according to the U.S. Department of Transportation, increasing population is only 13% responsible for an increase in driving (measured in total vehicle miles traveled, or VMT). As we attempt to build our way out of traffic congestion, we build more and more freeways, which promote sprawl, allowing people to live farther and farther from where they work. The result: increased trip lengths, and increased total VMT. Another factor is increases in trips taken. New roads attract new traffic via the phenomenon of "induced demand." Decreases in vehicle occupancy and switching to driving also contribute to increased VMT.

A recent study by the Surface Transporation Policy Project found that road building does not correlate well with reduced congestion. In fact, cities in which road building kept up with population growth ("high road building" cities) had essentially the same level of traffic congestion as cities in which road building significantly lagged population growth ("low road building" cities).