Short title: The Fundamental Law of Road Congestion The Fundamental Law of Road Congestion: Evidence from US cities

We investigate the effect of lane kilometers of roads on vehicle-kilometers traveled (VKT) in US cities. VKT increases proportionately to roadway lane kilometers for interstate highways and probably slightly less rapidly for other types of roads. The sources for this extra VKT are increases in driving by current residents, increases in commercial traffic, and migration. Increasing lane kilometers for one type of road diverts little traffic from other types of road. We find no evidence that the provision of public transportation affects VKT. We conclude that increased provision of roads or public transit is unlikely to relieve congestion. (JEL R41, L91) We investigate the effect of lane kilometers of roads on vehicle-kilometers traveled (VKT) for different types of roads in the United States (US). For interstate highways in metropolitan areas we find that VKT 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. Our investigation is of interest for three reasons. First, an average American household spent 16