We’ll spare you the calculus in the report. Here’s the upshot: “Roads cause traffic.”
Duranton and Turner: If you build it, you will sit in traffic on it. Photo: Arch and the Environment
Professors Gilles Duranton and Matthew Turner analyzed travel data from hundreds of metro areas in the U.S., 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.
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.