For an interesting read about the correlation (or lack thereof) between traffic congestion and economic growth, check out this post by Michael Lewyn on Planetizen: http://www.planetizen.com/node/43543
"Both auto and transit lobbies occasionally suggest (as did Graves) that without more transportation funding, economic life as we know it will end.
If this were true, the most congested regions would be the most economically stagnant ones, and the least congested regions would be booming. But this is hardly the case. According to the Texas Transportation Institute, the two large regions with the lowest per-capita travel delay are Buffalo and Cleveland (2)- hardly economic powerhouses."