I had the impression (from talking to traffic engineers) that it was pretty much common knowledge in the field that newly constructed highways only alleviate congestion for a very short time (a year or two), after which it returns to previous levels on both the old and new routes.

A quick glance at Wikipedia found this reference: http://link.springer.com/article/10.1007%2FBF00166218 which appears to be a decent summary.