The New York Times > Business > Economic Scene: Does Highway Spending Really Pay Off?: "On average, they write, 'one dollar of annual highway spending reduces the annual congestion costs to road users only 8 cents.' This is not a return on a one-time investment but a continuing expense; we have to keep spending that dollar to get the 8 cents.
Dr. Winston cautions that the specific number may change as the working paper is revised, but that the general point - a very small effect - seems certain."