But Martin Wachs, in a new report released this month by the Brookings Institution (see www.brook.edu/es/urban/publications/wachstransportation.htm), shows that the gas tax has been paying for less and less of the total cost of the national road system. Although Wach's report is at times overly narrow, it is valuable because he shows what many people do not know: that the gas tax pays for only about a third of the cost of the road system. This has major implications for mass transit funding, which is so often criticized for "not paying for itself."