By my estimation, most people in this country think that gas taxes and other vehicle-related user fees cover the cost of building and maintaining our nation’s roadways.

Those people are wrong, unfortunately, to the great detriment of our national transportation policy. The belief that roads are self-supporting is a big part of the reason that we continue to build new capacity even as our roadways deteriorate, and fail to invest in more cost-effective and efficient forms of transportation such as: literally everything else.

The Center for American Progress has a new report out that takes a close look at exactly how wrong this self-sufficiency argument is, and what share of our nation’s major roadways are bringing in less money than they cost to maintain. What they find is that, for the most heavily traveled roads (interstates and principal arterials), at least 48 percent don’t even bring in enough revenue to cover basic maintenance. In urban areas with populations over one million, that number jumps to 64 percent. And that’s just maintenance—if you’re asking how many get enough drivers to pay off initial construction and maintenance, you can probably expect much larger numbers.

The “Loss” row shows the number of miles in each geographic area where interstates and principal arterials are bringing in less revenue than it takes to maintain them. The number that are able to pay for maintenance and up-front construction costs are probably even lower. (Screenshot from CAP report.)

There are several reasons this is probably a drastic under-estimate of the number of roads that can’t cover basic repair costs. For one, this report assumes a cost inflation rate of just one-percent per year for the next 30 years, which is low even by the most conservative estimates. If the real inflation rate of construction work is closer to 3 or 4 percent, which is not an unrealistic expectation, the share of underfunded roads probably increases to around 60 or 70 percent.

The report also only looks at a small number of the total roads in the U.S. It makes the point that because interstates and principal arterial roads tend to get a disproportionate share of total vehicle miles driven (and therefore generate a disproportionate amount of gas tax revenue), local and other lower-trafficked roads are probably in even worse financial shape. A lot of roadway degradation is due to factors other than driving, like weather, so these roads still require ongoing maintenance, even if traffic is relatively light. With less users, and therefore revenue, then the already-underwater major roadways, governments are likely filling gaps with other funds for the large majority of local roads.

Total vehicle-miles traveled in the U.S. is flat, and per-capita VMT is on the decline, so this is unlikely to change any time soon, particularly if we can’t increase the gas tax at the local and federal levels. For states that begin taking into account the externalities of driving, such as carbon emissions and other forms of pollution, health and safety impacts, and opportunity costs relative to more productive uses of land, the calculus will undoubtedly come out even less favorable to roads.

[This article was originally published by Better Institutions]