The federal highway program. Highways are heavily, heavily subsidized by state and federal dollars. That's why their true cost isn't born on drivers, and it's something that greatly exacerbates the problem of urban planning the U.S that leads to vast sprawl, urban decay, hell for walkers/bikers and environmental degredation. If all highways were toll roads, most would cease to exist and our cities would become denser, doubly so if all those highway dollars were diverted to public transit.