Michael Webber:

The Highway Trust Fund depends on federal fuel taxes for its finances. And those taxes have remained stuck for two decades at 18.4 and 24.4 cents per gallon for gasoline and diesel, respectively. State taxes tack on another 31.1 cents per gallon on average.
 Keep this fact in mind: There were about 260 million Americans in 1993 when the tax was last raised. Today there are over 315 million. And we travel more miles than we did two decades ago. That means the transportation infrastructure has to do more with less per-mile spending, adjusted for inflation. That’s why we see crumbling bridges on the news, outdated traffic-light patterns and clogged roads.
 And, as we move into cities and use mass transit we will drive less. As cars become more fuel efficient they require less gasoline. At the same time, alternatively fueled cars such as electric vehicles don’t pay gasoline taxes at all, and others, such as natural gas vehicles, pay a lower rate on average, so the current system subsidizes their use. That means our gasoline purchases — and our gas taxes — are declining, putting a strain on our trust fund.