The problem: Americans now finance their roads by paying federal and state taxes on each gallon of fuel. (The federal tax is 18.4 cents per gallon; the state tax is 24.5 cents.) But as people switch to high-mileage cars, the revenue per highway mile traveled is declining. Pretty soon, the federal and state governments won't have enough money to maintain highways and build new ones.

The solution: charge people a tax per mile traveled rather than per gallon of gasoline. That way, transportation revenues would not be as dependent on fuel consumption.

In theory, we like the idea. It would tax people based on their use of the roads, which sounds equitable. Unfortunately, it would remove some of the financial incentive for people to drive fuel-efficient vehicles.

The people pushing this idea, including a lawmaker from Texas who met with his fellow wizards in the Utah Legislature the other day, say that the tax could be collected by installing electronic transmitters in each vehicle that would allow the owner to be billed for each mile driven.

But that's where this plan starts to run into unanswered questions. For example, how much would it cost to administer such a system? One of the beauties of the fuel tax is its simplicity. It's easy to administer and hard to evade.

Then there's that other wrinkle on the per-mile tax, called congestion pricing. The idea is that the state would charge people more per mile to drive during rush hour. The goal would be to encourage people to drive during off-peak hours. That would increase the efficiency of the highway system and reduce the need to build more roads.

In theory, we like this idea, too, but it hasn't been tested very widely.

The Utah Foundation reports that Oregon ran a pilot program in 2006 that replaced the state's 24-cent-per-gallon gasoline tax with a travel tax of 1.2 cents per mile. GPS devices were installed on participants' cars, and the tax was calculated and paid when drivers filled up at participating gas stations. The system also tested congestion pricing. In both cases, drivers reduced the miles they drove.

But, the foundation reported that Oregon is unlikely to implement such a system permanently because of high costs to refine the technology, privacy concerns and resistance from auto manufacturers and service stations.

Of course, states could just raise the per-gallon gas tax to compensate for greater fuel efficiency. Sometimes the simplest solution is the best.