CHARLOTTE, N.C. -- They don t make cars like they used to. And that s a problem, if you depend on gas taxes to fix old roads and build new ones.

The above chart from the University of Michigan shows the average gas mileage of new American cars bought over the last six-and-a-half years. In a relatively short time, fuel economy has made huge improvements, and federal regulations will only require cars to be more efficient in the future. Better gas mileage helps cut down on pollution, and can save drivers money. So how can that be construed as a bad thing?

It s bad if you use gasoline taxes to fund your highways department, like North Carolina does. According to a presentation made Wednesday to the state Department of Transportation in Raleigh, gas tax revenues are down because fuel-efficient cars are using less gas. Also, construction costs continue to go up, heavy trucks continue to tear up roads, and the state thinks federal funds that go toward highway construction and maintenance will start to dry up in the decades to come. Also, consider that only Texas maintains more miles of roadway than North Carolina does, and that the population here will only grow, and you can see the problem: lots of people using more roadways, but less money to go around.

So what do you do?

One idea, presented Wednesday to NCDOT, would tax drivers by distance at a half-cent per mile. The Mileage-Based User Fee, as it s called, could raise $495 million per year, according to a study prepared for NCDOT by consultants from North Carolina State University. The state says by treating roads like a utility, people will only pay for what they use and will look for ways to be more efficient.

But there are several problems. Because the plan doesn t reward drivers with hybrid or electric cars that use little or no gas, it would give people less of an incentive to buy them. It would also cost more to collect the taxes, and to accurately figure out how many miles you re driving, you d need a GPS tracker to be installed on your car or to have you mileage tallied up at your yearly inspection. All of this would be, according to the study, extremely unpopular. The NC State report calls it strong initial public resistance, and we re already seeing some of that on social media:

If it went into effect today, a driver who puts 15,000 miles a year on his or her car in North Carolina would have to pay an additional $75 a year in taxes. But if you take the latest figures from the chart above into account, a 15,000-mile-per-year driver in an average car would already be paying $221.46 a year in state gas taxes.

The proposal is just that, a proposal. NCDOT is considering several ways to raise more money, including raising the gasoline tax, adding more toll roads and charging drivers more to travel in congested areas, a move that would raise money while also reducing congestion. But the half-cent-per-mile tax plan would raise the most money and be the most stable long-term, according to the report.

So why not just raise the gas tax North Carolina already has? That could be a catch-22: At 37.5 cents per gallon, North Carolina already has one of the highest gasoline taxes in the nation. By driving up the overall cost of gas even more, people would simply drive less, which would ultimately lead to even less money coming in. As the bonus chart below shows, driver have steadily continued to drive more-and-more miles since 1950. And then the recession hit:

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