It’s no secret that America’s highways and bridges are a sham and the overall transportation system is in desperate need of funding.

What is troubling is Congress’ latest solution.

We pay for our highways and bridges through a tax on gasoline, which puts the cost of the transportation system squarely on users. Currently, the federal tax on a gallon of gas is 18.4 cents. That tax hasn’t changed since 1993, when gas was $1.16 per gallon, making the tax rate 19%.

But it’s not levied as a percentage, which is why the tax hasn’t changed in more than two decades.

That’s a problem.

Heavier vehicles tend to use more fuel, and therefore pay more tax. Smaller cars, which tend to be light, pay less.

Yes, there is the added twist of hybrid and electric vehicles, which rack up lots of miles while paying almost no tax since they buy little gas. But, as a percentage of all cars, there aren’t many of those on the road.

The problem is that vehicles in general get better mileage today than they did in the 1990s.

In 1993, we used 137 million gallons of gas to drive 2.3 million miles. Vehicles averaged 20.6 miles per gallon. In 2013, we used 169 million gallons to drive 3.0 million miles, and got 23.4 miles per gallon.

We traveled 30% farther, but only used 23% more gas, thereby shortchanging the highway funding mechanism.

To make matters worse, the 18.4 cents paid on every gallon doesn’t go as far as it used to (pun intended). To maintain its buying power, the tax would have to be 30 cents today. So we are paying less tax per mile driven, and the revenue doesn’t go as far.

It’s not surprising then that the transportation system is woefully underfunded. It’s gotten to the point where Congress had to top off the fund on several occasions over the last six years because it couldn’t pay its bills.

Possible answers to the funding woes are obvious. Congress could raise the tax. That would be a start, but would leave the funding vulnerable to the same problem in the future.