State Rep. Joe Rice, D-Littleton, has got the right idea when he says "we've got to figure out something besides the gas tax" to pay for roads. Hybrid vehicles and, later, a growing fleet of fully electric autos are going to destroy the link between the miles you drive and the amount of fuel tax you pay.

Highways won't merely be underfunded. They'll hardly be funded at all.

So Rice and other Democratic lawmakers are proposing a pilot project to examine whether the gas tax in Colorado could be replaced by a fee pegged to the amount of miles you drive.

But how, you may wonder, would officials know how many miles you travel? They'd be told by an onboard GPS device that would be installed in your car.

Now don't panic. This idea is not a clunker for the reasons you're probably thinking (although I'll get to a reason that should give you pause). The government would not be able to track your whereabouts. Your privacy would remain intact.

"Like all other GPS receivers, these onboard units receive GPS signals," Bob Poole of the Reason Foundation explains in his newsletter Surface Transportation Innovations. "They do not send location data back to the satellites. There is no real-time 'tracking.' "

Poole and others intrigued with billing for miles traveled appreciate the fact that it would function like a user fee. People who drive a lot ought to pay more to maintain and improve the system than those who drive less - and motorists generally accept this principle of "user pays."

If there is a threat from the GPS system, it's has to do with the ease with which it could be adjusted to discourage or reward certain lifestyle choices that are reflected in the use of a car.

When you pay the fuel tax, the pump that takes your credit card doesn't know or care what kind of car you're driving, when you do most of your driving or how many miles you've racked up that month. It treats everyone the same. But with an electronic gadget in your car that notes when and where you drive, bureaucrats will enjoy much more power. They'll be able to adjust driving fees according to the time of day or type of car. They'll be able to create zones in which the tax per mile is higher than it is in others.

As Jim Witty, an Oregon official with that state's Department of Transportation, told The Denver Post, "Because this involves computers, you could do any kind of formulas you want." And Witty seemed to like the idea.

Charging for road use based upon the level of congestion is certainly defensible. Unfortunately, many groups that weigh in on transportation policy these days - particularly those with an environmental or so-called "smart-growth" agenda - no longer wish to facilitate the use of the private vehicle. They want to impede it. And they will seize upon this new method of financing roads as a means to that end.

So by all means, let's experiment with mileage pricing. But let's also be wary.

Vincent Carroll is editor of the editorial pages. Reach him at carrollv@RockyMountainNews.com.