Buy a hybrid, and you qualify for a federal tax credit of up to $3,400. Your employer may give another rebate of as much as $10,000. You get to drive free in HOV (high-occupancy-vehicle) lanes, and you don't have to feed parking meters.

We're headed for a big problem: These entitlements accelerate the growth of the market for hybrid cars. That's no big deal when hybrids account for 1 percent of the car market, 206,000 out of 17 million vehicles sold in 2005. But the numbers are doubling every year, according to HybridCars.com.

What happens when hybrids are 10 percent of the market, and then 25 percent? The majority of cars may have some hybrid functionality in the next decade. Can all those cars fit in HOV lanes? Can city governments stand to lose the parking revenue?

When you get used to a perk, you start to think of it as your just due. Think of the auto workers who figured they were entitled to lifetime paid medical insurance even as U.S. automakers, crippled by high labor costs, struggled for survival. (The workers had a point, at least contractually, since nobody put a gun to the heads of the automakers who signed those sweetheart union deals a generation ago.)

My biggest concern is the free ticket into the HOV lanes in states such as California and Virginia. These lanes serve two purposes: easing highway congestion and saving fuel. A single-occupant hybrid doesn't do much for congestion, and it saves fuel only some of the time. Hybrids are less economical on the highway than in city driving, so two single-occupant, 40-mpg hybrids burn more fuel than a 30-mpg car with two people aboard.

Ralph Davis, Virginia's deputy secretary of transportation, told the Washington Post that in Virginia, hybrids in HOV lanes "are contributing to the eroding performance on I-95 and I-395." As a result of a backlash from owners of traditional cars, hybrids sold after July 1 won't be able to use HOV lanes on some of the state's most congested roadways. Older hybrids will be grandfathered in; new ones will have different-color license plates so the cops can spot HOV cheats. Similarly, California has a cap of 75,000 hybrid permits, of which 57,000 have been issued, with another 8,000 applications pending as of June 2006.

With hybrids, I suspect we've reached the point where buyers don't need much urging. Gasoline at $3 a gallon is a pretty good incentive.

Also, the government put a cap on the number of hybrid-incentive cars an automaker can sell. This will affect Toyota, which will likely sell 100,000 Priuses, while Ford and its sibling Lincoln-Mercury will sell about 20,000 hybrids altogether, and remain eligible to give incentives all year long. In theory, that gives Ford an incentive to work hard developing hybrid technology, but Ford actually is sourcing some of its hybrid technology through Toyota and Honda.

If the public wants perks for hybrid owners, so be it. I suspect most non-hybrid car owners don't mind some incentives. But they don't want the government giving away the candy store, either.

One thing in favor of hybrids clogging HOV lanes is that their mileage might actually improve. Hybrids that average 20 mph in congested, stop-and-go traffic get better mileage than hybrids traveling a steady 60 mph, as on the highway. But I'm not sure that's going to play well with motorists who are sandwiched between a hybrid Prius and a hybrid Ford Escape.