A person convicted more than once of driving under the influence of alcohol usually faces the suspension or revocation of his or her driver's license. But many people who lose permission to drive still get behind the wheel, so a growing number of states have taken a more forceful response: instead of telling the driver to hang up the keys, they take away the car.

Law enforcement officials say most of the people whose licenses are suspended or revoked keep on driving. No one knows how many, and the problem is referred to as an "invisible violation," since an officer has no way to tell that an unlicensed driver is on the road, short of pulling the car over and running a check.Unlicensed drivers constitute a high percentage of a large number of people. For example, a survey by the National Sheriffs' Association found recently that in Salt Lake County, Utah, with a population of 850,000, at any given time an average of 50,000 drivers were suspended.

The problem is particularly frustrating for police.

"It wouldn't surprise me a bit if two-thirds of them still drive," said A.N. Moser Jr., executive director of the association, referring to people nationwide whose licenses were suspended or revoked.

Asked if confiscating cars was too extreme, Moser said, "If that's their vehicle they're driving, I'd seize that and sell it so fast it would make their heads swim." Repeat drunken drivers impose a high cost on society in deaths, injuries and property damage, he said, and the alternative - locking up the driver - is costly, too.

Twenty-one states permit confiscation of vehicles, according to the National Highway Traffic Safety Administration, some for 30 days or longer, and some permanently. A recent study in California found that taking the car away does not solve the problem, but it can help.

The California Department of Motor Vehicles followed 6,300 drivers whose licenses had been suspended or revoked. Among first offenders, they found that of those whose cars were not seized by the courts, within a year about 25 percent were convicted for driving without a valid license. Among those whose cars were confiscated, about 20 percent were convicted for driving without a license.