In most states, some people convicted of drunken driving can start their cars only after blowing into a device attached to the vehicle that detects alcohol, shutting the car down if it does. These alcohol interlocks are a bit clunky and very intrusive.

But with improvements to the technology, where a simple touch of the steering wheel might measure a driver's blood alcohol concentration level, advocates say every car, not just ones owned by those convicted of drunken driving, could someday be equipped with an interlock.

The advocates said that if the technological and privacy hurdles were overcome — which could take many years, if not decades — the interlocks could save thousands of lives a year.

"It's better to prevent somebody from breaking the law, and maybe killing or injuring someone, than to arrest them after the fact and try to prevent them from doing that again," said Anne McCartt, senior vice president for research at the Insurance Institute for Highway Safety. The institute estimates that 9,000 lives could be saved if drivers with a blood alcohol level of 0.08 or more were prevented from driving.

The National Highway Traffic Safety Administration said that 40 percent of traffic deaths were alcohol-related. More than 17,500 people died in 2006 in accidents involving alcohol, 13,470 of them in crashes where at least one driver or motorcyclist was beyond the legal blood alcohol concentration limit of 0.08 gram a deciliter, according to the agency's data.

The toll has led Mothers Against Drunk Driving to push for increased use of interlock technology, urging courts to use interlocks for first-time drunken driving offenders, not just for repeat offenders. New Mexico has been a leader in the interlock effort, recently mandating that first-time offenders receive interlocks. Louisiana and Illinois also adopted harsher interlock laws, said Kathryn Heineman, the St. Louis affiliate director of MADD.

Barry Steinhardt of the American Civil Liberties Union does not object to interlocks as an alternative to incarceration for convicted drunken drivers. But he said that putting them in all cars might be a problem depending on how the requirement is applied and how accurate the system is.

"The question is going to be, Who's going to be barred from driving and why?" he said.

Sarah Longwell, managing director of the American Beverage Institute, a Washington trade group representing restaurant and bar owners, said that it was legal for people to drink moderately and still drive home, but added that right could disappear if interlocks that turn out to be over-sensitive were required for all cars.

"This is about taking incremental steps toward mandating this equipment for all cars," she said. "This will eliminate people's right to drink moderately and responsibly before driving."

The interlocks, made by seven major companies in the United States, consist of a breath analyzer wired to the car and a data-gathering system. People ordered to use an interlock typically pay about $125 to have it installed and a monthly fee of $60 to $75 for maintenance and use of the device.

To start a car, the driver must puff a breath into the unit. To avoid cheating, the breath puff is measured and must be given in a uniquely identifiable way that would be hard for a person who is not the driver to duplicate. Inside the unit, a small fuel cell converts any alcohol into electrical energy, which is measured and recorded. If no alcohol is detected, the driver can start the car. If alcohol is detected, the system turns off the power to the ignition.

The breath puff isn't just for starting cars. While driving, the driver must periodically blow into the system to keep the car running. Typically, the data is downloaded every 30 days and is available to probation officers and court officials.

Automakers and researchers have long sought a simpler interlock technology, and Saab last year equipped 100 test cars with a key fob breath analyzer it called an Alco-key. Volvo has demonstrated a similar technology in a seat belt buckle that tests for alcohol and then must be buckled to start the car.

Nissan demonstrated a system in Japan in which sensors in the headrest sample the driver's breath. The Japanese media reported in January that Toyota was working on a steering wheel that uses skin-sensing technology to monitor for alcohol. A Toyota spokesman in the United States said the company was working on Breathalyzer-type technology, but would not elaborate.

Robert Lange, executive director for vehicle safety at General Motors, said skin sensing offers the most promise because alcohol is a complex organic molecule, while human skin is transparent to light. That makes alcohol easy to detect in the bloodstream. But Lange said that even the most promising technologies for interlocks still aren't ready for widespread use. Most would impose unacceptably intrusive and time-consuming tasks on otherwise sober drivers, and are too delicate for the 10-year life cycle and harsh environment of cars, he said.

"To date, the technologies available to detect alcohol in the blood of prospective drivers are too burdensome for imposition upon a sober driving population," Lange said.

GM experimented in the 1970s and '80s with interlocks that used simple tasks to identify impaired drivers, like repeating a briefly displayed set of numbers or keeping a pointer centered on a shifting target. Lange said such systems worked, but also failed about 20 percent of the sober drivers in the tests.

Nicole Nason, the NHTSA administrator, said her agency was financing research on interlock systems but recognized that it might take a long time before an acceptable interlock was available for all drivers.

"To be frank, we are years and years away from this technology," she said in a telephone interview. "We are still in the beginning research phase."

She said the agency recognized the opportunity to reduce alcohol-related driving deaths, which have stayed at about the same annual level over the last decade. But she added that the agency won't recommend widespread use of interlocks built into new vehicles until the technology is reliable and unlikely to generate false alarms.

"It would be extremely frustrating and the public would be infuriated," she said.

Paul Marques, senior research scientist at the Pacific Institute for Research and Evaluation, a health and safety research organization in Calverton, Md., said the interlocks were highly effective but were too rarely ordered by judges and prosecutors.

"Once you put interlocks on all first offenders, and all repeat offenders," Marques said, "then you can begin to bring down the casualty levels."