No one disputes that Michelle Zimmermann (search) lost control of her 2002 GMC Yukon as she drove on a two-lane highway in Massachusetts one snowy afternoon last January. Her friend died after the SUV slammed into a tree.

Zimmermann claims she was driving within the posted 40 mph speed limit, but like millions of other Americans the 33-year-old didn't know that her vehicle had a "black box." Monitoring her driving, it recorded the last few seconds before the crash.

Bolstered by data that they say indicates Zimmermann was driving well above the speed limit, prosecutors have charged the Beverly, Mass. woman with negligent vehicular homicide. She has pleaded innocent and faces up to 2 1/2 years in jail if convicted.

An estimated 25 million automobiles in the United States now have so-called event data recorders (search), a scaled-down version of the devices that monitor cockpit activity in airplanes. Like aviation recorders, automobile black boxes mainly receive attention after an accident.

What the devices record increasingly finds its way into courtrooms as evidence in criminal and civil cases, leading some privacy advocates (search) to question how the recorders came to be installed so widely with so little public notice or debate.

"It's like having a government agent driving around in the back seat of your car," said Bob Weiner, Zimmermann's defense attorney and a former prosecutor. "I think it's a tremendous invasion of privacy."

Most people apparently don't even know whether the vehicles they drive are equipped with event data recorders. Nearly two-thirds of people surveyed by an insurance industry group knew nothing about them.

"The real issue is one of notice, and the problem arises from the fact that information is being collected about people's driving behavior without them knowing," said David Sobel, general counsel of the Electronic Privacy Information Center (search). "If drivers knew about the device, they could at least then begin asking questions."

Automakers and regulators have ignored basic privacy questions, leaving individual courts to decide such issues as who owns the information and whether a warrant is required to access it, he said. Some studies have questioned the data's reliability and accuracy.

Prosecutors, police and accident reconstructionists say the boxes yield information no different from what can be gleaned from crushed metal, skid marks and other evidence at the scene. Now, they say, calculations can be backed up.

"It's appearing in prosecutors' cases in support of the normal reconstruction," said W.R. "Rusty" Haight, director of the Collision Safety Institute.

A number of recent court cases across the country have involved event data recorders.

In early June, Edwin Matos of Pembroke Pines, Fla., was sentenced to 30 years in prison for slamming his car into a vehicle driven by two teenage girls, killing both. Data from the recorder showed he was driving more than 100 mph just seconds before the crash.

In April, Arlington Heights, Ill., police officer Charles Tiedje received a $10 million settlement after data from the hearse that struck his squad car contradicted claims that the driver blacked out. The device showed the supposedly unconscious driver accelerated and braked in the moments before the October 2000 crash.

The devices' primary function is to monitor various sensors and decide whether to fire air bags. But secondary and more recently installed features in many recorders store data from a few seconds before a crash.

Though capabilities vary widely among carmakers, most recorders store only limited information on speed, seat belt use, physical forces, brakes and other factors. Voices are not recorded.

General Motors Corp. has been using recording-capable devices, called Sensing and Diagnostic Modules (search), since the 1990s to help improve safety and gather statistics. GM spokesman Jim Schell said consumer privacy has always been a top concern.

"We collect the data with the permission of the owner or the person who is leasing the vehicle," he said. "When that data is collected, we take great care to assure confidentiality."

The modules helped GM figure out why some air bags were deploying inadvertently, leading to a recall in 1998 of more than 850,000 Cavaliers and Sunfires.

But there's a lot more interest in the data beyond engineering — namely, from lawyers.

GM and, more recently, Ford Motor Co. now allow outsiders to access the data by buying a $2,500 reader built by Santa Barbara, Calif.-based Vetronix Corp. The company says its primary customers are accident reconstructionists, law enforcement and insurance companies.

So far, about 1,000 of the devices have been sold, primarily in the United States and Canada. The company hopes to reach deals to cover data from other car makers.

The National Highway Traffic Safety Administration (search) has been studying data recorders for years, trying to determine whether the auto industry should standardize the equipment. Any decision could be years away, and there's no guarantee privacy would be addressed then. Agency spokesman Tim Hurd said state courts should decide what's admissible.

Haight, a former San Diego police officer, dismisses the privacy concerns because driving — and crashes — are public.

But Sobel argues that drivers at the very least have a right to know that their actions might be recorded. He also fears that data recorders will converge with other devices — such as locators and voice recorders — now finding their way into cars.

"It's hard to say that there is general public acceptance of this when the public has no idea about it," he said.