Is your car spying on you?

It was only a matter of time. For several years, electronic devices in cars have monitored acceleration and braking to save fuel and improve safety. Now, they're saving some of that data to give automakers and police a better idea of how you drive.

So far most of the devices record the last five seconds of readings before a crash, for example, a little like flight-data recorders in airplanes. The information has proven extremely useful to auto designers and accident investigators. It's also being used to prosecute drivers.

"The problem is most people don't realize these devices are in their vehicle," says Eric Skrum, spokesman for the National Motorists Association in Madison, Wis. "That information can be used against you, and there's no sort of regulation about who owns that information."

Already, drivers have had data from their own cars used to convict them. Last month, Danny Hopkins of New York was sentenced to 5 to 15 years in prison for killing Lindsay Kyle after the black box in his Cadillac CTS indicated the car was going 106 miles per hour five seconds before the crash. Investigators originally thought the car was going only 65 to 70 miles per hour. In St. Louis, Clifton McIntire of Phippsburg, Me., pleaded guilty to manslaughter last month after the black box in his GMC pickup revealed that he was going 85 miles an hour before he slammed into the back of a Toyota.

Today an estimated 30 million cars contain these "black boxes" - they're actually silver - known as event data recorders (EDRs). Most record simple data such as whether airbags deployed or if passengers wore seatbelts. But most cars from General Motors and Ford, as well as some Toyotas and Hondas, track even more information, including vehicle and engine speed, and whether the driver was accelerating or braking.

Automakers say they want this information to help improve safety equipment. "The main purpose of the EDR is to get data after a crash to help us understand how the airbags worked," says Alan Adler, manager of product-safety communications at General Motors in Warren, Mich. "The privacy of our customers is very important to us, but [the device] doesn't record anything that isn't true."

Without EDRs, investigators frequently don't have enough data to pinpoint the cause of an accident, says Joe Osterman, director of the Office of Highway Safety at the National Transportation Safety Board (NTSB) in Washington. That was the case when an elderly man killed 20 people when his Buick plowed into a farmers market in Santa Monica, Calif. in 2000. The driver said he was braking. Witnesses and investigators said he was accelerating.

While what exactly happened in the moments before the tragedy remains a mystery, the NTSB went on record afterward saying EDRs should be mandatory in all cars sold in the United States.

The NTSB, however, doesn't have the authority to mandate black boxes. The National Highway Traffic Safety Administration does. It proposes that the recorders become standard equipment starting in 2009 models, retain the last eight seconds of data before a crash, and include added data from electronic stability control and antilock braking systems.

Civil libertarians worry that such data will be used more broadly in the future.

"This is another example of where technology has outstripped the law and certain assumptions of how the world works," says Jay Stanley, director of communications for the Technology and Liberty Project at the American Civil Liberties Union in New York.

Some safety experts also worry about the wrong people using the data. While Mr. Osterman of the NTSB favors police investigators using black-box data in criminal investigations, he worries that private experts hired in civil litigation may have biases and could take the data at face value instead of cross-checking it.

"The data can be misleading if you're not a seasoned accident reconstructionist," adds Bob Kreeb, an engineer at Booz Allen Hamilton in Washington who chaired a committee of the Society of Automotive Engineers to set standards for the data gathered from black boxes. "So it needs to be interpreted and validated."

Installing black boxes with five seconds worth of memory was as simple as adding a memory chip to existing computer systems in cars. Increasing the memory to several months' worth of data would not be difficult at all, Mr. Stanley says. "If GM decided tomorrow to track three months of data instead of five seconds, there's nothing that would make them have to tell anybody," he adds.

In fact, Davis Instruments of Hayward, Calif., sells a black box called CarChip that will record throttle position and engine parameters for up to 300 hours of driving. Parents can use it to monitor their teenagers' driving habits, for example.

Progressive, an auto-insurance company, is running a pilot program with 5,000 drivers in Minnesota using a device similar to CarChip. It records up to six months of driving data, including vehicle mileage, time of day, and speed. The program, called TripSense, lets drivers choose whether to hand over data from their recorders to the insurer. Based on their habits behind the wheel, they can get discounts on their premiums of 5 to 25 percent.

But once any data is collected, some worry that it might be subpoenaed. If a police officer pulls you over while you're not speeding, "will your EDR tell him that five miles or five days earlier you were?" asked AutoWeek magazine's Bob Gritzinger in a November article.

Recorder data may also present problems for drivers with automobile warranties. Some wonder if vehicle manufacturers are using safety data to void warranties. Some people in Internet chat rooms have alleged Mitsubishi is doing just that to those who drive its racy Evolution VIII in amateur weekend races.

Even if not true, the existence of such stories shows people's concerns about this kind of technology, says Stanley. "If it's not controlled, it allows powerful institutions to increase their control over ordinary individuals," he says.

For example: When AutoWeek conducted handling tests on a mundane Chevy Malibu Maxx hatchback earlier this year, the recorder automatically alerted GM OnStar officials, who called the car to make sure the driver was OK after a particularly severe cornering maneuver. The driver was, but later said he resented the intrusion.