NEW YORK (CNNMoney.com) -- Everyone's familiar with the idea of "black boxes" in commercial airliners. They keep a record of everything the aircraft does so that, in the event of an accident, investigators can reconstruct what happened in the minutes leading up to the crash.

Ironically, most people are unfamiliar with the "black boxes" in their own cars.

In 2005, it was estimated that about 64 percent of passenger cars on the market came equipped from the factory with "Event Data Recorders" that kept a computer record of various things a driver was doing in the moments just before and after a serious impact. The number today is certainly much higher and the devices are becoming more advanced.

A recent rule passed by NHTSA requires manufacturers to tell people, in the owners manual, if their car has one of these data-recording devices. As it turns out, that formation has been in most owners' manuals for years. It's just that few people read their owners manuals.

The EDRs don't record voices and they only record a few seconds of data about what the car was doing right before and after the crash. Proponents point to the life-saving potential of that data which can be used to assess the performance of a car's safety systems and even to research unusual crash scenarios.

Despite the positive potential, many people still have concerns about their privacy. After all, the car they drive every day could turn snitch.

What cars have Event Data Recorders?

If you've purchased a new car in the past two or three years, chances are excellent that it has one. All General Motors passenger vehicles have them, for example. Ford, Toyota and other manufacturers have been including these devices in their cars for years.

How do I know if my car has an Event Data Recorder?

Check your owner's manual. Look in the index for "Event Data Recorder." Several states already require car manufacturers to disclose the use of EDRs in the manual. Toyota, Ford and GM all disclose the presence of EDR systems in vehicle owners' manuals and detail exactly what data is recorded and stored by them.

What type of data does the EDR collect?

It varies a lot. At most, the EDR will provide data about various driver inputs - steering wheel movements, gas and brake pedal use - and the car's own movements - acceleration, deceleration, skidding, swerving, etc - in the few moments before a crash and for a few moments after.

Essentially, the EDR can provide, for an investigator trained to understand the data, a "snapshot" of what a car and its driver were doing in a crash.

The EDR does not provide any indication of where a car has traveled or its location at the time of an accident.

Could my car really get me in trouble?

While the crash scene and witnesses could probably provide enough evidence, without the EDR, to say who was legally at fault, data from the EDR could provide supplemental information.

For example, the EDR might indicate that you weren't just speeding a little, you were really speeding. Or, while you might have failed to yield while making a left turn, the other driver's EDR might show that he made no attempt to avoid hitting you.

How can I remove or disable the EDR?

You can't. The data produced come from various vehicle sensors, such as the anti-lock brakes and electronic stability control. The data recording function is so thoroughly integrated into a car's electronics that there is no way to completely disable it without also disabling safety features in a way that would violate federal law.

Besides law enforcement, what good is this data?

The original purpose of EDRs was for car manufacturers to monitor the performance of various safety systems, such as airbag and seatbelt pretensioners, in accidents. This was done by monitoring their performance in the manufacturers' own fleet of vehicles when those vehicles are involved in wrecks

"It has helped us discover things that have led to recalls," said GM spokesman Alan Adler.

The systems are also in vehicles that are sold to the public, however. Aside from its potential use in law enforcement, that data could also be used to improve vehicle safety systems and even improve road designs, proponents say. For example, the data might show that airbags aren't deploying when they should or that a certain type of road marking is causing a confused response in drivers.

The National Highway Traffic Safety Administration recently passed a regulation requiring that the data from these systems be standardized in terms of what is measured and how. That will make it easier for researchers to compare data from vehicles - data that will be downloaded only with a vehicle owner's permission, NHTSA says - so that researchers can make the best use of it.

Who can gain access to this data?

From a technical standpoint, law enforcement and professional investigators can download data from Ford and General Motors vehicles, said Hasseeb Ghumman, an accident reconstruction engineer with Accident Reconstruction Plus, a company based in New York and Georgia.

Systems used by other manufacturers, such as Toyota, can only be accessed by the manufacturer, he said.

From the legal standpoint, several states have laws saying that a car's owner or lessee (the "owner" of a leased car) owns the data contained in the car's EDR.

Ford, GM and Toyota say that they have always treated the data as the car owner's property and that they would never download data from an individual's car without their permission or a court order requiring them to do so.

But the carmakers' policy on the issue wouldn't stop police who may have their own devices to extract the data. That's why those state laws are still important.

Is it infallible?

Sensors and computers don't necessarily tell the whole story. Accident investigators will still look at old-fashioned skid marks and dents to determine what really happened in a wreck, said Ghumman.

Even if the sensors collecting data about the actions of the car and its driver perform perfectly, errors can still be made in the collection and interpretation of that data.

Environmental factors could cause the sensors to provide misleading information. For example, if the car is skidding on a slick surface or if the wheels leave the ground altogether, the free-spinning wheels could provide misleading data about the car's speed. Again, said Ghumman, an accident reconstructionist would have to look at all the available data, survey the scene of the crash and gather all the evidence.

New rule: Car buyers must be told about 'black boxes'

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