When two cars collide on an isolated road, there may be more witnesses present than just the drivers and passengers. Silent – and, in most cases, unknown – witnesses can lie within the cars themselves.

The same electronic sensor that triggers the deployment of a car's airbags can record and store information on a car's speed, whether the seatbelts are fastened, and whether the driver hit the brakes before a collision.

The device is known as an event data recorder (EDR), or simply a "black box," because it serves a similar function to a flight data recorder in an aircraft.

The information the black boxes record includes:

the car's speed;
the engine's speed;
whether the brakes are applied;
the position of the gas pedal.

It also records other information, such as whether the driver was wearing a seatbelt and the force of the collision.

Because the memory of the black box is limited, it only retains this information for a few seconds. After a collision, the black box contains a record of what was happening in the last seconds before the impact.

Black boxes in the courts

The EDR was originally intended as a diagnostic tool to determine what caused a car's air bag to activate, but now insurance agents and police can use a car's black box to reconstruct what happened before an accident.

Black boxes are installed in millions of cars across North America – most cars that have air bags – but most drivers don't even know they're there.

And there are no rules governing how information taken from black boxes can be used in Canada.

Ontario Provincial Police have used data from black boxes in court cases even without formal rules.

California is the only jurisdiction in North American with any rules on the use of black box information. In that state, the information can only be used with a car owner's consent or with a court order.

A court case in Quebec may influence how black box evidence is treated in Canada.

In April 2001, Eric Gauthier was driving his new Pontiac Sunfire in east end Montreal. His car collided with another and the driver of the other car, Yacine Zinet, died at the scene.

Gauthier told police that Zinet's car was speeding and ran a red light. No one else saw the accident.

However, the black box in Gauthier's car said it was Gauthier who was speeding, travelling at between 130 and 160 km/h when the speed limit was 50.

Gauthier was convicted of dangerous driving, but was cleared of the more serious charge of criminal negligence causing death.

He will be sentenced in March.

Privacy concerns

Defence lawyer and columnist Jordan Charness questions whether courts should be allowed to use black box information, especially when most people don't know the devices are there.

"You're taking something that wasn't designed to do this job and you're using it for a completely different application," said Charness.

Privacy experts are worried about how the black box information could be used.

"The prospect that we're all under constant scrutiny has social effects and legal effects that we haven't even contemplated," said Stephen Keating of the Privacy Foundation at the University of Denver.