General Motors began installing black boxes in the 1970s as part of the airbag system. The devices continuously monitor information such as speed,
G-forces and seat-belt tension to determine when a crash is imminent. But unbeknownst to most drivers, black boxes preserve these details for the seconds preceding an airbag "event." In recent years such data have been cited by prosecutors in criminal trials and by
carmakers in product-liability suits--to show that drivers were speeding, for example, or not wearing seat belts.