We've had to have them in fire engines for a couple of years now. In some cases they can be a blessing and others a curse. I'm not sure how detailed the regular car ones are but for fire apparatus they pretty much tell you anything, who was buckled in, G forces, braking distance, speed, how hard you were braking ect, ect. You wreck a rig and kill someone the info they pull off that black box could potentially get you sued or a criminal trial, likewise it could save you from one.
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