Hey guys. I might be able to shed a little more light on the subject. I am currently a member of an accident investigation team and by next year I will be a accident reconstructionist. There are several types of crash data recorders on vehicles these days. Here are the ones that are currently in use by different manufactures.
ACM=Air Bag Control Module
SCM=Sensing and Diagnostic Module
PCM=Power Train Control Module
RCM=Restraint Control Module
The data that these things can record can be up to 20+ pages of information. They record things like Vehicle Speed, Engine RPM, Throttle position, Brake position, Seat belt status, Delta-V's (the amount of force placed upon the car by an outside force), Steering wheel location, Seat location. There is just a ton of information theses things record.
Each company out there uses one or more of theses types of devices. Almost every car has one of these in it including semi's. The first thing to understand is that a accident investigator will not base their investigation on the informatoin from these. Its a computer and it can malfunction. For example at a crash scene I mirror the module and dont look at it until I am almost done with my investigation. All the evidence that I need for an investigation is on the scene. I wouldnt need the module information for any part of the investigation, however it does help confirm my findings from my investigation. They have been very helpful in allowing investigators to figure out if the accident was caused by a vehicle issue or a driving issue. I hope this helps clear things up. I was against them until I became an accident investigator. These devices can actually help people who have been in an accident. As an example you are driving your impala that has one and the guy that hits you is driving a car without one. It can show how much force the other guy hit you with and show that you tried to avoid the collision by applying the brakes or tried to swerve to avoid hitting the other guy.
Now do understand that if you are in the wrong well then the information could be used against you. But if I go into court I am not going to just walk in with the module priint out and say this is what happened. I am gonna use crush, road, photo and other types of evidence. The easiest thing to do is drive using your head.