very interesting

 

well, after an accident, if someone gets badly injured or killed they will measure things, like skid marks, location of parts that flew off the car, etc. That can tell them, pretty accurately how fast the cars were going.
Cops who have seen a lot of accidents can usually just look at how long the skid marks are to tell how fast the car was going.

 

Most new cars have this capacity, normally it's used for the manufacturers collection of datanot forcourts to determine fault of an accident.