Police accident reconstruction officers can tell exactly how fast you were going loser. It is all about physics and evidence at the scene such as skid mark and not the kind in your britches Brucey.
In addition to skid marks and the like, vehicular computers record data for some seconds of speed, brake applied, air bag deployment and other useful (to the police) information. (I think the data is over-written in memory so all that is available is a few seconds before the event. I also recall that air bag deployment stops data recording so that the accident data is preserved and not over-written.)
I would assume the computer has been taken by the police and the data downloaded and analyzed.