Of course, there is the issue of can the recorded speed be
trusted if one or more wheels has left contact with the ground
and was spinning freely?

A 'black box' that only records a few minutes, and cannot
broadcast it in any way, doesn't seem too bad privacy wise, if
the only time people investigate it is after a crash the car
won't be driving away from.

There is a thing called a 'CarChip' which perhaps this black
box is based on. The CarChip records a lot more than would be
needed for figuring out fault in an accident, though.

http://www.davisnet.com/drive/products/carchip.asp