About 80% to 90% of reports would state it was operator error on the part of one or more of the drivers involved. Cars are pretty safe when people aren't drooling, lead-footed maniacs.
Almost all crashes are ultimately caused by driver error. Doing FAA/NTSB-style investigation on them would be pointless, because that's exactly what they'd find: people were driving recklessly, inattentively, etc. And there's nothing that can be done about that, because we as a society absolutely refuse to have serious driver training and any standards for driver conduct. We can't even agree on whether the left lane is for passing or not! Try driving in Germany sometime and you'll find that what we accept from drivers here in the US is really quite awful.