What goes into a death investigation on a freeway?

Q: Why do fatal car accidents require complete freeway shutdowns for investigations? The delays can be hours affecting thousands of motorists. Couldn’t there be a requirement that drivers sign a waiver that forfeits the requirement for a complicated, hours-long investigation? Aren’t speed, reckless and distracted driving the usual causes?
– Kenneth Rosen, Tarzana
A: No matter how good the ball game is on the radio, or the music coming out of the speakers — Honk