I agree with Kat - it's usually at least 24 hours and then the reaction to tensing from being hit really clobbers you.

Following too close is a factor at times, but if you're hit that hard from behind, all bets are off.

Even your insurance provider has an interest in settling this quickly, so don't sign anything any time soon. Look into the small print first. If the driver who hit you is under-insured or un-insured, then your policy kicks in.

SRS