You can not regulate safe driving anymore than you can legislate effective parenting, but the most fool proof way to reduce traffic fatalities is to reduce the speed of collisions (both for single car accidents or multiple car accidents). A 55 mph regulator on cars would save the most lives. A crash at 55 mph on the free way would be far less catastrophic than a crash at 80 mph. And less likely, since it increases contollablility of the vehicle and gives people more time to respond.

Opinion coming up, I think people drive more dangerously the safer they feel their cars are. Seat belts, roll bars, air bags, side air bags, etc. make people act more dangerously. This means that safety improvements in cars have a minimal, null or perhaps even negative impact on injuries and fatalities. I have no evidence to back this opinion up what so ever.