I think that's the wrong question. The question should be "what level of carnage is acceptable on our highways?"
You can set the speed limit on interstates to 30, and people will still die if they drive without seatbelts, drunk, and on the wrong side of the road.
Despite what state patrols would have us believe, speed alone does not kill, else every race car driver and pilot would be dead, because they go a heckuvalot faster than we drive even if we're far over the speed limit.
Running into things is what kills, and the goal should be to minimize running into things. My stance is that better driver training and harsher penalties for driver negligence is the way to solve that.
Instead of targeting the demonized-thing-of-the-week, whether it's the speed limit or cell phones, we should target driver ability and behavior.
I'd much rather an 80mph speed limit with well-trained drivers who knew how to handle their car no matter what, than 55mph with a bunch of dopes who barely passed driver's ed, and have no idea what to do when they hit that patch of black ice and go into a skid.
In my state, Minnesota, a driving instructor got in trouble some years ago because he spent too much time teaching people how to drive and not enough time sitting in a classroom. This guy was fantastic. He'd even take his students out to the go-kart track, toss oil on one corner, and teach his kids how to recover from a skid. He got himself a car with locking casters in place of the rear wheels, and would get them out in a parking lot doing turns, and suddenly unlock the casters and make them get out of skids.
They didn't pass until they could do such things in their sleep, and I guarantee his former students are much safer drivers today as a result. But the state cracked down on him because he taught people how to drive a car instead of teaching people how to drive a desk.
This is the same state that every year comes out in a big media blitz and tells us that speed kills, while ignoring the fact that bad driving is what kills people.
It's also the same state that will observe you driving like an idiot, and then pull you over and give you a drunk-test on the side of the road, and if you pass it they will write you a BS ticket, if that, and then let you go, when what they should be doing is targeting driving like an idiot instead of "drunk/distracted" driving.
At the end of the day, when you T-bone my car and kill my wife, I don't give a hoot if you're drunk, distracted, or just stupid. You shouldn't have been on the road. We shouldn't only take people off the road if they test positive for mind-altering substances. Sometimes... Often, really, people do not need substances in order to make them mentally incapable of driving well - they're mentally incapable of driving well all by themselves, and should be taken off the road right alongside the drunks.