Speed doesn't kill, but, it certainly contributes. Similar to parachuting, if your chute doesn't open it's not the fall that'll kill ya it's the sudden stop at the end.
I've said it in other threads, if you think it's safer going a posted 55MPh than the flow of traffic you are nuts! Get on a 3lane (each direction) hwy and go the posted 55mph. (Needs to be somewhat populated). Then stay in the right lane and count, each minute, how many cars come zipping by. I've done it, and there were a lot. If you've never done that before, do it for 10minutes. It'll freak you out how many people come right up on your ass only to swerve over at the last second to pass. You'll get all kinds of dirty looks and angry gestures. So, not only do you have to worry about speeders hitting you but you have to worry about road rage too.
Then go 5mph over the limit. And see how many cars you pass. I did it for 10 minutes and I think I caught up to 1 other driver going the speed limit.
I've done this in multiple cars at various times and the results are generally the same. It's simply NOT safe, IMO, to do the speed limit on a highway where traffic is (99%) exceeding it already. So, you speed and take your chances and hope you aren't the one in the herd who's going to get picked off or you do the limit and hope you don't meet up with any a-holes.
It's hard to do a similar test on city roads because of the frequent stop lights --you generally don't get a chance for other traffic to overpass you (but some will!). If you try it though, you might realize that some roads are timed for the speed limit so you don't ever have to stop and OTHER roads are NOT timed for the speed limit--they'll turn red just before you get there. I'd say that's a case where the road was designed for a higher speed and someone got it lowered but the traffic engineers weren't called back to adjust the lights. (Then there are other lights that aren't timed at all, so it doesn't matter what speed you go).
Bottom line, drive carefully and don't be an ass.