I think a lot of the reason people speed is that they don't anticipate suprises, since they get complacent about yet another uneventful trip in the rolling living room. Everyone thinks 'it won't happen to me', and can't imagine how easily their life could end (maybe we need to see more graphic images from car crashes). They drive at a speed they can comfortably handle as long as everything stays predictable. Then, another driver does something stupid, or there's an obstacle in the roadway, and suddenly they're out of control.

I find that speed limits are usually very reasonable, and in residential areas, I often drive under the speed limit due to poor sightlines and the potential for a child or pet to run out from behind any parked car. On car-only freeways, I agree that the limit is lower than it needs to be in terms of driver ability, but I think it's reasonable in terms of making any collisions survivable.