Comment by AnthonyMouse

The trouble with that one is that it's the one caused by people driving too slow, and isn't rare. When people are driving at significant speed differentials to each other, it causes a lot of lane change maneuvers. Which are dangerous because each one is a chance to move into a lane without realizing there is someone in your blind spot, or that the new lane has an obstruction ten feet in front of you, or focus your attention on what's happening in the lane you're intending to change into and fail to notice an immediate hazard in the lane you're still in. So you get more accidents, and those accidents are still happening at highway speed.

Meanwhile a significant proportion of accidents happen not because you couldn't stop fast enough but because you couldn't stop at all. Another driver side swipes you and rams you off the road and you're now three feet from a head-on collision with a utility pole; if you can even get your foot to the brake before impact it's not going to save you. You're driving on an apparently clear road in winter and come across a patch of black ice; your brakes have no effect and you're hitting whatever's in front of you at full speed. It's night, you're tired and you only realize there is a completely dark disabled vehicle in one of the travel lanes when you smash into it.

In cases like this the difference between 30 MPH and 50 MPH can mean a lot, but the difference between 55 MPH and 80 MPH is basically just "you're still dead".