Fembear, you do not appear to be taking any note of reaction times, as well as braking times in your argument that speed limits were set generations ago, and are therefore largely irrelevant. I am sure that my reaction times are just about the same as those of my grandmother's, or my great-grandmother's. To give you a clue about how a person's reaction times impact the total stopping distance (not just braking distance), see this table:
20mph: braking distance: 19ft, reaction distance: 44ft, total stopping distance: 63ft
30mph: braking distance: 43ft, reaction distance: 66ft, total: 109ft
35mph: braking distance: 59ft, reaction distance: 77ft, total: 136ft
40mph: braking distance: 76ft, reaction distance: 88ft, total: 164ft
So, if you're happily driving along at 35mph and someone walks out in front of you, you react. You slam on your brakes. And you will travel a further 27 feet than if you had been driving at 30mph. That basically means that the person who stepped out in front of you will be dragged a further 10 odd yards down the road than if you had been observing the speed limit. Hence the higher the speed, the higer the fatality rates.