Theoretically, for every speed limit you travel at, halve it, add it to the speed limit and that is the amount of distance you are travelling per second in feet.
At 30mph you will be travelling at 45 feet per second
At 70mph it will be 105 feet per second
If you think that any shoulder or rear observation check, which all motorcyclists should do regularly, takes an average of two seconds, at 30mph you'll have travelled 90 feet blind.
Also it takes us about one second to think about reacting to something, for example children stepping out into the road.