No doubt that impact protectors behave in some complicated, non linear fashion. Especially at the extreme low and extreme high ends of their functional ranges. And I'm sure that brakes get more than a little wacky at their functional extremes as well. Clearly, 30 mph and 60 mph stops don't stress modern brakes much. Anyway, I guess my point was simply that one shouldn't expect that stopping distance vary linearly with speed simply because of the nature of kinectic energy. In your example we should expect it to be 4x longer from 60 mph as it is from 30 mph. Who would have thought that a Tipperary Air Esprit vest would have me thnking back to high school physics!?! Fun stuff!