futureworldblog replied to your post: Sometimes I find cool things in my old engineering textbooks

Excellent. I wish I could tell my partner this. He drives too fast, and too heedlessly, much of the time. And this is why engineering is useful …

Roads (in the U.S. anyway) are designed to be sufficient for the “85th Percentile Design Driver.”  That is, 85% of the driving population fall within a set of ranges that are used to design roads.  For instance, a design driver:

  • Has an average Perception Reaction Time (PRT) of 2.5 seconds
  • Takes 3 seconds to adjust from dark to light
  • Takes 6 seconds to adjust from light to dark

And some other factors, but the PRT is the big one; that’s what’s used to determine sufficient stopping sight distances (the horizontal distance covered by the driver to come to a safe, complete stop after seeing an obstruction ahead).

So unless he has ridiculously slow reaction times or shitty brakes, he should be fine, though speed limits are also there for a reason.