[jrmg]

Momentum is an unintuitive thing. See, for example http://www.brake.org.uk/events/15-facts-a-resources/facts/12...:

A vehicle travelling at 20mph (32km/h) would stop in time to avoid a child running out three car-lengths in front. The same vehicle travelling at 25mph (40km/h) would not be able to stop in time, and would hit the child at 18mph (29km/h). This is roughly the same impact as a child falling from an upstairs window. The diagram below illustrates the impact at various speeds. The greater the impact speed, the greater the chance of death. A pedestrian hit at 30mph has a very significant one in five chance of being killed. This rises significantly to a one in three chance if they are hit at 35mph

Or the charts here:

https://one.nhtsa.gov/nhtsa/Safety1nNum3ers/august2015/S1N_S...

[STRML]

That's assuming a 1.5s reaction time. Surely that's quite slow and accounts for the bulk of the difference.

[jrmg]

The reaction times quoted at the link associated with the main quote are actually: a reaction time of 0.67 seconds, which assumes the driver is alert, concentrating and not impaired. Driving when tired, distracted or impaired significantly increases reaction times, so the thinking distances above should be regarded as minimums.

The NHTSA assumes 1.5 seconds because: A typical reaction time to perceive a threat such as a deer or a child running into the road is about 3/4 second, and then you add another 3/4 second to decide to act and move your foot to the brake pedal. I agree it's unintuitively long, but I also know that reaction times have always been slower that I think they are when they've been tested...

[dsfyu404ed]

1.5sec?!? You've got to be senile and on your phone to pull that off.