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...
