About how many feet will the average driver going 50 mph travel from the moment he sees danger until he hits the brake?

Accident reconstruction experts routinely testify that the perception + reaction time for people is about 1.5 seconds. Perception + reaction is the time it takes from when a person perceives an event that requires action to the time they take that action -- hitting the brakes in this case.

A car going 50 mph is going 73.33 feet per second. So, 73.33 * 1.5 seconds = 110 feet.