1 SIGHT DANGER
2 APPLY BRAKES
Ability to stop is crucial. One's zone of safety (i.e., distance between probable safe stopping point and the object to be avoided) decreases as new factors are added. Thus, as shown above, a one-second reaction time, only one-quarter second slower than normal, cuts 30-mph driver's zone by 11 feet; another 10 mph of speed further reduces zone by 35 feet. Add defective brakes, a hill and poor visibility and there may not be enough space in which to avoid a hypothetical deer on highway. The problem is compounded when two or more cars must stop (lower half of chart). In situation where the leading driver sights a deer and stops just in time (line A), the car following at a safe distance (line B) is able to halt 20 feet from the first. But in another case (C), in which the following driver has an extra half-car-length margin and is going just 5 mph faster than he was before, he cannot avoid a crash, nor can he when he has unwisely crowded to within two lengths of the leader at 40 mph.