The following excerpt was taken from the 1994 edition of AASHTO's A Policy on Geometric Design of Highways and Streets (pp. 118-119).
Brake reaction time is the interval between the instant that the driver recognizes the existence of an object or hazard in the roadway ahead and the instant that the driver actually applies the brakes. This interval includes the time required to make the decision that a stop is necessary.
Under certain conditions, such as emergency conditions denoted by flares or flashing lights, operators accomplish these tasks almost instantly. Under most other conditions the operator must subconsciously associate the object ahead with stationary objects adjacent to the roadway, such as walls, fences, trees, poles, or bridges, to determine that the object is also stationary or moving at a slow speed. These determinations take time, the amount of which varies considerably depending on the distance to the object, acuity of the operator, the natural rapidity with which the driver reacts, atmospheric visibility, the type and the condition of the roadway, and the type, color, and condition of the hazard. Vehicle speed and the roadway environment probably also influence reaction time.
Normally, an operator traveling at or near the design speed is more alert than one traveling at a lesser speed. An operator on an urban facility confronted by innumerable possibilities for conflicts from parked vehicles, driveways, and cross streets is also likely to be more alert than the same operator on a limited-access facility where such conditions should be almost nonexistent.
(The discussion continues about various studies that have been conducted to determine a reasonable brake reaction time for a below average driver.)
. . . In the determination of sight distance for design, the reaction time should be larger than the average for all drivers under normal conditions. It should be large enough to include the reaction time required for nearly all drivers under most highway conditions. For approximately 90 percent of the drivers in the first study mentioned, a reaction time of 2.5 s was found to be adequate. A reaction time of 2.5 s has thus been assumed . . .
A reaction time of 2.5 s is considered adequate for more complex conditions than those of the various studies, but it is not adequate for the most complex conditions encountered by the driver. (Decision sight distance is used in the most complex situations.)