PIEV THEORY 

According to this theory the total reaction time of the driver is split into four parts:

1. Perception
2. Intellection
3. Emotion
4. Volition

(1) Perception Time :

Perception time is the time required for the sensations received by the eyes or ears to be transmitted to the brain through the nervous system and spinal chord.

(2) Intellection Time :

Intellection time is the time required for understanding the situation. It is also the time required for comparing the different thoughts, regrouping and registering new sensations.

(3) Emotion time :

Emotion time is the time elapsed during emotional sensations and disturbance such as fear, anger, or any other emotional feelings such as superstition etc. with reference to the situation. Therefore the emotion time of a driver is likely to vary considerable depending upon the problems involved.

(4) Volition Time : 

Volition time is the time taken for the final action.

It is also possible that the driver may apply brakes to take any avoiding action by the reflex action, even without thinking.

The PIEV time of a driver depends on several factors such as physical and psychological characteristics of the driver, type of the problem involved, environmental condition and temporary factors (e.g., motive of the trip, travel speed, fatigue consumption of alcohol  etc.). The total reaction time of an average driver may vary from 0.5 second for simple situations to as much as 3 to 4 seconds or even more in complex problems.