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Explanation of PIEV theory

Short answer

PIEV theory splits driver reaction time into Perception (see object), Intellection (understand danger), Emotion (decide action), and Volition (apply brake). Total design time is 2.5 seconds (IRC). Example: at 60 km/h, vehicle travels 0.278×60×2.5 ≈ 42 m during PIEV time before braking begins.

Explain/define PIEV theory (with example).

The PIEV theory explains the driver's total reaction time. It divides the time between the instant the driver sees an object and the instant the brake begins to act into four parts.

  1. P - Perception: the time taken to see and receive the signal/object through the senses.
  2. I - Intellection: the time to understand and interpret what the object means (is it dangerous?).
  3. E - Emotion: the time to decide on a reaction (to brake, steer or ignore), affected by fear, fatigue or alcohol.
  4. V - Volition: the time to carry out the decision, i.e. the muscles act to apply the brake.
t=tP+tI+tE+tVt = t_P + t_I + t_E + t_V

The total time ranges from 0.5 s to over 4 s; the design value is 2.5 s (IRC, NRS 2070) used for SSD, which covers about 85-90% of drivers.

Example

A driver driving at 60 km/h sees a child run onto the road: he sees the child (P), recognises the danger (I), decides to brake (E), and moves the foot to the brake pedal (V). During 2.5 s the vehicle travels 0.278×60×2.5≈420.278\times60\times2.5 \approx 42 m (the lag distance) before braking starts.

Use: PIEV time decides the lag distance in the sight distance, and the location of signs and signals.

✓ Answer checked by Claude Opus 5.5 (high effort)

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