Perception-Reaction Time (PRT)
Definition
The elapsed time from when a driver can first perceive a hazard to when the brakes begin to apply. Olson and Farber established a standard 1.5-second value for alert drivers; design standards commonly use 2.5 s.
- Measured interval
- Hazard perception to onset of braking
- Alert-driver value
- 1.5 seconds (Olson and Farber)
- Design standard value
- 2.5 seconds
- Field
- Accident reconstruction
Common questions
Why do reconstructionists use a higher value than the 1.5-second research figure?+
The 2.5-second design value builds in a margin for drivers who are older, distracted, or otherwise slower than the alert drivers studied in the original research, making it more conservative for engineering and litigation purposes.
How is PRT actually used in a reconstruction?+
It is combined with pre-impact speed and braking deceleration to calculate total stopping distance and to assess whether a driver could plausibly have avoided a hazard once it became visible or perceivable.
Does PRT include the time to physically press the brake pedal?+
Yes, PRT spans from the moment the hazard could first be perceived through the driver's decision-making to the point the brakes actually begin to engage, not just the mental recognition step.
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