According to AASHTO, what is the assumed perception-reaction time for drivers?

• A. 1.5 seconds
• B. 2.0 seconds
• C. 2.5 seconds
• D. 3.0 seconds

Answer: (C)

The assumed perception-reaction time for drivers, according to AASHTO (American Association of State Highway and Transportation Officials), is set at 2.5 seconds. This time frame is used in various roadway design and traffic engineering calculations to estimate the time it takes for a driver to perceive a stimulus, such as the need to stop or swerve, and to initiate a response to that stimulus.

The concept of perception-reaction time incorporates both the time taken to recognize a hazard and the time required for physical response actions, such as applying the brakes. AASHTO’s standard duration of 2.5 seconds is rooted in empirical studies of driver behavior, providing a baseline that reflects average conditions under which drivers are expected to react.

This metric is crucial for roadway design, traffic signal timing, and various safety analyses, as it influences the layout of intersections, stopping sight distances, and overall traffic flow efficiency. By using this standardized reaction time, engineers can ensure that roadways are designed to accommodate expected driver behaviors under normal circumstances, thereby enhancing safety and operational effectiveness.