According to the AASHTO "Green Book", what value of perception-reaction time is recommended for calculating Stopping Sight Distance?

The value of perception-reaction time recommended by AASHTO's "Green Book" for calculating Stopping Sight Distance is 2.5 seconds. This time frame is considered reasonable for most driving conditions, taking into account the time it takes for a driver to perceive a hazard and decide to act.

The choice of 2.5 seconds is rooted in research that aims to reflect normal driver behavior under typical conditions where drivers may be facing unexpected situations. It includes both the perception phase, where a driver recognizes the need to stop, and the reaction phase, where the driver decides to apply the brakes and begins the braking process.

Using this value in the calculations for Stopping Sight Distance ensures that roadway design accounts for adequate sight distances that provide a safe stopping distance for vehicles. This consideration is critical in reducing the likelihood of stopping-related accidents and improving overall roadway safety.