What design perception-reaction time does the AASHTO Highway Safety Manual recommend, to capture the abilities of most users?

The recommendation of a 2.0 seconds perception-reaction time by the AASHTO Highway Safety Manual is based on empirical data reflecting the average response time it takes for drivers to perceive a stimulus, process that information, and initiate a response while driving. This duration encompasses the typical reaction time of most users under various driving conditions, allowing for an effective and realistic framework for highway design and safety assessments.

Using a value of 2.0 seconds acknowledges that while individuals can vary in their reaction times based on factors such as age and cognitive abilities, this figure captures the average capabilities of a wide range of drivers, including those who are reasonably experienced and attentive. Furthermore, this timeframe also aligns with road safety design principles aimed at minimizing accidents by ensuring that roadway features accommodate typical human responses without underestimating reaction capabilities.

Options that suggest longer reaction times or specific age ranges (such as 3.0 seconds or targeting different age groups) do not reflect the generally accepted timeframe used in road safety design, which prioritizes a balanced approach to capture the majority of drivers' characteristics. The notion that a single value should not be used under any conditions is contrary to the need for practical guidelines that serve to enhance roadway safety standards; hence, the AASHT