In highway geometric design and traffic engineering, what standard driver reaction time (perception–reaction time) should be assumed for computing stopping sight distance under design conditions?

Introduction / Context:
Stopping Sight Distance (SSD) is a core safety parameter in highway design. It ensures a driver can perceive a hazard, react, and brake to a stop before reaching the object. The assumed perception–reaction time (also called reaction time) directly influences SSD and thus affects vertical curves, horizontal curves, and placement of roadside features.

Given Data / Assumptions:

• Design is for normal drivers under ordinary conditions.
• SSD comprises two components: distance traveled during reaction plus braking distance.
• Standard practice adopts a conservative average reaction time constant for design.

Concept / Approach:

Highway design manuals (e.g., many national standards) commonly assume a perception–reaction time around 2.5 seconds for design purposes. This value covers typical alert drivers, accounts for recognition and decision lags, and builds in conservatism for real-world variance without being overly restrictive.

Step-by-Step Solution:

Verification / Alternative check:

Using t_r = 2.5 s aligns SSD with crash data and the operating speed environment. Sensitivity checks show SSD increases linearly with t_r, so using the standard value maintains consistency across facilities.

Why Other Options Are Wrong:

• 5 s or 10 s: Overly conservative; not representative of normal alert drivers.
• 0.5 s or 1.5 s: Underestimates human reaction, yielding unsafe SSD.

Common Pitfalls:

• Confusing perception–reaction time with braking time.
• Ignoring environmental factors (night, rain) that are handled by friction and visibility considerations, not by radically changing t_r.

Final Answer:

2.5 secs