Difficulty: Easy
Correct Answer: 2.5 s
Explanation:
Introduction / Context:
Perception–reaction time (PRT) is a key input to stopping sight distance (SSD) and other safety-related geometric design elements. In complex traffic situations, drivers need more time to perceive, interpret, decide, and act.
Given Data / Assumptions:
Concept / Approach:
SSD combines perception–reaction distance (v × t) and braking distance. A standard, conservative value of PRT around 2.5 s is used globally for design consistency under complex or unexpected situations, accommodating variability among drivers and conditions.
Step-by-Step Solution:
Adopt a conservative PRT to compute SSD = v × t + v^2/(2a).For complex conditions, t ≈ 2.5 s is recommended.Therefore, choose 2.5 s.
Verification / Alternative check:
Empirical studies show PRT distributions with means near 1.0–1.5 s, but tails extend higher. Using 2.5 s for design covers a large portion of real-world variability and accounts for distractions and decision complexity.
Why Other Options Are Wrong:
Common Pitfalls:
Final Answer:
2.5 s
Discussion & Comments