Perception–reaction time for stopping sight distance calculations For ordinary design conditions, what perception–reaction time should be assumed for a vehicle moving at 80 km/h?

Introduction / Context:Stopping sight distance (SSD) depends on perception–reaction time and braking performance. A standard design value of reaction time is used to ensure adequate safety margins for typical drivers.

Given Data / Assumptions:

• Design speed = 80 km/h.
• Ordinary (not exceptional) conditions and average drivers.

Concept / Approach:Design guidelines commonly adopt a perception–reaction time of about 2.5 s for highway design to include perception, decision, and initiation of braking.

Step-by-Step Solution:Identify standard value used in SSD calculations.Adopt t_r = 2.5 s.Select the matching option.

Verification / Alternative check:Using 2.5 s at 80 km/h gives a reaction distance of roughly (80 * 1000 / 3600) * 2.5 ≈ 55.6 m before braking, aligning with design tables.

Why Other Options Are Wrong:

• 1.0–2.0 s: too short for conservative design.
• 3.0 s: overly conservative for ordinary conditions; may be used in special cases.

Common Pitfalls:Confusing driver perception–reaction time with brake lag or vehicle deceleration parameters.

Final Answer:2.5 s