Stopping Sight Distance (SSD) – standard design expression Which equation correctly expresses the safe stopping sight distance D on a highway, for a vehicle at speed V (km/h) with perception-reaction time t (s), coefficient of longitudinal friction f, and grade G?

Introduction / Context:
Stopping Sight Distance (SSD) is the minimum visible distance required for a driver to perceive, react, and bring the vehicle to a stop safely. The SSD expression combines the distance traveled during perception-reaction and the braking distance on a given grade.

Given Data / Assumptions:

• V in km/h, t in seconds.
• f = coefficient of longitudinal friction between tire and road.
• G = grade (use + for adverse downgrade, − for upgrade) in appropriate form consistent with the 254 constant.

Concept / Approach:
SSD has two parts: perception-reaction distance and braking distance. Perception-reaction distance equals speed in m/s times t. Braking distance follows energy and friction–grade balance, which, with unit conversions, gives the 254 constant when V is in km/h.

Step-by-Step Solution:

Verification / Alternative check:
On level grade (G = 0), the formula reduces to D = 0.278 * V * t + V^2 / (254 * f), matching standard design tables.

Why Other Options Are Wrong:
Options B, C, D distort units and physical meaning. Option E omits braking distance and is therefore incomplete.

Common Pitfalls:
Using grade in the wrong sign or units; forgetting that adverse downgrade increases SSD, while upgrade reduces it.

Final Answer:
D = 0.278 * V * t + V^2 / (254 * (f ± G))