Stopping distance with brake efficiency and reaction time If V is the speed in km/h, t is the driver's brake-reaction time in seconds, η is the brake efficiency (0 to 1), and f is the longitudinal friction coefficient, the total stopping distance S on level road is:

Introduction / Context:
Stopping sight distance combines perception–reaction distance and braking distance. Real vehicles do not always exploit full tire–road friction due to brake condition and balance, so an efficiency factor η reduces the effective deceleration.

Given Data / Assumptions:

• V in km/h; t in seconds.
• f = coefficient of longitudinal friction.
• η = brake efficiency (0 < η ≤ 1).
• Level road (no grade term included).

Concept / Approach:
Total distance S = perception–reaction distance + braking distance. The first term is v * t, with v = 0.278 * V. The second term derives from uniform deceleration a = η * f * g. Using standard highway units introduces the constant 254 when V is in km/h and distances are in metres.

Step-by-Step Solution:

Verification / Alternative check:
When η = 1, the expression reduces to the familiar SSD formula on level grade: S = 0.278 * V * t + V^2 / (254 * f).

Why Other Options Are Wrong:
Option B multiplies friction by η in the numerator, which unrealistically shortens distance. Option C divides f by η inside the denominator, effectively increasing deceleration instead of reducing it. Option D omits braking distance entirely.

Common Pitfalls:
Using η > 1; forgetting that adverse grades require replacing f by (f ± G).

Final Answer:
S = 0.278 * V * t + V^2 / (254 * η * f)