Stopping sight distance (SSD) at 80 km/h: What is the minimum stopping sight distance to be provided on a highway designed for 80 km/h under standard assumptions used in Indian practice?

Introduction / Context:
Stopping sight distance (SSD) ensures a driver traveling at the design speed can perceive, react, and brake to a complete stop before reaching an object on the road. It is a fundamental control for crest vertical curves, horizontal sight clearance, and roadside design.

Given Data / Assumptions:

• Design speed V = 80 km/h.
• Perception-reaction time t ≈ 2.5 s (typical design value).
• Longitudinal friction factor f ≈ 0.35 (representative value for design).
• Road is level for SSD calculation; grade effect ignored for simplicity.

Concept / Approach:
SSD consists of two parts: the distance traveled during perception-reaction and the braking distance. Common highway design uses the formula in km/h units: SSD = 0.278 * V * t + V^2 / (254 * f), where V is in km/h.

Step-by-Step Solution:
Compute reaction distance: 0.278 * 80 * 2.5 = 55.6 m.Compute braking distance: 80^2 / (254 * 0.35) = 6400 / 88.9 ≈ 72.0 m.SSD ≈ 55.6 + 72.0 = 127.6 m, rounded conservatively to the nearest standard value ≈ 120 m to 130 m.Standard tabulated SSD for 80 km/h is commonly taken as about 120 m in many design summaries.

Verification / Alternative check:
Design manuals often list SSD values: for 80 km/h, a value near 120 m is widely used for quick checks, aligning with the calculation above (depending on chosen f and rounding).

Why Other Options Are Wrong:

• 80 m, 100 m: underestimate SSD and reduce safety margin.
• 150 m, 200 m: conservative but exceed typical standard provisions for 80 km/h under normal assumptions.

Common Pitfalls:

• Mixing units (m/s vs km/h) leading to miscalculation.
• Ignoring upgrades/downgrades; grades increase/decrease braking distance.

Final Answer:
120 m