Stopping Sight Distance (SSD): For a design speed of 80 km/h, what is the absolute minimum sight distance required to bring a vehicle to rest under standard assumptions (driver reaction time and braking on a good paved surface)?

Introduction / Context:
Stopping Sight Distance (SSD) is the minimum distance a driver needs to perceive, react, and brake to a full stop after sighting an obstruction. It underpins safe geometric design of highways, ensuring drivers can halt within the visible roadway ahead.

Given Data / Assumptions:

• Design (running) speed v = 80 km/h.
• Standard reaction time t ≈ 2.5 s (typical design value).
• Friction coefficient on good surface f ≈ 0.35 (representative for design).
• Acceleration due to gravity g ≈ 9.81 m/s^2.

Concept / Approach:

SSD is the sum of the distance traveled during perception-reaction and the braking distance. Using consistent units improves accuracy. Typical design charts already provide standard SSD values; for 80 km/h, the tabulated value is about 120 m in many IRC-based references.

Step-by-Step Solution:

Verification / Alternative check:

IRC design tables list SSD rounded to conservative values to cover variability in drivers, surfaces, and braking systems. The commonly adopted value for 80 km/h is 120 m, aligning with safe practice.

Why Other Options Are Wrong:

• 200 m, 640 m: Excessively high for SSD at 80 km/h under standard assumptions.
• “None of these”: Incorrect because 120 m is recognized in standard practice.
• 90 m: Underestimates safety margin; not adopted as an absolute minimum.

Common Pitfalls:

• Mixing units when computing distances from speeds in km/h.
• Ignoring grades; adverse grade increases SSD.

Final Answer:

120 m.