Sight distance assumptions: For calculating stopping and overtaking sight distances on highways, the standard height of the driver’s eye above the road surface is commonly assumed as

Introduction / Context:
Sight distance calculations require standard geometric assumptions to ensure consistent and safe design. Two key parameters are the eye height of the driver and the object height on the roadway. These values affect crest vertical curve length, setback on horizontal curves, and passing zone placement.

Given Data / Assumptions:

• We need the commonly adopted driver eye height in highway design.
• Values are expressed in centimeters for convenience.
• Applies to stopping and passing sight distance computations.

Concept / Approach:
IRC practice commonly uses a driver’s eye height of about 1.2 m (120 cm) and an object height (for SSD) of about 0.15 m. These standardized values create a conservative and uniform basis for geometry selection, improving safety.

Step-by-Step Solution:
Recall the standard assumption: eye height ≈ 1.2 m.Verify against typical design charts and examples for SSD and OSD.Select 120 cm from the options.

Verification / Alternative check:
Worked examples in geometric design texts repeatedly use eye height ≈ 1.2 m; adopting this value yields conventional crest curve lengths and horizontal curve sight clearances.

Why Other Options Are Wrong:

• 90–110 cm: too low versus standard design assumption.
• 150 cm: higher than typical and would increase required crest curve lengths unnecessarily.

Common Pitfalls:

• Mixing units between meters and centimeters.
• Using different eye heights for nighttime headlight sight distance without adjusting luminance assumptions.

Final Answer:
120 cm