In highway design, the maximum comfortable retardation (deceleration) assumed for moving vehicles is:

Introduction / Context:
Stopping sight distance calculations in highway engineering depend on a driver’s perception-reaction time and an assumed comfortable (service) deceleration. Selecting a realistic maximum comfortable retardation is essential to ensure safe stopping without causing instability or passenger discomfort.

Given Data / Assumptions:

• The query asks for the standard comfortable deceleration used in geometric design.
• Dry pavement and normal tyre-road friction are assumed.
• Design is for typical passenger vehicles, not emergency braking with ABS at peak friction.

Concept / Approach:
For stopping sight distance (SSD), the second term is braking distance, which uses the assumed average deceleration a. A widely adopted value in geometric design is about 3.4 m/s^2, representing a comfortable, repeatable deceleration that drivers can achieve without wheel lock and excessive jerk.

Step-by-Step Solution:
SSD = vt + v^2 / (2a), where v is speed, t is reaction time, and a is comfortable deceleration.Comfortable deceleration a is taken as approximately 3.4 m/s^2 for design.Among the choices, 3.42 m/s^2 matches this value.

Verification / Alternative check:
Field measurements of service deceleration typically range 2.5–4.0 m/s^2 for routine, controllable stops. The selected 3.4 m/s^2 lies safely within this band and is consistent with standard practice for SSD computations.

Why Other Options Are Wrong:

• 4.42, 5.56, 7.80, 10.00 m/s^2 represent increasingly severe/emergency decelerations, not appropriate for routine design comfort and consistency.

Common Pitfalls:

• Confusing peak friction-limited emergency deceleration with comfortable service deceleration.
• Using a higher a reduces SSD unrealistically and can compromise safety.

Final Answer:
3.42 m/s^2