Stopping capability at design speed: factors influencing stopping sight distance The driver's ability to stop a vehicle traveling at the design speed depends primarily on which of the following factors?

Introduction / Context:
Stopping Sight Distance (SSD) is a fundamental highway geometric design parameter. It represents the distance a vehicle needs to stop safely after the driver perceives a hazard. Understanding the underlying factors ensures safe alignment, visibility, and cross-section design.

Given Data / Assumptions:

• Vehicle is traveling at the design speed.
• We are considering human and mechanical factors that influence SSD.
• Dry pavement with typical tire–pavement friction.

Concept / Approach:
SSD = distance traveled during perception–reaction time + braking distance. Perception time and brake reaction time govern how far the vehicle moves before brakes are effectively applied. Braking distance depends on brake efficiency and available friction between tires and the road surface. All these elements together determine whether a driver can stop in time.

Step-by-Step Solution:
Perception time + brake reaction time → human factor distance (coasts at initial speed).Brake efficiency and tire–road friction → deceleration magnitude during braking.Higher friction and effective brakes reduce braking distance; longer reaction time increases total SSD.Therefore, all listed factors jointly control stopping ability.

Verification / Alternative check:
Highway design manuals compute SSD by assuming a perception–reaction time and a coefficient of friction, along with grade; brakes must be capable of achieving the assumed deceleration.

Why Other Options Are Wrong:
Each single factor alone is incomplete; safe stopping requires favorable human response and mechanical performance with adequate friction.

Common Pitfalls:
Underestimating reaction time; ignoring wet or polished surfaces that reduce friction; overlooking grade effects (uphill or downhill) on braking distance.

Final Answer:
all the above