Determinants of maximum safe speed: which roadway and operational factors primarily govern the maximum safe driving speed on a highway?

Introduction / Context:
Maximum safe speed is the highest speed at which typical drivers can travel while maintaining control and stopping within visible distance under prevailing conditions. It is not a single number but a function of highway classification, geometry, surface, and visibility. Recognizing these factors helps set realistic speed limits and design speeds.

Given Data / Assumptions:

• Functional class: expressway, arterial, collector, local.
• Surface condition: dry/wet, macro/microtexture, skid resistance.
• Geometry: curve radii, gradients, super-elevation, and vertical curves.
• Sight distance: daytime and nighttime (headlight) conditions.

Concept / Approach:
Design speed is chosen considering terrain and function (option a). Safe operating speed depends on tyre–road friction and surface condition (option b). Curve radii and superelevation limit lateral acceleration and, hence, speed (option c). Sight distance must exceed the stopping distance at operating speed; poor visibility reduces safe speed (option d). Together, these determine the practical safe speed envelope for a given site and time.

Step-by-Step Solution:
Relate functional class to design speed standards. Assess pavement friction and drainage effects on stopping distance. Evaluate curves and grades against lateral acceleration and comfort criteria. Check that sight distance ≥ stopping distance; otherwise reduce speed. Conclude all listed factors jointly govern maximum safe speed.

Verification / Alternative check:
Speed–flow–density relationships and crash data consistently show sensitivity to geometry and surface conditions, validating the multi-factor dependence of safe speeds.

Why Other Options Are Wrong:

• Any single factor alone cannot capture all constraints on safe operation.

Common Pitfalls:

• Using posted speed limits as the only determinant without considering wet or night driving conditions.

Final Answer:
All the above.