Vehicle dimensions and design controls: Which roadway design feature is least affected by the vehicle length (wheelbase or overall length) of typical highway vehicles?

Introduction / Context:
Vehicle dimensions (wheelbase, overall length, width) influence numerous design elements. Understanding which features depend strongly on length versus those controlled by other factors helps in prioritizing inputs for alignment and cross-section design.

Given Data / Assumptions:

• Conventional passenger cars, buses, and trucks.
• Standard design speeds and lane widths.
• Focus on length effects, not width or height.

Concept / Approach:

Vehicle length affects turning path and swept path envelopes. It directly influences minimum turning radius (through wheelbase and steering geometry) and extra widening on curves (to accommodate off-tracking). It also enters overtaking sight distance calculations because the overtaking manoeuvre duration depends on the lengths of the overtaking and overtaken vehicles. By contrast, shoulder width primarily relates to safety refuge, breakdown space, and lateral support; it is not set by vehicle length per se (though overall width/clearance standards matter).

Step-by-Step Reasoning:

Verification / Alternative check:

Design manuals provide explicit formulas for extra widening and overtaking sight distance with vehicle length; shoulder width appears as a tabled standard not keyed to vehicle length.

Why Other Options Are Wrong:

• A, B, C: each is affected by vehicle length to varying degrees.
• “None of these” is incorrect because shoulder width is the exception.

Common Pitfalls:

• Assuming shoulder width must increase with vehicle length; in practice, lane width and clearances govern.

Final Answer:

Width of shoulders.