Among the factors that influence highway geometric design, which one most critically governs the selection of horizontal/vertical alignments, super-elevation, sight distances, and gradients?

Introduction / Context:
Highway geometric elements—such as curvature, grades, sight distance, and super-elevation—must accommodate driver behavior and vehicle dynamics at the intended operating speeds. While vehicle dimensions matter, speed drives the limiting safety and comfort criteria for most geometric controls.

Given Data / Assumptions:

• The facility is designed for a target or design speed.
• Standard relationships tie speed to stopping/overpassing sight distance, minimum curve radius, and required crossfall.

Concept / Approach:
Key formulas—SSD, OSD, minimum R, and e—explicitly include speed. For example, e varies approximately with V^2, SSD depends on V and perception–brake time, and minimum curve radius is proportional to V^2. Hence, speed dictates many minimums and thus is the most critical factor among the listed options.

Step-by-Step Solution:
Relate each geometric control (SSD, OSD, R_min, e) to speed.Note that vehicle dimensions adjust widths/clearances but rarely control curvature/grades directly.Conclude speed is the governing factor.

Verification / Alternative check:
Design standards organize geometric criteria by design speed class, underscoring speed's primacy in layout decisions.

Why Other Options Are Wrong:

• Width/length/height: influence lane width, clearance, and structure gauges, but not the core curvature/grade limits to the same extent.

Common Pitfalls:

• Overemphasizing vehicle dimensions and underestimating the role of speed in sight distance and curvature.

Final Answer:
Vehicle speed