Question

The design of horizontal and vertical alignments, super elevation and gradient is worst affected by

• A. Length of vehicle
• B. Width of vehicle
• C. Speed of vehicle
• D. Height of vehicle

Hint:

Design speed is the single most important parameter in geometric highway design. All other geometric elements, including sight distances, radii of curves, superelevation, and gradients, are dependent on the chosen design speed. A higher design speed generally requires larger radii, longer sight distances, and flatter grades.

Answer 1

The Correct Option is C

Step 1: Understand the elements of highway design.

• Horizontal alignment: Refers to the plan view of the road, including curves.
• Vertical alignment: Refers to the profile of the road, including grades (gradients) and vertical curves.
• Superelevation: The banking of a curve to counteract centrifugal force and ensure vehicle stability.
• Gradient: The longitudinal slope of the road.

Step 2: Analyze the impact of each vehicle characteristic on these design elements.

• (1) Length of vehicle: Primarily affects turning radii for large vehicles, minimum length of vertical curves (for sight distance), and passing sight distance. While important, its effect on the fundamental design of alignments, superelevation, and gradients is less direct and universally critical compared to speed.
• (2) Width of vehicle: Primarily affects lane width, shoulder width, and clearances in tunnels or underpasses. It has a minor direct impact on horizontal or vertical alignment design in terms of curvature or gradient.
• (3) Speed of vehicle: This is the single most critical factor affecting the design of all listed elements.
  • Horizontal alignment and Superelevation: Design speed directly dictates the minimum radius of horizontal curves and the required superelevation to safely negotiate the curve without skidding or overturning. Higher speeds require larger radii and/or higher superelevation.
  • Vertical alignment and Gradient: Design speed affects stopping sight distance and passing sight distance, which in turn dictate the length of vertical curves and permissible gradients. Higher speeds require longer sight distances and flatter gradients.
• (4) Height of vehicle: Primarily affects vertical clearances (e.g., under bridges, in tunnels). It does not significantly influence the design of horizontal/vertical alignments, superelevation, or gradient from a safety or operational perspective.

Step 3: Conclude the worst affecting factor.
Speed is the most dominant factor in determining the geometric design of highways, including horizontal and vertical alignments, superelevation, and gradients, because it directly impacts safety requirements (like sight distance) and dynamic forces (like centrifugal force). $$\boxed{\text{Speed of vehicle}}$$