Which of the following statements is/are TRUE in the context of the geometric design of highways?

The correct answer/option is: The coefficient of friction used for the design of horizontal curves is lower than the coefficient used in the computation of the sight distances.

Let us analyze the statements step-by-step to understand why this option is correct and why the other statements are incorrect:

1. Coefficient of Friction in Highway Design:
In highway geometric design, two types of friction coefficients are considered:
- Longitudinal Coefficient of Friction: This is used in the computation of sight distances (like Stopping Sight Distance, SSD). It represents the friction acting along the direction of travel during braking. According to standard highway design codes (such as Indian Roads Congress - IRC), the design value for the longitudinal coefficient of friction ranges from 0.35 to 0.40 depending on the design speed.
- Lateral Coefficient of Friction: This is used in the design of horizontal curves to counteract the centrifugal force causing lateral skidding. The design value for the lateral coefficient of friction is taken as 0.15.
Comparing the two, the lateral friction coefficient used for horizontal curves (0.15) is significantly lower than the longitudinal friction coefficient used for sight distance calculations (0.35 to 0.40). Therefore, the statement is TRUE.

2. Centrifugal Force and Pavement Raising:
To counteract the centrifugal force on horizontal curves, super-elevation is provided. Super-elevation is achieved by raising the outer edge of the pavement with respect to the inner edge, not by raising the middle of the pavement with respect to the edges (which is camber, used for drainage). Thus, this statement is FALSE.

3. Grade Compensation:
When a sharp horizontal curve occurs on a steep gradient, the tractive effort is reduced due to both the gradient and the curve resistance. To keep the tractive effort within limits, the gradient is compensated by decreasing the gradient (ruling gradient) along the horizontal curve. Increasing the gradient would make it steeper and more difficult for vehicles. Therefore, grade compensation is achieved by decreasing (reducing) the gradient, making this statement FALSE.

4. Summit Curve Length under Unidirectional vs. Bidirectional Flow:
The design length of a summit curve is determined based on the sight distance requirements:
- For bidirectional flow (two-way traffic on a single lane or undivided road), the driver must be able to see oncoming traffic, requiring a larger sight distance (such as Intermediate Sight Distance or Overtaking Sight Distance, or at least double the SSD if passing is permitted).
- For unidirectional flow (one-way traffic on divided highways), the oncoming traffic hazard is absent, so the sight distance requirement is limited to the Stopping Sight Distance (SSD) for stationary obstacles.
Since the required sight distance for bidirectional flow is equal to or greater than that for unidirectional flow, the design length of the summit curve for bidirectional flow will be greater than or equal to that of unidirectional flow under identical conditions. Therefore, the statement claiming unidirectional length is greater is FALSE.