Question

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

• A. The coefficient of friction used for the design of horizontal curves is lower than the coefficient of friction used in the computation of the sight distances.
• B. Centrifugal force at horizontal curve is counteracted by raising the middle of the pavement with respect to the edges.
• C. Grade compensation is achieved by increasing the gradient of the horizontal curve.
• D. Under identical conditions, the design length of the summit curve of a road having unidirectional flow will be greater than that of the same road having bidirectional flow.

Hint:

Check design friction values, camber vs superelevation, the direction of grade compensation, and how sight distance sets summit curve length for one-way vs two-way roads.

Answer 1

The Correct Option is A

Each option touches a different rule of highway geometric design. Let's test them one at a time against standard practice.

1. Friction for horizontal curves vs. sight distance: The side friction factor used to design horizontal curves is kept low, around 0.15, so vehicles do not skid sideways even in wet or worn-tyre conditions. The longitudinal friction factor used for stopping sight distance is set higher, around 0.35 to 0.40 depending on speed, because braking friction is generally available in larger amounts than side friction. Since 0.15 is lower than 0.35 to 0.40, this statement is correct.
2. Raising the middle of the pavement to resist centrifugal force: Raising the centre of the pavement relative to its edges is camber, and camber exists to shed rainwater off the road surface. Centrifugal force at a curve is countered by superelevation, tilting the whole cross-section so the outer edge sits higher than the inner edge. The statement mixes up camber with superelevation, so it is wrong.
3. Grade compensation by increasing the curve's gradient: A vehicle turning on a horizontal curve already faces extra resistance from the turning action itself. Grade compensation eases (reduces) the gradient at such a curve so the combined resistance does not exceed what would be felt on the ruling gradient alone. Increasing the gradient works against this purpose, so the statement is wrong.
4. Summit curve length, unidirectional vs. bidirectional: A one-way (unidirectional) road only needs enough sight distance for a driver to stop safely, using a small object height. A two-way (bidirectional) undivided road often needs the driver to see an oncoming vehicle or find a safe overtaking gap, needing a longer sight distance. Since a longer required sight distance calls for a longer summit curve, the two-way road tends to need the longer curve, not the one-way road as the statement claims, so it is wrong.

Only the first statement, about the friction factors, holds up against standard geometric design practice.

Let's summarize:

• Design friction for horizontal curves (about 0.15) is lower than that for stopping sight distance (about 0.35-0.40): true.
• Camber, not superelevation, raises the pavement centre; superelevation raises the outer edge: so that option is false.
• Grade compensation reduces, not increases, curve gradient: false.
• Two-way roads generally need longer, not shorter, summit curves than one-way roads for the same conditions: false.

So the only true statement is (A).