Hairpin bend on a mountain road: which of the following geometric provisions are correct for a standard one-lane hairpin (switchback) on a highway in hilly terrain?

Introduction / Context:
Hairpin bends (also called switchbacks) are sharp 180-degree turns used on hill roads to gain elevation within limited horizontal distance. Highway geometric design in mountainous terrain sets minimum values for curve radius, transition length, and widened roadway at the apex to maintain safety, comfort, and operability for mixed traffic and heavy vehicles.

Given Data / Assumptions:

• Single-lane hairpin bend on a hill road.
• Standard practice values are being referenced for minimum inner radius, transition length, and apex widening.
• The alignment may require a compound circular curve to fit topography while keeping speeds low and sight distance adequate.

Concept / Approach:

The inner curve radius ensures that even at very low speeds, vehicles can negotiate the turn without excessive side friction or encroachment. Transition curves help vehicles gradually develop superelevation and change in curvature. Extra width at the apex accommodates off-tracking of longer wheelbase vehicles (like buses and trucks) and the lateral placement variability of drivers.

Step-by-Step Solution:

Verification / Alternative check:

Model the swept path for the design vehicle (for example, a bus) on the given radius and evaluate lateral clearance and mountable shoulder needs. The listed values provide satisfactory envelope clearances in standard practice.

Why Other Options Are Wrong:

• Each individual statement reflects accepted minimums; hence the combined correct choice is “All the above”.

Common Pitfalls:

• Providing standard two-lane widths at hairpins without extra apex widening can cause encroachment and side-swipe risks.
• Omitting a transition leads to sudden curvature entry, increasing skidding risk on wet surfaces.

Final Answer:

All the above.