Selecting highway alignment in hilly terrain – primary control features In mountainous regions, the alignment of a highway is most fundamentally decided based on which topographic feature?

Introduction / Context:
Highway alignment in hilly and mountainous terrain must reconcile geometric design needs with topography, geology, and construction costs. While many constraints exist, the fundamental control often comes from the locations where a ridge can be crossed efficiently.

Given Data / Assumptions:

• Terrain: hilly or mountainous.
• Key objective: achieve feasible gradients and minimum earthworks.

Concept / Approach:
Saddles and passes are the naturally lowest crossing points along ridgelines. Choosing an alignment that threads these locations reduces the summit levels to be negotiated, keeps gradients reasonable, and minimizes excessive cutting or tunneling. Other factors (river crossings, unstable areas, hard rock) influence local adjustments, but the controlling decision is usually the choice of pass.

Step-by-Step Solution:
Map ridge lines and identify saddles/passes.Connect valleys using these low points to hold ruling gradients.Iterate to avoid unstable slopes and excessive crossings where practical.Therefore, the primary determinant is the selection of saddles or passes.

Verification / Alternative check:
Historical mountain roads and modern highways alike are laid through natural passes to minimize climb and earthwork, confirming the primacy of this criterion.

Why Other Options Are Wrong:
Hard cutting, river crossings, and unstable areas are important constraints to mitigate, but they do not by themselves dictate the overarching alignment like the availability of a pass does.

Common Pitfalls:
Overfocusing on minimizing distance instead of gradient; neglecting geohazard mapping when selecting passes.

Final Answer:
saddles or passes