Exceptional gradient on hill roads: An exceptional grade of 1 in 12 may be provided on hill roads if the aggregate length of such stretches within a kilometre does not exceed

Introduction / Context:
Steep grades in hill roads must be strictly limited to maintain vehicle performance and safety. “Exceptional” grades are steeper than ruling/limiting values and are allowed only over short, controlled lengths.

Given Data / Assumptions:

• Exceptional gradient = 1 in 12 (≈ 8.33%).
• Control is by total length per kilometre of road.
• Design applies to mountainous terrain with typical traffic mix.

Concept / Approach:
Standards cap the cumulative length of exceptional gradients in any kilometre to prevent extended high power demand, overheating, and safety problems. A typical cap value used is 100 m per km for such steep stretches, with adequate transitions and sight distance maintained.

Step-by-Step Solution:
Identify the grade class as “exceptional”.Apply the aggregate length restriction per kilometre.Select the limit: 100 m per km.

Verification / Alternative check:
Vehicle gradeability charts show sustained operation above ruling grade rapidly degrades performance; hence the need to confine exceptional grades to short lengths.

Why Other Options Are Wrong:

• 45–90 m per km: more restrictive than typical allowances.

Common Pitfalls:

• Placing exceptional grades near sharp curves, compounding resistance.
• Failing to provide escape lay-bys or adequate signage on steep segments.

Final Answer:
100 m per km