Types of gradient and selection

Gradient (grade) is the rate of rise or fall of the road along its length, expressed as a percentage (or 1 in n).

Types of gradient

1. Ruling (design) gradient: the maximum gradient used in the design of a road; heavy vehicles can climb it without loss of speed. IRC: plain/rolling 3.3% (1 in 30), mountainous and steep terrain 5%.
2. Limiting gradient: a steeper value used where the topography makes the ruling gradient costly. IRC: 5% (1 in 20) in plain/rolling terrain, 6% in hills. It should not be used over long continuous lengths.
3. Exceptional gradient: the steepest value allowed in short stretches (not more than 100 m) under unavoidable conditions. IRC: 6.7% (plain/rolling), 7% (hills). Nepal's hill roads allow higher values depending on the road class in NRS 2070.
4. Minimum gradient: the least grade needed for longitudinal drainage on kerbed roads, in cuts or on flat terrain: about 0.2% (1 in 500) for lined (concrete) drains and 0.5-1% (1 in 200 to 1 in 100) for unlined earth drains.
5. Average gradient: the ratio of the total rise or fall to the length of a section; used for assessing hill roads.
6. Momentum gradient: a steep grade following a downgrade, which a vehicle climbs using its momentum.

(Figures follow IRC; refer to NRS 2070 for the exact table by road class and terrain.)

Factors in selecting gradients

1. Terrain - plain, rolling, hilly, or mountainous.
2. Design speed and the class of road.
3. Traffic volume and composition - a high proportion of trucks needs flatter grades.
4. Construction and maintenance cost - flatter grades require more earthwork in hilly ground.
5. Vehicle characteristics - power-to-weight ratio, tractive resistance, brake performance.
6. Drainage needs and climatic conditions (snow, ice, rain).
7. Length of grade - long grades need a flatter gradient; provide level stretches for rest.
8. Horizontal curves - provide grade compensation.
9. Safety: sight distance on vertical curves, braking on long downgrades.