Test: Highway Geometric Design -2 - Civil Engineering (CE) MCQ

Mechanical widening 
where, n = no. of lanes
For 10.5 m wide pavement,
n=3

Ruling gradient is the maximum gradient within, which the designer attempts to design the vertical profile of a road.

Vertical curve:

Vertical alignment is the elevation of the centerline of the road. It may be in the form of a circular arc or a parabola

The vertical curve is divided into two categories

1) Summit curve:

It is a crest curve with convexity upward.

The deviation angle between the two interacting gradients is equal o the algebraic difference between them.

When an ascending gradient meets with descending gradients the deviation angle will be maximum

N = n1 – (-n2) = n1 + n2

Ideal shape of summit curve is circular. 

But Square parabola is the best shape of the summit curve due to good riding qualities, simplicity of calculation, and uniform rate of change of grade.

2) Valley curve:

The maximum possible deviation angle is obtained when a descending gradient meets with an ascending gradient.

The overtaking sight distance is not included in the valley curve.

The cubic parabola is generally preferred.

The headlight sight distance available at the valley curve should be at least equal to the stopping sight distance.

When a fast moving vehicle travels along a summit curve, the centrifugal force will act upwards, against gravity and hence a part of the pressure on the tyres and spring of the vehicle suspensions in relieved. So there is no problem of discomfort to passengers on summit curves, particularly because the deviation angles on roads are quite small. The only problem is designing summit curves is to provide adequate sight distances. The stopping sight distance or the absolute minimum sight distance should invariably be provided at all sections of the road system and so aiso on summit curves.