Road design considerations
Two other features need to be considered when designing roads. These are the line of sight and superelevation. These two features can contribute significantly to the safety of your road resign.
Line of Sight Curve
Safety requirements dictate minimum sight distances in zones where passing is permitted and in non-passing zones to allow adequate stopping distance if there is an obstruction on the roadway.
In the image above, the sight distance AS (or C) can be summarised in the following equation.
This is a practical but not exact equation. This assumes that the value of m is usually small in comparison to R, however C is not the true stopping distance given that cars travel on either the inner or the outer lane. The result of this equation includes safety parameters commonly adopted in road design.
Superelevation
The effect of centrifugal force on a vehicle as it passes through a curve must be countered by raising the outer edge. The difference in height between the outer edge and the inner edge of the road is known as superelevation. The outer edge is raised incrementally through the transition curve until the beginning of the circular curve where it remains constant until the exit transition curve where the outer edge is incrementally lowered. Roads are slightly more complicated than railway conditions and a road normally does not have a level cross section to allow water to drain away to road edges. However, it is common practice to raise the outer lane to level (with the centreline) before beginning the superelevation.
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1. horizontal |
The equation for approximating the superelevation is as follows.
Where:
V = velocity in metres per second.
R = the radius of the curve in metres.
g = gravity. That is, 9.8m/sec2
e = the superelevation in radians
f = the coefficient of friction (nominally 0.35 on bitumen for 50km/h, 0.25 for 80km/h and 0.12 for 100km/h).