A consistent alignment which provides no ‘surprises’ for the driver is more important than an absolute design speed standard in terms of safety. A practical approach needs to be taken for alignment because the lay of the land and the earthworks balance dictates what can be done. The coordination of horizontal and vertical geometry is important to ensure:
- Safety and visibility
- Good traffic flow
- Acceptable level of visual impact of the road.
The diagram below shows how different curve radii and values of superelevation affect the design speed.
Alternatively, the minimum radius (Rmin) required for a given design speed and superelevation can be derived from the following formula:
v = design speed in km/hr
n = superelevation (m/m or %/100)
f = coefficient of friction (use 0.08 for gravel roads).
The design speed (v) is determined by the desired road standard and the terrain.
The superelevation (n) is the slope of the road surface and is expressed as a decimal. A maximum of 0.0626 (6.26%) is appropriate for logging trucks. For speeds of less than 26 km/hr, superelevation is not needed and normal crossfall is used.
The minimum road curve radius for highway trucks (even at very low speeds) should not be less than 18 m. At a lower radius, the trailer inside tracking becomes very significant, and the truck minimum turning circle may not be able to be accommodated. For off-highway trucks and stem trucks the minimum curve radius will be greater than 18 m and can typically be 40 m.
Curve radius, superelevation and design speed relationship