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Geometric Design: Professional Practice


The following excerpt was taken from the 1994 edition of AASHTO's A Policy on Geometric Design of Highways and Streets (pp. 142, 151-152).

There is a practical limit to the rate of superelevation . In areas subject to ice and snow the rate of superelevation cannot be greater than that on which vehicles standing or traveling slowly would slide down the cross slope when the pavement is icy. At higher speeds the phenomenon of partial hydroplaning can occur on curves with poor surface drainage that allows water to build up. Skidding occurs, usually at the rear wheels, when the lubricating effect of the water film reduces available lateral friction below the demand being made by cornering.

When traveling slowly around a curve with high superelevation, negative lateral forces develop and the car is held in the proper path only when the driver steers up the slope or against the direction of the horizontal curve. This direction of steering is an unnatural movement on the part of the driver and possibly explains the difficulty of driving on roads where the superelevation is in excess of that required for normal speeds. Such a high rate of superelevation is undesirable on high-volume roads, as in urban or suburban areas, where there are numerous occasions when vehicle speeds are reduced considerably because of highway traffic volume or other conditions.

Also some trucks have high centers of gravity and some cars are loosely  suspended on the axles. When these vehicles travel slowly on steep cross  slopes, a high percentage of the mass is carried by the inner tires.

(p. 151) Maximum Superelevation Rates

For a given speed, the maximum superelevation rate and the assumed value for the maximum side friction factor in combination determine the maximum curvature. The maximum rates of superelevation usable on highways are controlled by several factors: climate conditions, i.e., frequency and amount of snow and ice; terrain conditions, i.e., flat or mountainous; type of area, i.e., rural or urban; and frequency of very slow-moving vehicles that would be subject to uncertain operation. Consideration of these factors jointly leads to the conclusion that no single maximum superelevation rate is universally applicable and that a range of values must be used.

The maximum superelevation rate for open highways in common use is of the order of 10 percent or sometimes a maximum rate of 12 percent is used. . . .

. . . Where snow and ice are factors, tests and experience show that a superelevation rate of about 8 percent is a logical maximum to minimize slipping across a highway when stopped or attempting to slowly gain momentum from a stopped position.