With wide variation in vehicle speeds on curves, there usually is an unbalanced force whether the curve is superelevated or not. This force results in tire slide thrust which is counterbalanced by friction between the tires and surface. The counterforce of friction is developed by distortion of the contact patch area of the tire.

The coefficient of friction f is the friction force divided by the mass perpendicular to the pavement and is expressed as the following simplified curve formula:

f = V2/(127*R) - e/100

This coefficient has been called lateral ratio, cornering ratio, unbalanced centrifugal ratio, friction factor, and side friction factor. Because of its widespread use, the last term is used herein. The upper limit of this factor is that at which the tire is skidding or at the point of impending skid. Because highway curves are designed to avoid skidding conditions with a margin of safety, the f values should be substantially less than the coefficient of friction of impending skid.

The side friction factor at which skidding is imminent depends on a number of factors, among which the most important are the speed of the vehicle, the type and condition of the roadway surface, and the type and condition of the tires. Different observers have recorded different maximum rates at the same speeds for similar composition pavements, and logically so, because of the inherent differences in pavement texture, weather conditions, and tire condition.

In general, studies show that the maximum side friction factors developed  between new tires and wet concrete pavements range from about 0.5 at 30 km/h to approximately 0.35 at 100 km/h. For normal wet concrete pavement and smooth tires the value is about 0.35 at 70 km/h. In all cases the studies show a decrease in friction values for an increase in speed.

Curves should not be designed directly on the basis of the maximum available side friction factor. The portion of the side friction factor that can be used with comfort and safety by the vast majority of drivers should be the maximum allowable value used for design. . . .

In selecting the maximum allowable side friction factors for use in design, one criterion is the point at which the centrifugal force is sufficient to cause the driver to experience a feeling of discomfort and cause him or her to react instinctively to avoid higher speed. The speed on a curve, at which discomfort due to centrifugal force is evident to the driver, can be accepted as a design control for the maximum allowable amount of side friction. . . .