When centerline deflections exceed the values in Table 2.5, it is necessary to introduce
a horizontal curve to assist the driver. Curves are usually accompanied by superelevation,
which is a banking of the roadway to help counteract the effect of centrifugal
force on the vehicle as it moves through the curve. In addition to superelevation, centrifugal
force is also offset by the side friction developed between the tires of the vehicle
and the pavement surface. The relationship of the two factors when considering curvature
for a particular design speed is expressed by the following equation: