Superelevation and SideFriction
Most highways will change directions several times over the course of
their lengths. These changes may be in a horizontal plane, in a vertical plane, or in
both. The engineer is often charged with designing curves that accommodate these
transitions, and consequently must have a good understanding of the physics involved.
The superelevation of the highway crosssection and the sidefriction factor are two of
the most crucial components in the design of horizontal curves. The superelevation is
normally discussed in terms of the superelevation rate, which is the rise in the roadway
surface elevation as you move from the inside to the outside edge of the road. For
example, a superelevation rate of 10% implies that the roadway surface elevation increases
by 1 ft for every 10 ft of roadway width. The sidefriction factor is simply the
coefficient of friction between the design vehicle's tires and the roadway.
Whenever a body changes directions, it does so because of the application of an
unbalanced force. In the special case of a body moving in a circular path, the force
required to keep that body traveling in a circular path is called the centripetal force.
When vehicles travel over a horizontal curve, it is this centripetal force that keeps the
vehicles from sliding to the outside edge of the curve. In the simplest case, where the
road is not banked, the entire centripetal force is provided by the friction between the
vehicle's tires and the roadway. If we add some sideslope or superelevation to the
crosssection of the roadway, some of the centripetal force can be provided by the weight
of the car itself.
High rates of superelevation that make cornering more comfortable during the summer by
requiring less frictional force, can make winter driving ponderous by causing slowmoving
vehicles to slip downhill toward the inside of the curve. Because of this, there are
practical maximum limits for the rate of superelevation. In areas where ice and snow are
expected, a superelevation rate of 8% seems to be a conservative maximum value. In areas
that are not plagued by ice and snow, a maximum superelevation rate of 1012% seems to be
a practical limit. Both modern construction techniques and driver comfort limit the
maximum superelevation rate to 12%.
The sidefriction factor has practical upper limits as well. As was discussed in the
braking distance module, the coefficient of friction is a function of several variables,
including the pavement type and the vehicle speed. In every case, the sidefriction factor
that is used in design should be well below the sidefriction factor of impending release.
In addition to the safety concerns, drivers don't feel comfortable if the roadway seems to
rely heavily on the frictional force. Several studies aimed at determining the maximum
sidefriction factors that are comfortable for drivers have been conducted. Some of the
results from these studies are tabulated below. (AASHTO, 1994)
Speed (km/h) 
Comfortable SideFriction Factor 
40 
0.21 
50 
0.18 
5580 
0.15 
> 110 
< 0.10 
The sidefriction factors that are employed in the design
of horizontal curves should accommodate the safety and comfort of the intended users.
The module on horizontal curve minimum radii will bring the effects of
the superelevation rate and the sidefriction factor together. Both of these concepts
contribute to the final alignment of horizontal curves.
