Have you ever wondered what keeps a car from sliding off the road when you go around a curve? Or, have you ever noticed that it’s unnecessary to reduce speed for safety or comfort purposes when driving through a curve, even at high speed? The reason is superelevation.

Superelevation is the banking of the roadway through a horizontal curve and serves to counteract the lateral acceleration impacting a vehicle when it traverses through a curve. Think about the curve in a racetrack to understand superelevation. It enables vehicles to travel safely at higher rates of speed.

Tangent (straight) sections of roadways are constructed at a normal crown, meaning a roadway’s surface is typically sloped two percent away from the centerline on undivided highways or two percent away from the inside edge of pavement on divided highways. The tangent sections must be rotated or transitioned into full superelevation. When a roadway is superelevated, the entire surface slopes in one direction; this, combined with speed and friction, helps to hold the vehicle on the road.

There are two types of transitions: spiral curves and standard superelevation transition.

Spiral curves provide a gradual transition along the horizontal curve by introducing a curve with a gradually changing radius, which provides a more natural path for the driver to follow. The transition from normal crown to full superelevation is accomplished over the length of the spiral curve. There is no one set method to determine the required length of a spiral curve as it varies by agency. Some highway agencies use spiral curves, but they are more commonly used with railways since a train must follow the track’s specific path. Spiral curves aid in reducing wear, preventing derailments, and providing a smoother ride.

The most common method used by state departments of transportation is standard superelevation transition. It contains two separate components: tangent runout and superelevation runoff. The tangent runout portion of superelevation is the length needed to remove the adverse crown from the road, and it is the length necessary to transition the outside lane from the normal two percent cross slope to zero percent cross slope. Superelevation runoff includes the length needed to transition from zero percent cross slope to full superelevation.

Runoff length is based on the width of the travel lane, the number of lanes being rotated, the required rate of superelevation, and other factors. In contrast, runout length is based on the normal crown cross slope, typically two percent, and the required superelevation rate. Unlike spiral curves that accomplish superelevation over the length of the spiral, the location of the runoff length must be located at the beginning or end of the horizontal curve. Thus, it must be balanced between both the tangent portion and horizontal curve of the highway. The location varies according to different state requirements, from 50 to 80 percent.