Charles Cochran, a Project Manager on the Traffic [...]
By Mark Randall, P.E.
Similar to how NASCAR tracks utilize “banking” to help a car’s downforce and velocity through a turn, civil engineers use superelevation on regular roadways to do the same thing. While some tracks, such as the Talladega Superspeedway, are banked up to 33 degrees (an increase of 6.5’ in vertical elevation every 10’), typical drivers experience similar banking methods at much lower degrees. Superelevation is the “banking” of a roadway through a horizontal curve, and it serves to counteract the lateral acceleration that acts on a vehicle as it traverses a horizontal curve. Superelevating curves enable vehicles to travel at a higher rate of speed while also striving to ensure the safety of drivers.
Drivers who aren’t on the NASCAR track may experience such superelevation when driving through any type of a horizontal curve in the road. Exceptions would be in urban or highly congested areas. In these cases, instead of experiencing superelevation, drivers will experience another mitigating device, such as a lower speed limit.
It is important to remember that there are practical limits to the amount of superelevation that may be used on a curve. The limits are related to climate, neighboring land use, the amount of slow moving traffic, and constructability. Superelevation rates need to be limited in areas that are prone to snow and ice since steep banking can cause still or slow moving vehicles to slide off the road. Also, Superelevation rates in urban areas or in locations that are prone to congestion are often lowered or removed all together due to the difficulty of warping pavement at intersections. This is also due to the problems that are encountered by introducing high rates of superelevation in areas that tend to be lower in speed. Since higher speeds are required to “hold” the vehicle in position on curves with steeper superelevation rates, slow moving vehicles are forced to “steer up the slope or against the direction of the horizontal curve” which is counterintuitive to driver behavior.
In reference to these limits, superelevation guidelines were developed by the Federal Highway Administration (FHWA). They have been set forth in “A Policy on Geometric Design of Highways and Streets,” or the “Green Book” as it is more commonly known. State authorities refer to these policies for their design standards.