Design of Superelevation of Highway Curves: An Overview and Distribution Methods

Abstract

Superelevation is the banking of highway horizontal curves to assist the driver by counteracting the lateral acceleration produced by tracking the curve. Superelevation is expressed as a decimal, representing the ratio of the pavement slope to width, and ranging from 0.04 to 0.12. Proper superelevation allows a vehicle to safely turn at high speeds and will make riders comfortable. Centrifugal force is the outward pull on a vehicle traversing a horizontal curve. As a vehicle traverses a horizontal curve, centrifugal force is counter-balanced by the vehicle weight component due to roadway superelevation and by the side friction between tires and surfacing. Excessive centrifugal force may cause considerable lateral movement of the turning vehicle and it may become very hard to stay inside the driving lane. Superelevation and side friction are the two factors that help stabilize a turning vehicle. Inadequate superelevation can cause vehicles to skid as they travel through a curve, resulting in a run-off-road crash.\xa0 Trucks and other large vehicles with high centers of mass are more likely to roll over at curves with inadequate superelevation. There are practical limits to the rate of superelevation. High rates create steering problems for drivers traveling at lower speeds, particularly during ice or snow conditions. This paper presents an overview of the concept of highway superelevation, and AASHTO distribution methods that utilize both side friction and superelevation in the design of the highway horizontal alignments.