Desirable Length of Spiral Curves for Two-Lane Rural Roads

Abstract

Most design standards recommend the use of spiral curves in the transition design. The advantages usually connected with their use are a gradual increase in the centrifugal force, a convenient arrangement for superelevation, and a satisfactory roadway appearance. However, some studies show a potentially negative effect of a long spiral transition on the driver's curve perception and safety. An analysis conducted on paths surveyed along 12 transitions with and without spiral curves confirmed the negative effect of excessive spiral length on driver behavior. The results of this analysis showed that the most desirable spiral length, which offers advantages in comparison with a tangent-to-curve transition, is equal to the distance traveled during the steering time. Therefore, a model to estimate the desirable spiral length for transitions of sharp horizontal curves on two-lane rural roads was developed on the basis of the data collected in three studies. Starting from the radius of the impending curve, the model calculates the desirable spiral length and gives a good description of real driver behavior, as observed in field surveys. The choice of the spiral length based on this model appears to be useful because the estimated length is consistent with the real distance traveled by the vehicle during the steering action, and this correspondence ensures optimal operating conditions for drivers.