Influence of Vertical Alignment on Horizontal Curve Perception: Phase I: Examining the Hypothesis

Abstract

The importance of the driver’s receiving precise visual cues from the road environment cannot be overstated. If the visual cues are confusing or in any way cause the driver to incorrectly assess the approaching road environment, the crash risk of the driver may increase. Of particular concern are the perceptual problems induced by superimposing horizontal and vertical curves. To investigate the effect of overlapping vertical alignment on the perceived horizontal curvature, dynamic and static computer-generated three-dimensional presentations of the driver’s view of a road were created. In Phase I of the experiment, data were collected to test the hypothesis that overlapping crest curves made the horizontal curvature appear sharper and overlapping sag curves made the horizontal curvature appear less sharp. The results of both presentation methods (dynamic and static) were in agreement and showed that the hypothesis was valid. However, the hypothesis was more evident in the case of sag curves, which is a more serious issue with respect to safety. The probability of erroneous perception, as influenced by vertical curves, increases as ( a) the sight distance increases, ( b) the horizontal curve radius increases, and ( c) the length of vertical curve per 1% change in grade decreases. Driver characteristics did not appear to affect the horizontal curve perception.