Perceived speed and driving behavior in foggy conditions

Abstract

Control of speed and heading is the primary task for motor vehicle operators. While cognitive information is available from the speedometer, motorists probably rely more heavily upon extra-vehicle visual information. How is this accomplished in poor visibility conditions? Consistent with evidence that perceived speed decreases with reduced contrast, Snowden et al. (Nature, 1998) found that drivers underestimate their speed when operating a simulator in foggy conditions. The present study investigated whether this hazardous misperception and unintentional increase in speed also occur when driving an automobile on a rural public road. With the speedometer obstructed from view, 14 participants (mean age 35.79 yrs) drove around a closed road circuit three times under each of three randomly ordered visual conditions: clear view and two levels of reduced contrast (created by diffusing plastic filters on the windscreen and side windows). Dependent measures, collected on separate laps at 7 locations along the circuit, were: Verbal Estimates of speed; Adjustment of Speed to instructed levels (25 to 70 km/h); and Judgments of Minimum Stopping Distance (before colliding with a model wallaby). Actual speed was recorded at each judgment. All measures of perceived speed decreased significantly with reduced visibility: Travel Speed (58 to 49 km/h, F(2,13)=24.3; p<.0001); Verbal Estimates (50 to 44 km/h, F(2,13=7.95; p<.02); Minimum Stopping Distance (17.9m to 129.5m, F(2,88) 13.84 p<.0001). Linear regressions showed that speed adjustments were highly correlated with the target instructions (r2=0.95 to 0.96), with slopes declining from 0.91 to 0.80 with reduced contrast. These findings demonstrate that drivers travel more slowly in fog, and they estimate their speed with equal reliability, under clear and foggy conditions. Moreover, unlike laboratory findings, speed adjustment data revealed a tendency to overestimate speed in the fog.

Footnotes

 Supported by Franklin & Marshall College, Queensland University of Technology, and Queensland Transport.