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Abstract

Optic flow affects the walking speed during human pedal locomotion: when the ground texture is moved in a direction opposite to the walking direction, people slow down despite noticeable
changes in their physical effort. We hypothesized that in driving conditions, where no direct
matching between physical effort and produced speed exists, the role of the optic flow should
be even more pronounced. Similarly, fog is supposed to reduce the perceived driving speed, by
a reduction of the contrast of the visual scene. This, subsequently, should lead to a speeding
during fog [1]. We aimed to replicate these results using a large field-of-view, a real road
trajectory and a realistic exponential fog model. We used a driving simulation which was
back-projected on a large fronto-parallel projection screen (75 x 70 degrees). Participants were
trained to learn the relation between current driving speed (digital speedometer) and amount of
optic flow in the image plane. In the experimental phase, we varied environmental condition
(clear vs. foggy scene), speed signal (40, 60, and 90 km/h), and road texture motion. By adding
motion to the road texture, the apparent driving speed was increased to 150 or reduced to
67 of the actual driving speed with regard to the rest of the environment. After the beginning
of the trial, a speed signal indicated the required speed. The participants were instructed to
accelerate up to this speed, keep it for five seconds, and terminate the trial by button press.
The average speed of the last five seconds of each trial was included into the analysis. We
found a significant effect of the motion of the road texture on the produced driving speed. As
expected, participants increased the driving speed when the texture motion indicated a slower
speed and vice versa. We observed also a significant fog effect which is in opposition to the
previous results: when contrast was attenuated by fog the driving speed was reduced. Our study
raises serious doubts about the previous interpretation of the relation among contrast reduction,
perceived speed and driving behavior. In a realistic driving scenario, fog does not directly lead
to speeding. However, the large effect of texture motion, suggests that the optic flow, especially
originating from the road ahead, clearly determines the perception of the driving speed, even
in foggy conditions.