Investigating the role of visual and corresponding auditory stimuli in driving-related speed perception

Speeding is a main risk factor in traffic. Because drivers need to integrate a variety of information, investigating different perceptual modalities in speed perception can help develop interventions to mitigate speeding. Humans perceive traveling speed relative to the speed of the environment, which is also called “optic flow”. In spatial tasks, processing visual input often dominates processing of auditory input, but simultaneous presentation of information in both modalities can lead to intersensory bias. In the present study, we investigated such cross-modal interactions between visual and auditory information on a simulated road in two experiments (Experiment 1A and 1B). We applied the method of constant stimuli to assess speed perception. Participants were presented with short video sequences and decided via keypress which sequence they perceived as faster, so that it was possible to calculate the point of subjective equality (PSE). Stimuli were either unimodal visual (regular lights on both sides of the road on the asphalt) or bimodal visual-auditory (the same lights and added wind noise every time a light was passed), both either in static positions or animated in a way to give the impression of moving towards drivers. Both experiments showed that oncoming stimuli led to higher perceived speed (indicated by a lower PSE) than static stimuli. However, this influence on perceived speed was stronger in the bimodal visual and auditory than in the unimodal visual condition, suggesting the added influence of acoustic flow on the existing influence of optic flow. In sum, these findings suggest that earlier studies with unimodal visual stimuli underestimated the influence of oncoming stimuli on perceived speed and thus its potential role on mitigating speeding in traffic.