Light Levels and Driver Perception of Speed: A study examining egospeed under simulated day and night lighting conditions in a rural setting

摘要

International studies show that globally, drivers are statistically more likely to be involved in collisions during the night than they are during the day. However, the exact mechanisms behind this have not been fully explored. The research carried out in the course of this thesis examined the possibility that the difference in light levels between day and night periods had an effect on drivers’ perceptions of speed on a rural road. Three experiments were performed in order to test this hypothesised link between light levels and driving speed.The first and second experiments were designed to examine whether light levels had an effect on egospeed discrimination ability at 60 km/h and 80 km/h (Experiment 1) and at 100 km/h (Experiment 2). The experiments used a psychophysical technique (method of constant stimuli) to measure the point at which two different egospeeds presented under day and night conditions appeared to be the same (the point of subjective equality, or PSE). The value of the PSE relative to the standard condition (60, 80, or 100 km/h) indicated whether egospeed was being underestimated or overestimate. The results of Experiment 1 indicated that participants were able to discriminate very small differences in egospeed (close to 6% in some cases) but that lighting level (day vs. night) did not have a strong effect on their perception of egospeed. Some participants perceived themselves to be moving faster during the night condition compared to the day condition at both 60 km/h and 80 km/h, but the difference was only statistically significant at 80 km/h. The results of Experiment 2 indicated that participants perceived themselves to be moving faster during the day condition compared to the night condition at 100 km/h, but that this was not to a significant degree. Large individual differences were found at all three speeds examined in Experiments 1 and 2.The third experiment focussed on absolute estimates of egospeed rather than on differences, and was designed to examine whether light level had an effect on judgements of absolute speed at 60 km/h – 100 km/h, through the use of a magnitude estimation task. Participants were shown individual day and night scenarios, and were asked to estimate the exact speed at which they perceived themselves to be moving. The results showed that light levels did not have a statistically significant effect on the speed at which participants judged themselves to be moving, but that they were able to distinguish between the different speed conditions quite well. However, participants’ absolute estimates of egospeed were greatly underestimated.The overall findings from all of the experiments indicate that, in general, light levels do not affect drivers’ egospeed perceptions, but that observers are quite sensitive to small differences in egospeed, and that their ability to judge these small changes is quite robust to the influence of light level.