I Can (Not) See Clearly Now 1

I Can (Not) See Clearly Now 1

I Can (Not) See Clearly Now:

Driving Performance in Foggy Conditions

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I Can (Not) See Clearly Now: Driving Performance in Foggy Conditions

Of all the environmental or atmospheric hazards facing drivers, perhaps none are more common or more dangerous than thick fog on the roadways. Brooks et al. (2010) contend that motor vehicle accidents attributed to foggy conditions claim an estimated 600 lives annually and result in more than 16,300 injuries in the United States alone. These devastating numbers can speak to the unique and complex challenges of understanding and accommodating drivers’ perceptual changes in foggy conditions. Research has shown that response times, headway, and speed perception are all drastically impacted by foggy conditions, though the nature of the effect varies depending upon the density of the fog, the distance traveled, whether or not a lead car is present, and, if so, the degree to which the lead car is visible.

The Accident Scenario and the Processing of Images in the Driver’s Visual Field

In the scenario in which a driver rear-ends a lead car despite the perception of adequate distance and braking time, a number of factors are at play. In foggy conditions, regardless of the density of the fog, perceptual thresholds are increased across a myriad of neural structures, including reduced perceptions of speed and motion operating across both the sagittal and fronto-parallel planes (Caro, Cavallo, Marendaz, Boer, & Vienne, 2009). This has been primarily attributed, according to the authors, to a reduction in contrast, which results in higher perception thresholds related to relative motion.Contrast refers to the degree to which objects are able to be perceived or distinguished from the background. In foggy conditions, contrast is reduced due to the scattering of light in water droplets.

Contrast plays a pivotal role in the perception both of speed and of distance. This is because visual processing relies upon a myriad of physiological structures which are absorbing and processing an array of visual cues in the process of translating sensory input into the perception of distance and speed. Thus, visual processing is at one ambient, taking in visual stimuli from across the visual field, as well as focal, pertaining to the objects encompassing the central vision (Mestre, 2007). Binocular visual cues, particularly those which pertain to the peripheral vision, play a particularly important role in the perception of speed. Visual stimuli at the left and right edges of the visual field provide important cues essential to the perception of speed. Reflective paint at the margins of the roadway facilitate the perception of speed, as do other peripheral visual markers, from trees to pedestrians to lampposts and other vehicles. In foggy conditions, these binocular visual stimuli are reduced, if not entirely absent. The visual optical flow is dramatically reduced, requiring the drive to compensate by shifting to focal vision. With the reduction of the visual optical flow, the loss of peripheral visual cues, perceptions of speed become highly inaccurate; frequently, this leads drivers to overestimate their speed and to decelerate.

This, initially, would seem to enhance driver performance by accommodating the higher response times logic suggests would be required in low visibility conditions. However, if ambient vision facilitates the perception of speed, focal vision, while also playing a role in speed perception, plays an even greater role in the perception of distance (Mestre, n.d.). This focal vision corresponds to monocular vision, in which both eyes fixate on the same central object(s), receiving the same relative sensory input at the central vision. This contrasts with the binocular input in which visual stimuli from the left and right eyes differ and must be processed, especially at the sagittal and front-parallel regions of the brain, in order to construct a perception of distance and speed via the ambient visual field. In monocular, focal vision, speed and, especially, distance perception are predicated upon the ability to perceive the focal object(s) in the foreground of the central vision. The ability to perceive the outline of the object as distinct from the object’s further facilitates visual acuity and the perception of speed and distance. In regard to focal, monocular vision related to speed and distance, visual cues from the roadway, such as the luminescence of the center roadway lines and relation to a central focal object, such as a lead car, facilitate perception in foggy conditions where the ambient, peripheral optical flow is limited or absent. In particular, perception both of speed and distance appears to be more accurate when the subject in motion and the focal object(s) ahead are moving in opposite directions, such as when a lead car decelerates or the subject in motion gains on the focal object (Caro et al., 2009). When contrast obscures the subject’s ability to clearly distinguish the focal object from its background, and particularly when the object’s outline is no longer clearly visible, perceptions of speed and distance are dramatically reduced. This can occur when the distance between the focal object(s) increases beyond the subject’s perception threshold (depending on his/her individual visual acuity) or, more salient for the purposes of this study, when atmospheric conditions, such as fog, impede the ability to view the focal object and its outlines and to differentiate it from the background (i.e. low contrast conditions).

The Presence of the Lead Car

In the scenario presented above, the presence of a lead car is a salient issue in regard to the perceptual processes at play in these foggy conditions. Caro et al. (2009) assert that one of the most significant perceptual factors is the distance between the lead car and the following driver. The authors invoke Koschmeider’s law (p. 380) in theorizing that the greater the distance to the lead car in foggy conditions, the lower the contrast and, proportionally, the lower the threshold of perceived motion.

Significantly, however, a distinction exists not only in the perceptual effects of distance to the lead car but also in the extent to which the lead car is visible. More specifically, if the contrast in relation to the outline of the car is low, reducing its visibility, then a marked increase in response time occurs, according to the authors’ study. Further, response times are not only increased when the taillights alone are visible, but the angle of the lights also plays a significant role in headway and response time. If the taillights are too narrowly spaced, perception of motion and speed is further compromised. This suggests, the authors note, that vehicle manufacturers should take into account taillight placement when designing automobiles for safety in these inclement conditions.

Global versus Local Processing (or the Top-Down and Bottom-Up Processes)

Bottom-up processing refers to the accumulation and transformation of stimuli in a uni-directional form, from the retina to the visual cortex in the brain. Bottom-up processing pertains specifically to the absorption of sensory stimuli from the ambient and focal visual fields, the processing of optical flows across increasingly complex visual systems in the eye and the brain. Top-down processing, conversely, is contextual and relies upon the higher centers of the brain, specifically the cerebral cortex. Top-down processing is fundamentally constructivist, relying on context and learned information to absorb, parse, and translate visual stimuli into meaningful data (McLeod, 2007). As relates to the accident scenario, bottom-up processing may be attributed to the accident because the fog reduces contrast and thereby dramatically reduces the amount of visual stimuli accessible to the processing centers of the eye and brain. In relation to top-down processing, driving in conditions of severe fog, where visibility in general and contrast in particular are low, there is little context for the brain to “construct” an accurate perception of speed and distance. Drivers often have little experience with locomotion through such low-contrast conditions and the brain, therefore, has not learned to contextualize visual stimuli in such conditions or to construct accurate perceptions of speed and distance through these stimuli.

Distance and lead car visibility are not the only factors in shaping perception of motion, speed, and distance. Another significant are the global versus the local visual cues at play. Thus far, what has been emphasized are the local visual cues, the bottom-up visual processes that build from a singular image frame, such as the taillights or the outline of the lead car. However, the authors found that global perceptual cues also factor in. In ordinary driving conditions, global optical flow, such as peripheral visual input from the left and right margins of the visual field (i.e. the pavement, the objects at the periphery of the roadway) may have an interference effect that is absent when these stimuli are cut off due to the presence of thick fog. Because perceptual cues are reduced down to the local optical flow—the taillights or outline of the lead car—response times may decrease. This is particularly true, the authors note, when the lead car is decelerating. In such a case, the reduction in distance increases the perception of motion and speed and decreases response time. This holds true both in foggy and in clear conditions, but the impact is magnified in foggy conditions because of the heightened effect of distance on perceptual acuity (i.e. shorter distances to the lead car=reduced response times).

Other Factors

In addition to factors such as distance, visibility, and relative motion (acceleration versus deceleration), driving performance in foggy conditions is impacted by a range of psychological, environmental, and physiological conditions. For example, Caro et al. (2009) describe the push/pull hypothesis, which theorizes that in foggy conditions, drivers are more likely to endeavor to reduce distances with a lead car in order to maximize the visual cues provided by the car’s outline and taillights (the “pull” hypothesis) while also seeking to increase distances between themselves and vehicles behind them (the “push” hypothesis).

The research conducted by both Caro et al. (2009) and Brooks et al. (2010) reveals that drivers in foggy conditions tend to overestimate headway and speed, but Brooks et al. found that drivers under time pressures are more likely to increase their speed even in foggy conditions and despite this tendency to overestimate speed and headway. These drivers do, however, tend to compensate in such conditions with heightened alertness to available visual cues. Thus, their response/deceleration times, according to the authors’ research, are not negatively affected.

Physiological and Environmental Factors

A physiological factor which may have led to the accident described in the hypothetical scenario above might be the driver’s eyesight. If vision is already compromised, this may further reduce the driver’s ability to perceive the visual cues afforded the outline of the lead car or the taillights, thereby mitigating and eliminating any benefit derived from the loss of global optical flow. Environmental factors impacting the scene could relate to the road conditions themselves. Reduced distances between the vehicles may cut response times in these situations, but they also cut deceleration/braking times. If road conditions are slippery, due to ice, snow, or rain, then drivers may not have the ability to brake as quickly, regardless of how fast their response time in applying the brakes is.

Implications

Both studies reveal the complex perceptual processes involved when drivers encounter foggy conditions. The data suggest that, though it may appear counterintuitive, a reduction in distance between vehicles may be more effective in reducing response times by enabling greater accuracy in the detection of speed and relative motion. This is particularly true when the outline and angle of the lead car is clearly visible. Further, the studies suggest that when fog or relative distance obscures the outline and angle of the lead car, taillight placement becomes paramount. Narrow spacing between the taillights has a detrimental effect in reducing perceptual thresholds relating to speed, distance, and motion. This suggests that automotive manufacturers have a mandate to incorporate these findings into future vehicle designs to promote the public safety relating to a hazard that claims hundreds of lives annually.

Conclusion

Fog is one of the greatest atmospheric hazards drivers face. It can come out of nowhere and can appear at most any time, effectively rendering drivers blind. Each year in the United States alone, thousands motor vehicle-related injuries and deaths can be attributed to fog. It is incumbent upon researchers, legislators, automotive manufacturers, and drivers to understand how fog impacts perception. Specifically, due to the reduction in contrast, drivers are likely to experience higher thresholds for perceiving speed, motion, and distance. The use of a lead car can be effective in counteracting the diminished perceptual acuity borne of low-contrast, foggy conditions, especially if the outline and angle of the lead car are clearly visible or if taillights are positioned at a proper distance to provide the angular cues needed to more accurately perceive relative motion. Thus, it may benefit drivers to follow lead cars more closely in foggy conditions, though only with heightened alertness to the local optical flow. Likewise, automotive manufacturers, transportation officials, and legislators should take into account the robust body of research revealing the vital role rear vehicle design plays in accommodating these hazardous conditions.

References

Brooks, J. O., Crisler, M. C., Klein, N., Goodenough, R., Beeco, R. W., Guirl, C., . . . Beck, C. (2011). Speed choice and driving performance in simulated foggy conditions. Accident Analysis and Prevention, 43(3), 698–705. Retrieved from

Caro, S., Cavallo, V., Marendaz, C., Boer, E. R., & Vienne, F. (2009). Can headway reduction in fog be explained by impaired perception of relative motion? Human Factors, 51(3), 378–392.

McLeod, S. (2007). Visual perception theory. SimplyPsychology. Retrieved from

Mestre, D. R. (2007). Visual factors in driving. Human Factors for the Highway Engineer. Retrieved from