Weather and Accidents: Rain & Fog

Marc Green

Rain reduces driver perception in and is especially debilitating at night. It both directly affects perception (seeing through rain) but also produces visibility changes through its action on headlamps, windshields, the road itself and road markings. We normally see an object when light from a source, the sun, streetlamps, our headlights, reflects from the object back to the eye. Rain interferes with this process in several ways.

• Rain makes headlamps and other light sources less effective by filtering away some of their light output and reducing illumination on the road ahead. Lighting effectiveness may be further lowered by the film of dirty water which passing cars splash on to the headlamps.


• When light strikes the raindrops, only a portion passes through while the rest scatters. The rain therefore blocks some of the light reflected by objects and less reaches the driver's eye.


• Some of the headlamp light becomes "backscatter," light reflected back by rain to the driver's eye. The backscatter acts as a veil and reduces contrast of everything in the field of view. The rain then blocks light coming back from objects in the road while simultaneously lowering their contrast by the veiling effects of backscatter. Backscatter also creates glare, which is defined as light much brighter than the driver's level of dark adaptation. Glare creates visual discomfort and lowers the ability to see contrast. Glare can impair vision for all drivers, but is especially bad for older people.


• The poor visibility causes people to concentrate their attention directly ahead in order to see where they are going. This decreases the probability of seeing with the peripheral field, so that, for example, it would be harder to see a car or pedestrian approaching from the side.

Rain also impairs vision in other ways. Rain affects ability to see through the car windshield. Even with windshield wipers operating, the splashing of rain periodically blocks vision. The rain acts like a lens which scatters lights and distorts the visual scene image. The clutter and movement caused by the raindrops falling on the windshield further draw attention and mask objects on the road. Wipers are never 100% efficient and typically leave a smear of water across the windshield. Lastly, wipers only sweep part of the visual field clean of rain.

Rain also affects visibility by changing the amount of light reflected from the road back to the driver's eye. Rain makes road delineations, such as crosswalks, less visible. The paint used in making road markings has a reflectorized material. In dry conditions, headlight illumination bounces off the reflective paint back to the driver's eyes. However, water acts like a lens which disperses the lights, so that much of it is reflected in different directions. As a result, the lines become almost invisible to the driver.

The same effect makes the road appear darker. A normal road has a rough surface, which causes some of the head light beam to reflect back to the driver's eye. Rainwater fills in the rough irregularities and creates a smooth mirror-like finish on the road. The headlight beams then strike the road and instead of reflecting back, reflects forward. The road appears darker and therefore it becomes harder to see pedestrians who are wearing dark clothes - their contrast is lower. The mirror-finish of the road also causes light from any other source, streetlamps, business signs or headlights of other cars, to be more likely to shine in the driver's eyes and cause glare.

Lastly, water reduces the reflectivity of most materials. Less illumination falling on the object is reflected back, so the objects appear darker and have lower contrast. Pedestrians in dark clothes would become even darker and could be more difficult to see.

Fog

Most people have heard of the horrific chain reaction accidents which occur in fog. It is hardly surprising, given the effects of fog on perception. Fog obviously produces accidents because a driver cannot see as far ahead. However, foggy conditions also promote accidents because they affect perceptual judgments of speed and distance. The effects are the result of reduced contrast. We see objects, not based on their absolute brightness or darkness, but on their difference between the object brightness and background. Fog lowers contrast substantially, causing objects to become fainter and less distinct.

Contrast Lower

The physical cause is the same as the scattering effect in rain - light hits moisture droplets and scatters. As droplet size decreases, however, both the type and mount of scattering change. Smaller droplets cause more scatter and especially more backscatter. Fog is produced by suspension of very fine droplets in the air, so the amount of scattering is even larger and there is more loss of contrast and more backscatter from headlights. This is the reason that you should not use high beams in fog.¹ It may seem natural to switch to high beams so that you can see farther. In fact, the main effect is to scatter more light back to the eye and actually decrease visibility.

The amount of contrast lost depends on the distance of the object and the coefficient of scatter of light in the medium. This in turn depends on the size and density of the moisture droplets suspended in the atmosphere. These values vary with location of the fog, as droplets in urban areas are usually smaller due to the dust particles in the air.

When air particles become very small, the situation is more complicated because not all wavelengths are affected equally. Air molecules, for example, scatter short wavelengths (blue) more, making the sky blue. This is why some experts advocate yellow fog lights. They believe that blue scatters more and that removing blue (to produce yellow) reduces the ability of fog to scatter lights. However, this is a mistake for two reasons. First, fog droplets are still far too large to selectively scatter light of different wavelengths, so yellow scatters the same as blue. Second, filtering the fog lights to produce yellow lowers their output and actually reduces visibility.

Misperceptions of Speed and Distance

There are several perceptual side-effects of this lowered contrast. One is that we judge motion to be slower and have a harder time discriminating between motion and motionless objects. As my own research has shown, ability to judge motion, especially out of the corner of the eye decreases markedly in low contrast. Imagine a person driving along a freeway and faintly seeing a car ahead. It will be very difficult to detect that it has stopped because the lower contrast impairs speed judgments. Moreover, the driver has an expectation that cars do not stop on freeways. The driver may then fail to realize that the car ahead is stopped until it is too late.

Fog can also cause a driver to underestimate his/her own speed. One way that drivers judge their own speed is by their movement relative to objects in the visual field. When driving fast, for example, telephone poles on the side of the road zip by. In fog, perceived speed relative to other objects is apparently lower, so a driver will likely underestimate his/her own speed. The driver will likely misjudge the time required to stop.

Fog also promotes accidents by means of "aerial perspective," a visual effect which causes people to misjudge distance. People automatically perceive objects which are low contrast and indistinct as being farther away. We learn this because of an environmental regularity: light from distant objects, such as mountains that are far away, must pass through more air molecules and becomes more scattered. Distant objects are then less indistinct and fainter. The driver approaching the stopped car will judge it to be farther away than it really is and therefore underestimate the time and distance needed to stop.

Accommodation Error - The "Mandelbaum Effect"

Accommodation is the physiological process by which the eye brings objects of different distances into sharp focus. Fog causes accommodation error, resulting in blurred vision for more distant objects. This is caused by the "Mandelbaum Effect," the tendency of the eye to approach "resting" accommodation under conditions of poor visibility. Since resting accommodation is relatively short, a little over three feet, objects down the road will be out of focus.

References

Green, M. (1986) Contrast detection and direction discrimination of drifting gratings. Vision Research, 23, 281-289.

Endnotes

¹ Fog lights work by capitalizing on the basic principle, "angle of incidence equals angle of reflection." They are mounted low and project light directly along the road. As a result, there is little scatter back to the eye.

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