Weather and Accidents: Rain & Fog
Rain reduces driver perception in several ways 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 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.
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.
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 the eye to misaccommodate, 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
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