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 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.

Frsc Mokwa's photo.

Rain impairs vision in other ways. Rain affects ability to see through the car windshield. Even with windshield wipers operating, the splashing of rain and the windshield wipers themselves periodically block 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.