Review of the Mechanisms of Visibility Reduction by Rain and Wet Road

Rain drastically modifies the visual environment of road users, particularly at night. It changes visibility through its effects on headlights, windshield, pavement and markings. Rain lessens the performance of headlamps and other light sources by filtering part of their luminous power, thus reducing the illuminance on the roadway ahead of the vehicle. Rain affects the capacity of the driver to see through the windshield. Rain also affects visibility by changing the amount of headlight retro-reflected by the road surface toward the driver. The film of water on the pavement makes delineation and pedestrian crossing markings almost invisible by cancelling the retroreflective properties of the beads in the painting materials. The same physical phenomenon makes the pavement appear darker than in dry conditions. This is a brief list of commonplace facts on the visual effects of rain [10]. In the following, we seek to explicit the physical and psychophysical ground of these facts, based on scientific references when available. The outline of the paper is as follows: in the first section, we study the visual effects of the falling rain; in the second section, we tackle the visual effects of sprayed water; in the third section, we list the properties of wet materials, specially pavement markings; finally, we propose synthetic diagrams describing the effects of rain on roadway visibility.