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 masks objects on the road. 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 of the reflective paint back to the driver's eyes. However, water acts like a lens which disperses the lights, so that 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.