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

1. Review of the Mechanisms of Visibility Reduction by Rain and Wet Road Nicolas Hautière, Eric Dumont, Roland Brémond, Vincent Ledoux Université Paris-Est LEPSiS, INRETS-LCPC, Paris

2. Outline of the presentation • Objectives • Visual effects of the falling rain • Visual effects of sprayed water • Light reflections on wet materials • Summary • Perspectives

3. Objectives • In foggy weather, there is a standard dimension called « meteorological visibility distance » binding fog density and visibility distance: where k denotes the extinction coefficient of the atmosphere. • In rainy weather, we would like to build such a standard dimension. • However, the microstructure of rain makes it consequences on highway visibility very different from those of fog. • In this work, we reviewed the different mechanisms of visibility reduction by rain and wet road.

4. Rain is a population of water droplets falling. They interact with each other and with the environment. While falling, a rain drop undergoes rapid shape distortions: Rain drops come in a wide range of sizes. The Marshall-Palmer distribution usually binds rain particles density and sizes 25 mm/h 5 1 Visual effects of the falling rainThe nature and microstructure of rain shapes of rain drops Marshall-Palmer rain drop size distribution

5. Some experiments were conducted in an attempt to relate optical extinction on a long distance to rain density. The general relation found between the extinction coefficientks (m-1) and rain intensityR (mm.h-1) is: where a and differ with respect to the location and the optical devices used in the experiments. Light scattering in rain is rather limited. Using an analogy with fog: Visual effects of the falling rain Light scattering in rain Different experimental curves relating the atmospheric extinction coefficient and the intensity of the rain

6. To the best of our knowledge, there is no analytic model for the overall reduction of visibility induced by rain on the windshield. Some works focus on the appearance of rain drops: Several experimental studies on wiper usage focused on object visibility and seeing distance. In particular, [Bhise, 1981] investigated the visibility distance of target vehicles under natural downpours.The resulted model is: where rt characterizes the accumulation of rain water on the windshield and Lb the background luminance. Visual effects of sprayed waterVisual effects of rain on the windshield fish-eye lens effect created by a rain drop

7. The water sprayed by vehicles has undeniable effects on visibility. [Fournela, 2003] studied these effects. Unfortunately, no model came out of it, because of several experimental difficulties. Other studies showed that splash and spray is reduced by 95% on porous asphalt. The most rigorous works have been conducted for the development of heavy vehicle spray reduction devices. Even though these researches do not directly address driver visibility, their metering systems might be used to investigate this problem. Visual effects of sprayed waterVisual effects of water sprayed by other vehicles www.dynaplas.com.au

8. The water on a surface makes it specular because of the smooth air-water interface. Optical interactions on such a surface are governed by Fresnel equation for dielectric materials: A film of water on a Lambertian surface can also make the surface appear darker. This is mainly caused by internal reflections at the water-air interface. Subscattering effects may also exist. Light reflections on wet materials Water at the surface

9. Rain changes the visual aspect of the pavement. The pavement appears more specular or darker, depending on the observation angle. This can be dazzling for the driver, especially in daytime with the sun at grazing angles, or at night with opposing headlights. The visibility of retro-reflective road markings is also particularly impaired. In daytime, on wet roads, retroreflective materials sometimes appear darker than the pavement. When the water layer is higher than the size of the beads, markings may disappear. Light reflections on wet materials Consequences on highway visibility

10. Summary (1/2) • Daytime situation

11. Summary (2/2) • Nighttime situation Dry weather Rainy weather

12. Perspectives • Development of solutions to enhance the perception of the driver in rainy weather: • adaptive wipers, • anti-splash and spray devices, • porous pavement, • retroreflective markings • … • adaptive headlights. • The quantitative visibility models should enable to define scenarios and to compute the necessary power to compensate for the visibility loss, or to find alternative strategies to compensate for the loss of light backscattering by the pavement.

13. Thank you for your attention !