Like low pavement skid resistance, the aerodynamic phenomena of splash and spray are evident only when the pavement is wet. However, unlike low pavement skid resistance, splash and spray are separable and uniquely identifiable: (1) Splash is the mechanical action of a vehicle's tire forcing water out of its path. Splash is generally defined as water drops greater than 1.0 mm (0.04 in.) in diameter which follow a ballistic path away from the tire. (2) Spray is formed when water droplets, generally less than 0.5 mm (0.02 in.) in diameter and suspended in the air, are formed after water has impacted a smooth surface and been atomized. Several factors must be present: (a) water in the tire path (the thicker the water film, the greater the quantity of splash and spray generated); (b) pavement surface characteristics (thicker water films are retained on a nonporous surface); and (c) turbulent and high-velocity air masses generated by the combination of vehicle configuration and speed.
The Federal Highway Administration (FHWA) splash and spray research included driver simulator and wind-tunnel experiments, full-scale component, vehicle configuration, and countermeasure evaluations on test tracks, and cost-effectiveness analyses. The main findings were sixfold: 1. The driver simulator—A driver would not lose control of the vehicle due to the temporary visual obscurity due to the spray cloud, the turbulent air mass suspending the water droplets. 2. The wind tunnel—The areas were found where the spray cloud would be the densest, that is, the vehicle bow wave, around the wheels, and at gaps. 3. The full-scale component evaluations—Splash is water thrown away from the tire to the front (tire bow wave), to the side (side wave), and to the rear (tread pickup). Spray is water that the tire has picked up due to capillary adhesion and is stripped from the tire as it rotates and is atomized as it impacts the vehicle undercarriage. The splash from the tire that impacts the vehicle undercarriage will also be atomized and contribute to the spray cloud; however, only tread pickup is a major contributor to the cloud. 4. The full-scale configuration evaluations—these confirmed the wind tunnel findings. 5. The full-scale countermeasure evaluations—Spray can be effectively reduced by on-board countermeasures, such as baffles, dams, and drag shields, which reduce the size of the turbulent air mass. 6. The cost—effectiveness analyses-The most appropriate on-board countermeasures are cost effective on both regional and annual bases.
However, like improvement of skid resistance, the most effective and global countermeasures to reduce splash and spray are those applied at the pavement surface to reduce the amount of water in the tire path.