Water pressures build up under a tire as the speed increases. When these pressures become high enough they lift the tire completely free of the pavement, and when the entire tire footprint is riding on a water layer or film the tire is said to be hydroplaning.
Transverse safety grooving improves the drainage of pavements by providing smooth channels through which water can drain to the side. Water can flow in the channels with virtually no interference from wind or tires. Grooves 1/4 inch wide and 1/4 inch deep, located 1 1/4 inches on centers, have been found satisfactory for most pavements. Although extremely heavy rain can overload these channels, causing flooding of runways, grooves of these dimensions and spacing are adequate under most storm conditions. Spacing the grooves farther apart decreases the number of grooves under the tire footprint and makes grooving less capable of draining the pavement adequately.
On highways it is more common to cut grooves longitudinally so that grooving operations can be confined to one lane and cause less disruption of traffic. Longitudinal grooves do not help drain water to the side, but they do aid in draining from under the tire footprint. Safety grooving alleviates hydroplaning and can restore tire friction coefficients on wet pavements to near dry-pavement friction levels. Longitudinal grooving improves both cornering friction coefficients and braking coefficients on wet pavements. Spacings of 1 « inch or less are preferred.