Caution! Slippery when wet. But it is becoming common to do something more than warn motorists. Sawing grooves into pavements greatly reduces wet weather skidding accidents on roads and airports. During the past 15 years more than 5 million square yards of highway pavements have been grooved and accident records verify that grooving creates safer highways. Airline pilots agree that grooving of runways gives better control of aircraft on landing. On highway pavements grooves have usually been sawed in the longitudinal direction to improve directional control of vehicles, especially on curves. Although longitudinal grooving generally does not increase the coefficient of friction significantly (as measured by standard skid trailers) it does bring about a reduction in wet weather skidding accidents. Transverse grooves are more effective than longitudinal for increasing the coefficient of friction and reducing stopping distances. Airport pavements are always grooved transversely. This grooving and that at high-speed turnpike turnoffs is commonly one-fourth inch wide and one-fourth inch deep at one and one-fourth inch spacings. Grooving is done with machines equipped with cutting heads composed of multiple 12 inch diameter diamond tipped cutting blades. The diamond segment extends approximately three-sixteenths of an inch from the hard steel centers on which the segments are welded. After the cutting head has been set for a required depth, width and spacing of cuts, its position is usually controlled electronically with gages sensing the pavement surface. While cutting heads for precise grooving in lose quarters may have as few as 29 diamond blades and a width as small as 21 inches, high production models may have more than 80 blades and be able to cut grooves longitudinally in widths of up to 6 feet in one pass. Cutting heads usually rotate at about 2,000 revolutions per minute, with water being used as the coolant for the diamond blades.