Q.: We built a pavement that was supposed to be textured by transverse tining for skid resistance and good braking. However, we got caught by rain, which washed away the existing texture and prevented further tining. We are now required to groove it transversely. This requires closing two lanes, is a danger to the public, and is very slow. We would like to groove the pavement longitudinally but the state allows only transverse grooving. Do you have any information on the relative effectiveness of longitudinal and transverse grooving on skid resistance and braking?
A.: Walter B. Horne has written a technical report for the International Grooving and Grinding Association called Safety Grooving, Hydroplaning and Friction. In that report he says "Many highway authorities have long been puzzled by the seeming paradox that while longitudinal safety grooving does greatly reduce wet weather skidding accidents, friction coefficient measurements made before and after grooving often show little change. This result has led to the belief by many that grooving reduces hydroplaning but does not improve the coefficient of friction." However, he then points out that most highway departments measure friction coefficient by an ASTM standard method that requires the skid trailer to lock the test tire while traveling over the pavement at 40 miles per hour. He cites data from tests by the Pennsylvania Transportation Institute that show that, if the wheel is braked without locking, the friction coefficient is higher. Thus the braking effectiveness should be higher than most measurements made by the standard ASTM method indicate. Some of the data are also shown in Figure 4 of our article, "Skid Resistance Theory Related to Grooving Practice," published in the August 1983 issue, page 619.