The Illinois Depatment of Transportation (IDOT) has been involved in pavement friction testing with locked-wheel equipment since 1968. In 1980 IDOT started testing with a smooth (blank) tire in addition to the treaded (ribbed) tire. Since 1985 IDOT has routinely tested with both smooth and treaded tires and has accumulated a multitude of test data. In 1987, IDOT developed tentative guidelines for evaluating both smooth-and treaded-tire data in analyzing high wet accident sites. The history of friction testing equipment development in Illinois and the upgrading process used for constructing IDOT's current friction testers are described. A summary of various distributions of smooth-and treaded-tire friction test data collected since 1985 on IDOT's highway network is presented.