Fortunately, the tire industry and federal authorities have gotten together to let us know when car tire replacement should take place. During the late 1960s and 1970s, the Department of Transportation published a series for Federal Motor Vehicle Safety Standards which mandated, among other things, that tires must be replaced when there is one-sixteenth-of-an-inch of tread left on the tire. To reinforce this, the DOT’s National Highway Traffic Safety Administration (NHTSA) mandated that all tires have treadware bars that show when there’s one-sixteenth-of-an-inch of tread left. These bars are perpendicular to the standard tread. When you see them, you know it is time to replace the tire.
For more info, see How Often Should You Replace Tires