A lot of Asheville residents abide by the Penny Test.
It's easy: place a Lincoln penny into the tread groove of your worn tire. If you can't see all of Lincoln's head, then the tire tread should be fine. Unfortunately, this isn't a very accurate way to test whether or not your tire tread is at a safe level, and could result in some dangerous trips driving around Fletcher.
The idea behind the Penny Test is that the performance or your vehicle doesn't really go down in poor driving conditions (rain, slush and snow) until the tire tread wears down to around 2/32" (which is where you will see all of Lincoln's head).
This idea is simply wrong. Many states consider 2/32" of tread the level at which a tire's tread is worn out. However, 4/32" or more is needed to maintain good and safe traction on your tires.