According to tests conducted by Consumer Reports, tread can give up a significant amount of grip when it's at the half way point. Worn tires can be very dangerous on wet roads. When the grooves are not thick enough to channel water water, you are more likely to hydroplane and be a hazard to other drivers around you. Even on dry roads, bald tires are more prone to tire failure and more easily punctured.
Tires are considered bald when one or more of they are 2/32 of an inch deep. Some manufacturers have made spotting bald tires easier by putting a series of horizontal bars at the base of the grooves.Depending on their warranties, tires should travel roughly 25,000 to 40,000 miles before their tread reaches the halfway point. Don't know if your tires are bald? Click here to see how to conduct a penny test!