Flat or bald spots on tires are not only dangerous, they are potentially deadly when it comes to driving on wet roads. According to Consumer Reports, bald tires are more prone to hydroplaning on standing water. Hydroplaning is a state in which the vehicle no longer responds to the movements of the steering wheel. Bald tires also reduce effective braking.
Consumer Reports indicates it conducted tests that show even tires with 50-percent tread have lost significant gripping capability. The magazine reports that tires are actually bald when at least one groove is worn to 2/32 of an inch deep. New tires' grooves measure 10/32 of an inch deep. To help consumers spot bald tires more easily, many manufacturers have begun adding horizontal bars at the base of each groove. Once the bars are even with surrounding tread, the tires are considered bald.
Cars Direct reports that many states have enacted laws requiring motorists to drive on tires with a minimum amount of tread. It urges drivers who are unsure about their tires' tread to take their vehicles to a mechanic or tire shop to have the tread measured. It also suggests the do-it-yourself method that involves placing a penny in each tire groove and checking to see if the tread is deeper than the top of Lincoln's head. If it is, the tire has sufficient tread. If not, the tire is close enough to being bald that it should be replaced.