Tires must be replaced when the tread is worn down to 1/16 of an inch in order to prevent skidding and hydroplaning. An easy test: place a penny into tread groove. If part of the Lincoln's head is covered by the tread, you're driving with the proper amount of tread. If you can see all of his head, you should buy a new tire.
Built-in tread wear indicators. Or "wear bars," which look like narrow strips of smooth rubber across the tread will appear on the tire when the tread is worn down to one-sixteenth of an inch. When you see the "wear bars," the tire is worn out and should be replaced.
Visually check your tires for signs of uneven wear you may have irregular tread wear if there are high and low areas or unusually smooth areas. Consult your tire dealer as soon as possible.