Driving safety and aquaplaning indicator The driving safety indicator on the center rib of the tire indicates groove depth. The numbers stamped on the tread show the remaining groove depth in millimeters. The numbers fade one at the time as the tire wears down. The aquaplaning indicator on the center rib draws the driver’s attention to increased risk for aquaplaning. The indicator droplet disappears when groove depth wears down to less than 4 mm.