New generations Driving Safety Indicator The next-generation Driving Safety Indicator on the centre rib indicates tread depth. You can check the amount of remaining tread in millimetres by examining the row of numbers on the tread. The numbers and water drop symbol indicating a risk of aquaplaning disappear, as the tyre wears down. When the tread depth is below 4 mm, a red stripe that indicates low tread depth will appear around the tyre. When this happens, please purchase new summer tyres in order to ensure maximum safety.