Winter Driving Safety Indicator, DSI The snow flake symbols in the central tread blocks signal that the tyre is safe in winter use. When the snow flake wears off, it is time to change to new winter tyres. The Driving Safety indicator (DSI) indicates the groove depth. The row of numbers on the tread indicates the extent of the remaining tread in millimetres. As the wear progresses, the numbers disappear one by one.