DSI and WSI indicators The tread surface is equipped with the Driving Safety Indicator (DSI) that is patented by Nokian Tyres and improves winter safety. The numbers on the centre rib of the tyre indicate the depth of the main grooves in millimetres. The numbers wear out as the tyre wears down. The snowflake symbol of the Winter Safety Indicator (WSI) remains visible to a groove depth of four millimetres. When the symbol wears out, the possibility of slush-planing, for example, increases substantially and, therefore, the motorist should purchase new winter tyres to ensure safety.