DSI and WSI indicators Driving Safety Indicator (DSI) and the snowflake symbol (Winter Safety Indicator) improve driving comfort and safety. The numbers indicate the depth of the main grooves in millimetres. The numbers fade away as the tire wears down. For safe winter driving, the required groove depth is between 4 and 5 millimetres. The snowflake symbol remains visible until the groove depth is four millimetres. Once the snowflake symbol wears off, the driver should buy new winter tires to ensure adequate safety.