One would expect that the markings M+S or Mud and Snow on a tire would mean that it was designed for proper winter traction in all conditions. You might be surprised to find that it only defines a tire whose treads:

• Have multiple pockets or slots in at least one tread edge that extend toward the tread center at least 1/2 inch from the footprint edge
• Measured perpendicularly to the tread centerline, have a mimimum cross-sectional width of 1/16 inch< /li>
• Have edges of pockets or slots at angles between 35 and 90 degrees from the direction of travel
• Have a contact surface void area will be a minimum of 25% based on mold dimensions

It says nothing about the tire's rubber compound and it's ability to stick to compact snow and ice. In fact, at temperatures below 7 C. it can be expected to perform just like a summer tire in these conditions.
If you are like most tire buyers, my guess is that you choose an all season tire based on the manufacturer's mileage durability claims. This means a rubber compound that stays hard and does a poorer job of sticking to compact snow and ice.
Tires with the mountain and snowflake design are rated for their ability to provide traction in winter conditions where the temperature is below 7 C. Think of them as low temperature tires and choose them over all season tires when you drive on BC's winter road environment.
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