winter tires have dedicated rubber compound to stay pliable in sub zero temps. Plus, they have sipes in the tread to bite into the compact snow/ice when there is a thin layer of water over. You can literally stop 30-feet sooner from 30mph on ice/snow with winter tires over all-season tires, and even more if comparing to summer tires.

It's like this. You wouldn't try and walk on snow/ice with dress shoes. You would put on boots with specific tread. Otherwise you'll fall on your arse

http://www.tirerack.com/winter/tech/...hid=120#doneed

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