Tires: Replacing Winter Tires., winter tires, last winter

Question
How do you know when winter tires need be replaced?  My current winters look to have a lot of treadwear left on them, but they left me sliding on hard pack and ice a lot last winter.  The edges of the tread blocks are slightly rounded, as I think they were run all summer on the vehicle at least once.

Answer
Sean

As tires wear, their traction in adverse condition gradually decreases, and gets worse more rapidly as the tire wears.

Obviously you already know the tires have crossed the point where traction is an issue, so replacement is probably needed.