Inspired by Susan's tread separation, I searched the 'net and couldn't find anything on this topic. So I'm re-posting here where it might get more views.

Contrary to Sheldon Brown's assertion, I don't believe that a hard or dry-rotted tire is safe. Maybe for low-speed tooling around, but not when traction is important, particularly cornering traction (commuting in traffic, descending, group riding).

Input, anyone? My instinct would be that the shelf life of a bicycle tire would be somewhat shorter than that of a motorcycle tire, but I really have no idea if the tread is even made of the same materials.