DJ says they're legal in MI, but I honestly don't care if they're legal or not. I need them to be safe, and if they were illegal, I'd use them anyway. All the cops I've ever met around here have been very much interested in keeping people safe more than giving anyone a hard time (unless they NEED an attitude adjustment); I don't think any cop would tell you that you need to take your studs off.
If one did, I'd be sure to ask for their badge number and tell them that I was going to send a letter to their station indicating that they had instructed me to remove an important safety device from my bike, and any falls in traffic should be investigated to see if the studded tires that THEY told me to remove could have prevented my accident/injury/death. See what they say.
But that's not really going to happen, because no cop is going to tell you that.
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Work: the 8 hours that separates bike rides.