How safe is the drive from Maztalan to Canun?
I don't know about safety but it is a LONG drive if you mean Mazatlan And even if you mean Mazatlan 31 you can't really drive to Cozumel because Cozumel is an island. You would have to take a ferry to get there.
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That is a very long trip. Try to avoid driving at night. Use the main highways. You should be ok.