These are the warning labels on the back of a mobile electronic traffic sign--the kind that usually says something like, "Mass Ave under constr until 2019. Expect Delays. Seek Alt Route" or "Be Safe. Enjoy The Prom. Belmont PD."
I was struck by the strange regularity of the decreasing size of the labels. At first I hoped they were just different translations of the same "do not stop cooling fan with face" warning. But they were actually different warnings, of varying severity.
This one is currently on Garden St. near Harvard Square. Inactive, for now, but still dangerous.