ASL zone is a space where our language is used only in visual-spatial modality at some places such as schools for the Deaf, ASL classrooms and ASL instructors' offices, Gallaudet University campus, and Deaf gatherings where using voice is considered disrespectful. It's a known rule in Deaf culture.
That would mean that hearing people who do know ASL are expected to use ASL with other ASL speakers, whether hearing or Deaf.
A street zone sign can be seen at many Deaf schools, offices, Deaf clubs, and such.
Some schools for the Deaf have a street sign notice on a wall to remind.
A street sign at the Kentucky School for the Deaf. Photo by the KSD, June 2013. This one is really nice to push the unwritten rule into a formal notice.