County where I worked tried soething like this, but way more broad. They included in prohibited places any vehicle used to conduct job duties, including personally owned vehicles regardless of where parked. (So your own car at your house.) Also, the exception was written to only include deputy sheriffs in uniform on duty, which tossed out detectives, probation officers, bailiffs, and investigators for the prosecutor. By the time they got done listening to all of the complaints, it was simply dropped in embarassment.
