I was in a 60 officer department in the suburb of Washington DC. In my county alone there were 23 municipal departments, the County Police, the County Sheriff, US Park police, Maryland Park Police, Department of Natural Resources Police. Maryland State Police, Maryland Transportation Authority and several I am sure I am forgetting. I attended the academy in a different county and routinely interacted with members of departments in several other counties and states, including Washington DC.
I have never heard of any agency other than New York City and the Metropolitan Police of London having a policy where you could not wear a uniform off duty. Almost all of the agencies I named also have take home car programs where you use a cruiser off duty as if it were a personal vehicle.