The effectiveness of police stems largely from their team training, like soldiers - always backing each other up. Procedure is, never try to handle a perp alone. So of course, their radio is always on. Historically, the introduction of radio revolutionized law enforcement.
Let's say you're planning a job. Naturally, you want to interfere with police response. Or maybe you want to isolate a single patrol cop. How about a vehicle with a transmitter, tuned to police band, blasting noise? Could you nullify their communications? How long would it take to locate the transmitter? How complex would such a project be?
I figure this must have been a plot theme for a cops & robbers movie at some point -
-- Rich