I work with cops in PA all the time , there is no "required signage" for video or photos, they will and have brought successful charges based on evidence even from covert game cameras. You just have lazy/overburdened cops or a corrupt judicial system in your area.
Audio is a different matter, many states in the US (inc PA) prohibit audio recording without "informed consent", these are old laws dating back to the earliest portable recording equipment ,1930's/1940's ? The key issue it all comes down to is that while someone may realize they can be photoed/(now videoed) anywhere , they have a "reasonable expectation of privacy" in spoken communications".
So while a police car will have forward looking and back seat video capabilities, there is no audio recording, even if you lock two people who are under arrest in the back of a cruiser, anything they say is inadmissable . And when the officer exits the car to interact with an individual he cannot activate his lapel microphone unless everyone who will be heard consents. This is also the same premise that prompts the recording that states "this call may be recorded for quality control purposes" when you call many tech support lines or call centers for large companies.
As far as the cameras themselves buy the best resolution you can afford and record at high resolution , there is nothing more disappointing then getting something recorded and not being able to identify someone/thing because your cameras stink or you were recording at 360X240.
Ray