All marked cop cars whever I’ve been and have noticed have had large numbers painted on their roofs. Also on many taxicabs.
I wonder if these numbers are ever actually used. Does the sargeant ever say “take the chopper and see if you can find C109 out there somewhere”? The dispacher knows where all the units are at any given time anyway, right? Is this one of those holdovers from the old days?
Maybe if one is stolen (rare)?
Peace,
mangeorge
I think the numbers are mostly used in car chases. The “eye in the sky” helicopter can easily look down and coordinate the movement of the police cars below.
That’s always been my understanding.
The Doctor
Wiki has this to say about it:
The Doctor
When I was a kid, everybody called them “prowl” cars. Haven’t heard that term in many years. I think they say it in some old movies too.
But anyway, I can see why they’d care which agency is where, but not which unit in any one agency.
One thing for sure; it doesn’t hurt anything. 
Maybe in case of witness testimony? “How can you be sure it was officer Higgins’ car?”.
Many fire and EMS vehicles are similarly marked.
It could also be useful for helicopters to help coordinate groups of vehicles pursuing a suspect, particularly through city streets.
“Car 109, the suspect is approaching on your left.”
“Cars 67 and 25 will arrive for backup in a moment.”
“Car 54, you’re going the wrong way.”
Buses too in Sydney.
Really? I never noticed that (not that I’m usually in a position to, mind).
Sydney police cars have an abbreviation of their station or area command on the bonnet (CA for Campsie, BK for Bankstown, etc) and the car number on the roof. All Armaguard arnoured cash-carrying vehicles have a large ‘A’ painted on the roof, so they can be tracked by chopper in a robbery.
You must not have been many places. The cars in my department have 4 inch numbers on the side so each one can be identified by those using them. No numbers to be seen from above. I don’t recall any police cars in New Jersey with numbers on the top. I can understand it being done in California because of the use of police helicopters.
They do it in Philly and have since the '60s.
It’s basically just for identification.
They’re all marked on top around here, as well as fire and EMS vehicles, like danceswithcats said.
The fire engines and ambulances are marked so the Medi-Vac knows who to look for at the landing site at a multi-car accident site. The pilot needs to know exactly where to land, and knows which vehicle number to look for - that’s the spot where it’s safest to land.
Numbers on the top of vehicles are not only useful for tracking them from the air. They may also be used when viewing them from the upper floors of buildings, overpasses or other high places.
When buses go into the Port Authority Bus Terminal in New York, I believe there is a dispatcher on the upper floors of the building keeping track of the numbers of the buses that come up the ramp.
…and San Francisco.