Hmm... a model won't do if the script actually calls for a cop driving the car, or cops getting in and out of it, or throwing handcuffed perps into the back seat.
If you have no other choice, why not? Look at the picture of the guy standing next to the model "Focus". If that is masked around a real car, he can get into it. Just replace that model with a cop car.
My buddy Kelly, who uses the model cars above does show a cop car driving by in his latest movie, but it's a model. Like I said, you can use a real car and you could have the cops shoving the guy into it, and then mask that real car with a cop car model. You could also put the model's mask in the foreground and have the cops walk behind it and put the guy in. If you really think through your After Effects, Composite Lab Pro/Vision Lab Studio, etc. programs, this is a viable alternate for someone with no budget.
Certainly, a real cop car is preferable. I know I needed one in THE AWAKENING, but we couldn't get a cop car to save our lives, so we composited cop lights onto a real SUV. I know what it was like to not be able to get the vehicle. Our no-budget movie didn't have the rental option or even permits. I have no idea what the original poster, ckThirdOak, has for a budget. If he has money, my suggestions are moot.
With the model that we greenscreened, we shot the girl walking up to the SUV and throwing it, without the SUV there. When we added it in, you could see her through the driver window. It was pretty cool.
I like compositing better than keying.