What is the collective noun for police cars?

There isn't a specific, widely recognized collective noun for police cars.

However, you could use some descriptive phrases to refer to a group of police cars:

* A fleet of police cars

* A patrol of police cars

* A contingent of police cars

* A line of police cars

* A squadron of police cars (more informal)

The best choice would depend on the context and the specific situation you're describing.