I m sure Italian's police car is the fastest, they used Lambroghini Gallardo as a police car, that car normally travels on the highway between italy and the neighourhood countries!
I have heard that America in texas also use lambo, but i m not sure!
In singapore, the highway police used Subaru STi as police cars!
In my country, they used Audi S6, i m in south africa!
Go to this website and look at the lambo police car!
www.speedycars.net