in the usa you have states made up of counties made up of cities.
some city police have their own machines, usually light, then each county has a sheriff's department and they have some too, usually light and medium, then there are the state coppers, i'm sure they would have one or two of each.
then there are the federal marshalls and the fbi and the national guard. the mind boggles.
then there is the private medevac companies, hundreds of them. there are more helicopters over there than you can poke a stick at.
helicopters fill the air and each one has its own role to play.