SimCity 4: Rush Hour/Vehicles

In My Sim mode, you can have a free drive on any unlocked vehicles, or do any U Drive It missions that are avalible. This page tells you about the vehicles, including how to unlock them.

Police Car
The police car has a siren, which once turned on will cause all traffic to go to the side of the road to let you through. To unlock the police car, you need to have either the small or large police station somewhere in your city.

Police Van
This too has a siren, but will have different U Drive It missions than the police car. The police van can be unlocked by building either a large or deluxe police station in your city.

Fire Engine
It also has a siren, and can shoot water out of the hose by holding down the right mouse button over the area where the fire is (or just for fun, and make all the sims run away.) The fire engine can be unlocked by having either a fire station, or the fire department landing strip in your city.

Ambulance
It too has a siren, and the two U Drive It missions avalible for this vehicle are both to do with the commercial area. To unlock the ambulance, you need to build a hospital in your city.

City Bus
The city bus doesn't move particularly fast, and can be hard to turn around in a built up area. To unlock the city bus, you need a bus stop in your city. You should have at least two bus stops in your city, or else your sims will not be able to use them.