Cheat codes:
From any menu, type the following cheats to activate them:

rushhour = race with lots of traffic on the road.
empire = race on the Empire City track.
elnino = race with the El Nino car.
merc = race with the Mercedes CLK GTR.
gofast = make your car go super fast in Single Race mode.
cars = enable all cars including Pursuit Vehicles.
testdrive = Another cheat to drive a Mercedes
jag = race with the Jaguar Sport XJR-15 

Type the following cheats and click RACE to drive the different non-player cars.

go01 = Mazda MX-5
go02 = Toyota Landcruiser
go03 = Cargo Truck
go04 = BMW 5 Series
go05 = 71 Plymouth Cuda
go06 = Ford Pickup with Camper Shell
go07 = Jeep Cherokee
go08 = Ford Fullsize Van
go09 = 64/65 Mustang
go10 = 66 Chevy Pickup
go11 = Range Rover
go12 = School bus
go13 = Taxi - Caprice Classic
go14 = Chevy Cargo Van
go15 = Volvo Station Wagon
go16 = Sedan
go17 = Crown Victoria Cop Car
go18 = Mitsubishi Eclipse Cop Car
go19 = Ford Falcon XR8
go20 = Range Rover Cop Car/Ranger Vehicle
go21 = Cargo Truck (same as 03)