This can be unlocked by either winning Championship Races in the Career or in the Single Race mode at the main menu.
Finish in 1st place in 50 races for this to unlock, the quickest and easiest way to do this is by going into the Single Race menu and setting up the pre-race options with the following values:
Nitro Booster: On
Vehicle Damage: Off
Number of Laps: 1
Number of Opponents: 1
Opponents' Vehicles: Mixed
Oppenents' Strength: Easy
Complete 50 of these 1 Lap races, making sure you finish in 1st place each time.