Activate the following cheats by getting the corresponding numbers of stars in the shop. At the main menu, select "Options" > "Cheats" selection to turn the cheats on or off. Enabling cheats will disable high scores and rewards:

CHEAT FUNCTION - CHEAT CODE
Easy Mode - One star. In this mode, all civilian traffic is removed.
Nitro - Six stars. In this mode, all player vehicles travel faster.
Stay On Target - Five stars. In this mode, hitting sections of the environment will cause the mission to fail.
Weak - Three stars. In this mode, the player can never earn justice.