Instructions
Complete all missions in the main game. This includes taking down the wicked crime syndicate behind the intrigue at the heart of the game's story and winning all cash races. This may require 5-10 hours of play, depending upon your skill level.
Save your game and hold down the "X" or "A" button on your controller while in the main menu. Use the "X" button for PlayStation 2 and 3 and the "A" button for Xbox 360 and Wii systems.
Wait for the cheat menu to appear, then use the onscreen keyboard to enter the following code: "Pedestrian."
Begin any standard race or police chase level, then press the left and right triggers simultaneously, or the L2 and R2 on PlayStation 3 systems, to exit your car. While your pedestrian avatar may appear blocky and pixelated, you are now free to wander around at will on foot.