Note: This was done on Versus mode online. Start the campaign and jump on the wall that is away from the alley. When looking towards the street, go to the right. When looking down, you will see a white "board". Jump for the board and heal yourself, then jump down to the ground. On the other side of the street is a desk-like object in front of an alley. Jump over the desk. Only common infected can get over that desk. Stay away from the Hummer or you will randomly get killed.
This is only possible on Versus mode, and you must be a Hunter. First, get on the rooftop that is basically the building in the center of the map. Face the other end of the street; not where the gas trucker is found, but the other one. Aim for the light post, then jump and land on top of it. Jump up to the ledge on the building in front of you, then jump to the fire escape stairs. Run up to the top of the stairs and jump on the roof. Go all the way to the back and turn left. Start walking. You will pass over an alley, and a second one you must jump over. From there, continue beyond the buildings. Refer to this map. You will eventually fall down to a street. There is here nothing of importance, but you are outside of the map.