Use either a motorcycle or a very fast car. When the race starts, turn around and go into the tunnel behind you. Then, when the tunnel gets to the wide area with the pillars, turn around and nitro through the tunnel you just came from. Drive at full speed at the pyramid. When you hit the big pyramid, use a boost or a nitro to soar off and over the building after the pyramid that you hit. You should fly over the road behind the building and on top of the building on the other side of the street. You are now able to drive on rooftops.