This trick requires the Keys To The City expansion. Enable the "Empty Streets" code, then get onto a reasonably long straightaway. Face your character (and look) directly down the road in one direction. Spawn a ramp truck. Then, face and look in the other direction and spawn another ramp truck. Get into one of the ramp trucks and reverse into the other one (so that your ramp hits its ramp). Both trucks will instantly rocket into the air.