The Pedestrian Harvest powerup makes the killed pedestrians stick to the car. Once a pedestrian is run over, it'll stay glued to the car, and in the middle of the "dying" animation. The player may bring any number of pedestrians with them at a time. This powerup is useful for the "pile driver" bonuses, seeing as the pedestrians are stuck to the car, and they're not quite dead (they are in their dying animation), the player can ram them against the wall as many times as they want.
To obtain it with cheats, press Ctrl+3 when in cheat mode or type TRAMSARESUPER.
