Other consideration include number of vehicles in the systems, size and complexity of the layout, and the objective of minimizing the number of separate zones. For there reasons, the zones are normally much longer than a vehicle length. Zone control illustrated in figure 10.7 in its simplest from. When one vehicle occupies a given zone, any trailing vehicle is not allowed to enter that zone. The leading vehicle must proceed into the next zone before the trailing vehicle can occupy the current zone. When the forward movement of vehicles in the separate zones is controlled, collisions are prevented, and traffic in the overall systems is controlled.