A method and apparatus are provided for sorting or retrieving items to/from a plurality of destinations areas. The items are loaded onto one of a plurality of independently controlled delivery vehicles. The delivery vehicles follow a path to/from the destination areas that are positioned along the path. Along the path, the vehicles are scanned to determine if any item on the vehicles extends beyond a dimensional constraint. If it is determined that an item on a vehicle extends above the pre-determined threshold, the vehicle is controlled, such as by stopping or re-directing the vehicle. Once at the appropriate destination area, an item is transferred between the delivery vehicle and the destination area.