1. A method for avoiding collisions between manned vehicles and other manned vehicles or between manned vehicles and automatically guided vehicles within a defined operational environment, the method comprising: a) receiving information on the position of each vehicle within the defined environment at predetermined time intervals; b) determining a predicted trajectory and safety zone for each vehicle by calculating the velocity and direction of travel of each vehicle from position information at successive time intervals; c) determining any areas of intersection of a manned vehicle with other manned or unmanned vehicles to predict a potential collision; d) transmitting instructions to an automatically guided vehicle within an area of intersection to reduce the speed or to stop the automatically guided vehicle to prevent the predicted collision; e) transmitting a warning to one or more manned vehicles to alert the one or more operators of a predicted potential collision, so that the one or more operators can take appropriate action to avoid the predicted collision.