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, by a server, information on the position and orientation of each vehicle within the defined operational environment at predetermined time intervals from an optical position and rotational orientation system; b) determining, by a collision avoidance apparatus configured to communicate with the server, a predicted trajectory and a safety zone for each vehicle by calculating the velocity and direction of travel of each vehicle from the position and orientation information at successive time intervals; c) determining, by the collision avoidance apparatus, any areas of intersection of the safety zone of a manned vehicle with the safety zones of other manned or automatically guided vehicles to predict a potential collision; d) transmitting instructions by the collision avoidance apparatus to the server to an automatically guided vehicle whose predicted safety zone lies within an area of intersection of the safety zone of another vehicles to reduce the speed or to stop the automatically guided vehicle to prevent the predicted collision, the automatically guided vehicle reducing its speed or stopping in response to the instructions; and e) transmitting a warning by the collision avoidance apparatus to the server to one or more manned vehicles whose predicted safety zone(s) lies within an area of intersection of the safety zone of another vehicle to cause a driver interface of the one or more manned vehicles to alert the one or more drivers of a predicted potential collision, so that the one or more drivers can take appropriate action to avoid the predicted collision.