THE VIRTUAL COLLISION
Air traffic control is done in 3-dimensional space. Even in controlling traffic on runways, there is opportunity to cancel landing instructions of arriving traffic in order to prevent collision with traffic on the ground. Technically, a controller has caused a collision if traffic has been allowed to be closer than 1000 feet vertically or 5 miles horizontally. The important part of this statement is allowed to be. Instructions that could lead to a collision are not considered such if they are changed before a technical or actual collision occurs. Even having caused a technical collision, with the constant monitoring of radar, the instant communication of radio and 3-dimensional space, a quick-thinking controller or pilot can change the course of the traffic and prevent the collision.
A train dispatcher, on the other hand, controls traffic in a very narrowly defined 2-dimensional space. Historically as well as currently, a technical collision occurs for a train dispatcher at the moment conflicting instructions are issued. There are no mistakes allowed and dispatchers are required to turn themselves in for having issued such instructions. Should such conflicting instructions be issued, there is not usually the opportunity to correct the error because ground to ground radio communication, when available at all, is not as dependable as ground to air or air to air communication and because train dispatchers do not have all of their traffic readily visible on a radar-like display. To overcome the problems inherent to ground to ground communication, a train dispatcher monitors and uses several, possibly even 15 or 20, 2-way radio installations placed along the track at intervals allowing coverage of at least most of the territory. Even when dependable radio communication is available to any given train to which a dispatcher needs to speak, unlike the traffic of an air traffic controller which is all within radio range of not only the controller but off all of the other traffic involved, the dispatcher’s other traffic cannot usually hear both sides of the conversation and often cannot hear any of the conversation. A break in a train dispatcher’s concentration and the resulting error may not become apparent for hours and there are generally no objective listeners who can catch errors as they occur.