CTF or Controlled Traffic Farming is the practice of driving all machinery on the same wheel tracks year after year.
A typical system uses multiples of either 9 or 12m, with all wheel tracks on 3m centres. For example a farmer may use a 9m seeder, 9m harvester and 27m boomspray, this is known as a 3:1 system as the boomspray is 3 times the width of the other implements. Some farmers may use a 2:1 system, eg: 12m seeder, 24m boom or 18m seeder, 36m boom.
Other widths are sometimes used, eg: 10.5m and 15m, however 9m and 12m systems make the best use of available equipment. Some farmers use imperial sizes, eg: 30' and 90' with 120" wheel tracks. These systems make the best use of North American imported machinery which are made in imperial sizes.
2m systems are also common in high rainfall areas where 2m raised beds are installed and also traditional cotton farming. A 2m sytstem would typically use an 8m seeder and a 24m boomspray. The harvester would not be included in this system, although there has been at least one harvester set up with a 4m axle to harvest on 2m raised beds.
CTF Alberta (Canada) recently travelled to Australia to study CTF. Here is a link to a summary of their findings: http://www.precisionagriculture.com.au/assets/PA%20Newsletter%20-%20Summer%202010%20tn.pdf