Since 1965, the United Kingdom has gradually changed to the international system of metric weights and measures (the International System of Units or 'SI' system); although, despite having had almost 50 years to do so, the process is not yet complete. A limited amount of imperial (ie non-metric) units must still be used in certain limited circumstances. It is clear however that the United Kingdom is currently in a bit of a conceptual muddle given that these two systems are concurrently in use, both informally and formally; albeit to a limited degree in the latter case. A good example is the Department for Transport's (DoT's) Traffic Signs Manual, which refers to 'road works' signs with a '200yds' plate. However, it goes on to state in this guidance that such signs must be placed 200 metres (and not 200 yards) in advance of the road works: a discrepancy of some 17.12 metres (or about 18 yards and 2 feet). Mistakes like this are quite common. Indeed, there is still much debate in this area