Recognise these blue signs? If you've ever got the tube, anywhere, ever, you should do. They exist by the bucketload in tube stations, but what do their seemingly random numbers mean? Why are they there? What do they do? Does anyone know?
We touched on the issue briefly back in 2011 (clearly, this one's been puzzling us for a while), when we said:
"they’re code numbers for the emergency services, making it easier for rescuers to orient themselves in smokey conditions. The top number on each sign denotes the level below ground, while the remaining numbers define a location on that level."
So the signs are Station Identification Numbers (SID). Each station has a layout diagram, with the numbers allowing for very specific locations to be identified, e.g. for contractors to perform work and maintenance. At the entrance to each tube station there is also a red London Fire Brigade emergency box, which is opened in an emergency and contains station plans using these numbers.
In some instances you can also find these signs which have letters instead, instead of numbers. There is an "A/101" sign at Hounslow West, an anomaly to the usual convention of the first number relating to the level of the station - it's because when you go up from street level, each level uses letters instead of numbers, 'A' for the first level up, 'B' for the second level up, and so on.
Seen any other blue signs on the tube? Know any more about what they mean? Let us know in the comments.