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|A tiger tail merge on the M5 at Catshill Interchange|
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The phrase tiger tail is informally used, even in engineering departments, to describe a ghost island used, typically, on motorway entry and exit slip roads.
TD22/06 of the Design Manual for Roads and Bridges defines one as a ghost island layout at a diverge utilising TSRGD diagram 1042.1 to separate the points of exit to a slip road. So called because the vertical sign used to inform drivers of the layout incorporates an illustration that resembles a tiger’s tail.
The use of such merging layouts has its origins at the beginning of the motorway system. A crudely designed ghost island merge, using different coloured surfacing rather than hatched road markings, was in place at the merge of the M1 and M45 in 1959. There were fears motorists would find this layout confusing, and large 'traffic islands ahead' warning signs were erected. Their use was rather reserved until the 1990s when they began to appear at several junctions.
The vertical traffic signs used at tiger tail diverges were only prescribed in the 2016 edition of the Traffic Signs Regulations and General Directions. Prior to this, all locations had to be specifically authorised by the Highways Agency.