Directional T junction
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The Directional T is a junction design used where one of the routes terminates. The term directional T is perhaps misused in Britain & Ireland. It is correctly defined as a 3-way junction where each approaching route splits, with the left lanes staying left, whilst the right lanes fork right (left for left / right for right). However, generally speaking British & Irish Motorway planners have chosen to maintain a through route in such junctions, rather than breaking each mainline up. However, for ease of description any junction where the terminating route conforms to the Directional T principle has been classified as such here.
There are a variety of sliproad layouts in use in Britain and Ireland for this type of junction, with them crossing before or after crossing the through route, or even as they cross. This means that the Junction can appear to be half a Four Level Stack or even half a Whirlpool in layout.
3 Level Stack
The start of the M898 at Craigton Interchange on the M8 is one example of this design. More commonly, however, the sliproads are spread to allow them both to cross over the main route on their own bridges at some distance from each other. Whilst this incurs a higher land take, it reduces the costs of bridges and banks / cuttings, so is the preferred option in rural locations. It is also an easier design to build where the routes join at more acute angles.
Perhaps the best example of a half whirlpool in Britain is Airport Interchange on the M56, although the junction is incomplete, and was intended to have another road heading west. Masterton Junction, where the A823(M) terminates on the M90 is also incomplete, being an unfinished Octopus. As such, it too is effectively a half-whirlpool Directional T junction design.
Non Directional T junctions
A Non Directional T is obviously one where the terminating route does not conform to the left for left / right for right principle. Perhaps the most notable such junction is Ray Hall where the M5 terminates on the M6. Here, because it was thought at the time of construction that most traffic from the M5 would be continuing north, rather than circulating Birmingham, the M5 mainline continues to join the M6 north, thus left for right / right for left. Whilst this may prove a little confusing for some, it has proven to be the correct choice, with a substantially larger volume of traffic travelling M5 - M6N over M5 to M6E.