Four Level Stack
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A Four Level Stack Junction (sometimes called a Maltese Cross Junction) is a fairly common Grade Separated Junction design used where two motorways cross. The name comes from the vertical alignment of the various carriageways and sliproads, which all cross near the centre of the junction, creating a point where 4 levels of traffic pass under and over each other. These are the two main carriageways and two pairs of sliproads permitting the right turn actions. Due to the topography of the site and the alignment of the main routes, it is perhaps less common for there to literally be four levels of traffic at one point, but the name comes from the basic design, and is used at the Almondsbury Interchange where the M4 and M5 meet, amongst other locations.
The sliproad layout is designed so that each carriageway only has one off and one on slip, with the sliproads then splitting and merging to provide full access between the carriageways. The left turn slips simply turn through 90 degrees from one carriageway to the other, whilst the right turn lanes initially turn the 'wrong way' with the left turn lane, before doubling back round to pass under / over the centre of the junction and meet the opposite left turn sliproad.
Not all four level stack junctions are complete. Chiswell Interchange where the M25 and M1 meet, for instance, has no south facing slips for the M1. This makes it a hybrid junction, somewhere between a Four Level Stack and a Directional T junction design.