Four Level Stack
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This is the holy grail of interchange design in the UK - the Four Level Stack is not just the most high-powered in terms of traffic flow, it's also the most high rise, with (as the name suggests) four levels of traffic crossing at the central point. It's also just as rare as the Cloverleaf - there's only three in the whole country.
Early motorway plans in this country referred to this design - at the time nothing more than an idea - as a 'Maltese Cross'. This refers to the shape formed by the four right-turn sliproads that cross in the centre; a cross formed from four outward-curving lines.
In the 'business' these are often referred to as an 'Almondsbury', after the town near the earliest UK example, the M4/M5 junction.
Advantages
- Very high traffic flows are possible.
- Unlikely to jam up - merging and diverging is well arranged, with just one diverge and one merge for each through carriageway.
Disadvantages
- Very large land take.
- Visually intrusive due to height; in this country they are built in valleys to avoid having to construct lengthy approach embankments.
- Cost is comparable to a moon landing mission.
Variations
There are a shortage of variations on this interchange type - there are three of them and they all look the same. The A19/A66 junction, referenced in the Partially Unrolled Cloverleaf page, contains elements of the Four Level Stack. This seems to be about it!
Where to Spot Them
The original British model, opened 1966, is at Almondsbury, Bristol, where the M4 and M5 cross. The other two are on the M25 around London; one where it crosses the M4 and the other at the M23.
With thanks to Andrew Saffrey for information on this page.
