Four Level Stack

This might be the gold standard for motorway-to-motorway junctions. It is, in all probability, the junction that motorways put on their Christmas list.

Limited-access roads
2
Surface roads
0
Vertical levels
4
Bridges required
3+
 
Access between roads
Full in all directions
Number in UK
3
First built in UK
1966

The four level stack (or sometimes just the "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 road crossing each other. Like all the most impressive free-flowing junctions listed in these pages, it's also very rare: there are only three of them in the UK. That's really not very many, and other places are not so reticent to build them. There are seven within the city of Dallas, for example, and quite a lot more in its wider urban area.

Early motorway plans, long before one of these junctions had actually been built in this country, called this design a "Maltese Cross". This refers to the shape outlined by the four right-turn sliproads that cross in the centre: a cross formed from outward-curving lines. Today, among British highway engineers, they are sometimes referred to as an "Almondsbury", after the name of the earliest UK example, the M4/M5 interchange.

Almondsbury opened in 1966; the two others are the M25/M23 Merstham Interchange, opened in 1976, and the M25/M4 interchange at Iver opened in 1986. Some say it's just coincidence that they all appeared precisely ten years apart, but others tell stories around campfires late at night that more four level stacks were built in 1996 and 2006, and we just haven't found them yet.

Diagram showing traffic flows at a Four Level Stack interchange

Why build one?

Why not build one? I'd have one in my back garden if there was space.

A full four level stack uses up a large area of land and has a very high cost in terms of earthworks and bridge structures, so the case needs to be particularly good in order to justify the expense. That's why we only have three of them. They are employed where the volume of traffic interchanging between two motorways is expected to be very high, and (importantly) where the existing geography allows for it. Four levels of roadway stacked up on top of each other are very tall indeed, and to limit visual intrusion they are, in the UK, only built in locations where there is a natural valley and the size of the junction can be hidden.

The reason they are able to handle large volumes of traffic is that they arrange their sliproads so that exits and entrance points are as neat as possible. There is just one exit from each motorway, which then splits in two; two sliproads then merge together to form one, and just one sliproad then merges back in to each motorway at the end. There are no loops or tight corners, and the right-turn sliproads can be gently curved and therefore fast.

Ironically, of the three stacks in the UK, one has never fulfilled its potential. The M23 is not nearly as busy as it was expected to be, because it was never completed, and instead four of the sliproads at the impressive Merstham Interchange effectively form a local exit to Croydon. There are plenty of other places where that junction would be better deployed if only we could move it.

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.
  • No sharp corners or looped sliproads.
  • Intuitive to use: after exiting the motorway, take the left sliproad to turn left or take the right sliproad to turn right.

Disadvantages

  • Uses a large amount of land.
  • Need for a large number of complex bridge structures.
  • Visually intrusive due to height.
  • Cost is comparable to a moon landing mission.
Drawing of a Four Level Stack, based on Almondsbury Interchange near Bristol
Drawing of a Four Level Stack, based on Almondsbury Interchange near Bristol

Variations

There are a shortage of variations on this interchange type — there are three of them in the UK and they all look the same.

The A19/A66 junction, between Stockton and Middlesbrough, and referenced in the Partially Unrolled Cloverleaf page, contains elements of the Four Level Stack. There's also the M1/M25 Chiswell Interchange which is four levels high but only contains half the sliproads: it has no access to or from the M1 to the south, so it's really half a stack.

With thanks to Andrew Saffrey for information on this page.

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