Whirlpool

A complex but rather beautiful design that provides free-flowing access in all directions between two intersecting motorways.

The whirlpool (otherwise known as a turbine) is something of a halfway house: it involves more bridge structures than a cloverleaf or a partially unrolled cloverleaf, and therefore works out more expensive, but it's not nearly as complex or expensive as a four level stack would be. The result is something that will flow nearly as well as a stack, but might be smaller or less controversial.

One of its real charms is how much its size can be adjusted. If there's lots of space to play with, a large whirlpool can have fast, gently curving sliproads and room inside for landscaping. But if land is at a premium, it's possible to crunch it down to a much smaller size, creating a free-flowing junction that's packed tightly into an impressively small area. The only limitation is how sharp you're willing to make your corners and how steep you're willing to have your ramps.

One other, relatively minor, advantage is that the right-turn sliproads run parallel to each other, making it possible to have connecting links for emergency services to perform u-turns or to hop from one sliproad to another, or to provide contingency during maintenance works. That can be useful sometimes, and is an opportunity that most junction designs don't offer.

Why build one?

One of the principal reasons to choose a whirlpool is for a location where space might be limited and height is a definite problem. Other low-lying junctions, the cloverleaf and the partially unrolled cloverleaf, demand lots of space, but a whirlpool can be more compact without needing to climb skyward. That's an important consideration somewhere like the M25/M3 interchange, which is in the midst of the Surrey commuter belt. There aren't many hills around to disguise tall flyovers, but there are an awful lot of people with big houses and friends on the parish council. A whirlpool is an astute choice somewhere like that.

The M25/M11 interchange, meanwhile, has the benefit of being reasonably compact but also offers opportunities to jump between sliproads and perform u-turns for emergency and maintenance vehicles. At that junction, where pairs of right-turn sliproads run parallel, they are actually on the same road surface and separated only by painted markings. On three of the four approaches, the next junction is a considerable distance away, so that contingency can be vital.

But don't get carried away. M60 J12 Worsley Interchange might be the UK's most dysfunctional free-flow junction. It's a whirlpool that demonstrates just how small one of these layouts can be, but in almost every way it's actually too small. Its sliproads are narrow, its corners are worryingly tight, its merges are sudden and squeezed in between bridges. It causes a lot of problems. Sometimes, you can take a whirlpool too far.

Variations

This junction comes in two principal forms. The one illustrated above is a full whirlpool or turbine, and has only one entry point to each motorway, with the two approaching sliproads merging together to form one approach.

The alternative, which can reduce the number of bridges that are required, is sometimes called an octopus — because it has eight arms, of course. In an octopus junction, the layout is mostly the same, but each right-turn sliproad turns ninety degrees before curving inwards to merge with the motorway it's joining straight away. This means an octopus has one exit from each motorway carriageway, but two merges.

This vastly reduces the number of bridges as the sliproads don't need to cross each other, but makes for less orderly merging and a few more problems for traffic on the mainlines. The M25/M11 interchange is arranged like this, as isM74 J6 at Motherwell.