The Magic Roundabout

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There is a story that has travelled incredible distances and, perhaps more than anything else, given the unassuming Wiltshire town of Swindon cause to be discussed across the world. It's not a famous person or a particular achievement. In fact, it's far more mundane than that: it's a roundabout.

A roundabout. Or is it?

The Magic Roundabout is a fabled, almost mythical entity discussed in hushed tones by American road enthusiasts already wary of Britain's many fast, multi-lane roundabouts. Stories of it are swapped with fear and trepidation among those planning a trip to Swindon. Guide books that would otherwise have no reason to mention the place find room for a paragraph or two when this little junction is concerned.

Why? Well, because it's the roundabout that goes the wrong way, of course!

What is it?

In Swindon town centre, just next to the local football stadium, is a junction where five roads meet. Until the 1960s it was a standard roundabout - the usual British way of dealing with unusual multi-directional junctions - but even then it had traffic problems.

A direction sign for the Magic Roundabout. Scared?Thanks to a rather daring bit of design work, it is now a "Ring Junction": a vast skidpan of tarmac on which are painted five mini-roundabouts, one for each approaching road. They are linked by two-way roads forming a ring. Therefore, approaching it, you can turn either left or right on to the "roundabout" and proceed around it in whichever direction provides the shortest route to your exit.

You can also spin round and round in the wrong direction, if the idea of going against usual road convention gives you a sort of thrill. (I would never, ever be caught doing something so juvenile, of course.)

Standing at the side and taking in this majestic sight - or indeed driving up and waiting to join it - gives the onlooker a panorama of vehicles all swirling and swooping past each other in what appears to be randomised, terrifying chaos. Its name as a roundabout was County Island, changed to County Islands when it grew five more, but its appearance from the sidelines is so unearthly that the nickname "Magic Roundabout" stuck very quickly. It has now been made official and appears on direction signs on the approaches.

How does it work?

The logic behind the junction design is surprisingly simple. Take a look at its layout from above, and after your pulse returns to normal, you can see it's quite logical.

Pplan of the Magic Roundabout. Click to enlarge
Plan of the Magic Roundabout. Wand shown actual size. Click to enlarge

The most important element is, of course, that it works both ways. Because traffic can choose the shortest path between its entry and exit points, it spends less time on the junction. One of the most common movements - from Fleming Way to Queens Drive - now involves travelling about half the distance as it did when this was a conventional roundabout. And if traffic can get out more quickly, that adds up to fewer vehicles clogging up the junction at any one time.

The trade-off is that there are now a much greater number of points of conflict - places where one stream of traffic crosses another. This effect is offset by a simple priority rule on every approach to every mini-roundabout, which means that no vehicle should be held up for very long at any "give way" line. Additionally, every approach has space for several vehicles (usually three but sometimes more) to wait alongside one another: that way, when there is a gap and it's safe to move off, several vehicles can all go at the same time.

It looks like utter mayhem from the sidelines, but it's actually incredibly controlled. Traffic keeps moving almost all the time, waiting only a few seconds to join each mini-roundabout and thus steadily travelling at low speed across the junction. A normal roundabout would involve long waits to join; signals would involve bursts of movement and long enforced stoppages. As a result, it has been calculated that the Magic Roundabout has a greater throughput of traffic than anything else that it would be possible to install in the same space. Magic indeed!

How was it created?

RRL test track experiment. Click to enlargeMini-roundabouts were developed by the Road Research Laboratory in the early 1970s as an experiment into whether the success of the priority-to-the-right rule at larger roundabout junctions could be applied to locations where there wasn't enough space to install a full-size roundabout. By summer 1971, 33 were in use across Britain.

The exercise was, at first, nothing more than an attempt to adjust the priorities at urban junctions to maximise traffic flow. However, one RRL researcher, Frank Blackmore, realised that something much more interesting was going on. The design that was necessary in these constricted places, a very small central island and approaches that widened rapidly just before the roundabout itself, was actually providing better throughput of traffic than a conventional roundabout.

Trials of the new junctions - including "small roundabouts" where a fixed central island was only a few metres across - went incredibly well. At one experiment at the RRL's own test track (shown left), capacity was improved by 25 to 35% simply by reducing the roundabout's central island to 3m (10ft) diameter.

Blackmore pursued this discovery, noting that the design was also superior to signalised junctions, following a Peterborough experiment where an extra 1,000 vehicles could be handled every hour by a new small roundabout at a previously signalised T-junction. He started to wonder if several small roundabouts could be linked to improve more complex junctions. The first double mini-roundabout - now a fairly common sight - was at the southern end of the Truro Relief Road in Cornwall. Its signage was complex, but all the same, it worked (and, in fact, it still works on the same spot today). What if three roundabouts could work the same way? What if more could?

Two roundabouts... Three roundabouts... or Four roundabouts?
Two, three and four: can you see a Magic Roundabout forming? Click to enlarge

The first Ring Junction was installed to replace an existing large roundabout in Colchester and, like the four-way design above, had a square central island. Traffic turning left or going straight ahead went around the outside as usual, and "wrong way" circulation could only be used to turn right. Really it was still a big roundabout with gaps in the central island; the RRL called them "unhooked" right turns. Unfortunately, its accident record was terrible: it proved much more dangerous than the congested roundabout it replaced, and after fiddling with road markings and signage, the experiment was abandoned, with the junction returning to its previous state.

Overhead photo of experimental roundabout. Click to enlargeBlackmore didn't give up, though, and in 1972 gave Britain a new design of Ring Junction. It was supposed to be in Birmingham, but the Council there was unable to fund the scheme, and so the RRL was invited to try its new design at a congested roundabout near Swindon town centre. For the first time, traffic could flow both ways right around the central island, meaning that if one side was congested, the other side could take up the slack. After a few days of Police control, in which time RRL researchers logged events from a crane-mounted camera, the experiment was branded a success.

There was an initial flurry of excitement, with more Ring Junctions built at Hemel Hempstead, Denham and Sadler's Farm near Southend. The idea has lost favour now, despite offering greater throughput than any other design for a flat junction. They also have an excellent safety record, probably because all traffic is moving too slowly to do any real damage in the event of a collision. But the Magic Roundabouts (er, sorry, Ring Junctions) that do exist are still doing good service. County Island is one of the few places where the jams have never really returned despite forty years of traffic growth.

What's it like to use?

Despite having been voted the seventh worst junction in the UK, the Magic Roundabout's bark is worse than its bite. It looks difficult to negotiate and the direction signs on its approaches are nothing short of intimidating. But all it asks of any driver is to be observant and to always, always, always give priority to traffic coming from the right.

The only other thought needed is deciding which way to go in order to reach your exit. Left, straight ahead, left again? Or right, left, left, right? The junction is actually small enough that motorists can keep sight of their exit as they travel through it, so it's hard to get lost. And the real secret is to stop thinking of it as one junction, because it's not. It's really just five mini-roundabouts that happen to be close together. Deal with them one at a time, ignoring the rest, and you too will come to be touched by Swindon's magic.

Media elsewhere on CBRD

You can take a drive through the Magic Roundabout in CBRD's Video section.

Good to see the Magic Roundabout given the credit it deserves. There's a similar junction in Colchester - the Greenstead Roundabout. Named after the housing estate that borders it, the Greenstead Roundabout was a five-way standard roundabout that was the bane of the commuting lives of anyone living to the east of Colchester.Traffic lights and widening did nothing to improve matters and so, in the early '90s, the council installed five mini-roundabouts, similar to the Swindon model. Since then, whilst there are still snarl-ups at peak times, traffic flows have been much improved and journey times sped up as a result.

Good to see that Ring Junctions were still being installed as recently as the early 1990s - there's hope for a revival yet!

Image of Magic Roundabout with sign taken from an original by Chris Downer, used under Creative Commons Attribution-Share Alike 2.0 Generic licence.