- Number of limited-access roads
- Number of surface roads
- Bridges required
- in all directions
- Estimated number in UK
- First built in UK
- ca. 1974
The dumbbell is a hybrid between the diamond and the roundabout interchange, which makes it a very close relative of both. Its name derives from the way its two roundabouts linked by a single bridge resemble a weightlifter's dumbbell.
The UK's first full motorway interchange was a variation on the dumbbell: it was a folded dumbbell (or if you like, a parclo with roundabouts), with two of the sliproads looped to turn 180 degrees. It was built at M6 J31 Samlesbury, on the Preston Bypass, and opened in 1958. It has since been converted to something a bit more like a standard dumbbell. A few others with folded sliproads were installed in the earliest parts of the motorway network; one of those that remain intact is at M50 J1.
The main advantage of this type of junction is that it can provide most of the capacity of a roundabout interchange, but with the smaller footprint and the single bridge of a diamond junction. Smaller, cheaper and nearly as good.
Typically, a dumbbell can't quite match the capacity or speed of a full roundabout interchange unless the two roundabouts are made very large and the bridge is a dual carriageway, so its typical home is on expressways and upgraded A-roads, where it provides a reasonably high capacity junction without taking up too much room.
The earliest example of what we now consider a standard dumbbell, as pictured above, may be M62 J31 at Normanton, which opened in 1974, but which has been significantly altered since then.
Why build one?
We've already covered the argument about space. On any road scheme where space is at a premium, it may be possible to install a dumbbell interchange where a roundabout interchange wouldn't fit. Of course, it's possible — with careful design — to make a roundabout interchange very small, if the roundabout is only just big enough to span the main road. But a dumbbell can also win the day with a cheaper construction cost, requiring only one bridge instead of two.
There are some locations where a dumbbell has an advantage because several roads connect at the junction, and if there are significant traffic flows that don't interact with the motorway, and instead are just going between two surface roads, a dumbbell can keep that traffic off to one side, rather than have it circumnavigate a huge roundabout and cross the motorway twice.
The last reason might be that you're replacing an existing roundabout interchange. If the motorway is being widened, and the bridges have to be rebuilt to accommodate the increased width, it's possible to make the job simpler by building one new, wider bridge in the middle of the old junction, convert it into a dumbbell, and then demolish the old bridges. That's the history of the dumbbell junction at M6 J20 near Lymm, which used to be a two-bridge roundabout interchange and got one new bridge in the middle when the M6 gained a fourth lane.
- Cheaper than a roundabout interchange, with just one bridge and less land take.
- An easy-to-build step up from the diamond junction.
- Often takes up less space than other types of junction.
- Easily upgraded with traffic signals.
- Lower capacity than the roundabout interchange, with two roundabouts often functioning less effectively than one.
- Often installed today where a larger but more expensive junction would have been built in the past.
Most variations can be accounted for by the installation of a roundabout on only one side of a diamond junction. There are also numerous unique layouts that involve two or more roundabouts and which are best described as dumbbell derivatives.
You can also perform all the same sliproad gymnastics as with a diamond, by putting some or all of the sliproads on the same side of the roundabouts, turning some of them into loops and creating a folded dumbbell.