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A Dumbbell Interchange is a compact and comparatively low cost Grade Separated Junction design which only requires a single bridge across the major route. At either end of the bridge two small roundabouts provide access to the slip roads for all traffic. It has become increasingly common in recent years for new junction design, even between two major routes.
The majority of Dumbbell interchanges are built at locations where two routes cross each other at something approaching right angles. However, by increasing the size of one roundabout, it can also be used to allow two parallel routes to meet, with a spur across the major road. In such circumstances, the far, terminal roundabout may be converted into a 'teardrop' as there is no traffic making that final movement on the roundabout.
Dumbbells are also common where full movements are not necessarily required, for example where there are only south facing sliproads. Similarly, a number of former Diamond interchanges have been converted to Dumbbells in recent years, to try and reduce queuing on sliproads amongst other reasons.
A common variation sees one or both of the roundabouts turned into 'teardrops', effectively making the junction a single roundabout, squished in the middle to fit over the single bridge. Whilst this can slightly increase the distance some traffic needs to travel, it can also improve flow by removing some circulating traffic from the ends. This design is colloquially known as a 'dogbone' junction.
Folded Dumbbell interchange
A folded Dumbbell is one where the slip roads to one or both roundabouts are both on one side of the roundabout(s), one being straight, but displaced around the outside of the other which is folded with a 270 degree turn to connect the roundabout with the major route. It is therefore similar to a folded Diamond interchange. This design can be used either due to land constraints limiting the available room in a corner of the junction, or because there is another road joining in the normal position for the sliproad.