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Roundabout interchanges are built using two bridges since the circulatory carriageway needs to cross the mainline twice, once in each direction. Due to this, a more modern approach is to use only one bridge with two roundabouts, forming a dumbbell interchange, though these tend to have lower capacity and can be more confusing for drivers. Roundabout interchanges are rarely built these days; however, a notable exception is when the dumbbell taking the A6144 Carrington Spur onto the M60 was actually converted from a dumbbell to a roundabout interchange.
Three Level Stacked Roundabout Interchanges
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Sometimes, two dual carriageways will cross a roundabout interchange, which is known as a three-level stack(ed roundabout), or colloquially a stackabout. While this type of junction is often criticised for being cheap and under-capacity, it can work very well in some places, such as the M18/A1(M) junction. The main advantage of a stackabout as opposed to any other type of four-way interchange is that services and local roads can easily be plugged into it without resulting in weaving or an expensive mess of sliproads.
Coryton Interchange, between the M4, A470 and A4054, is notable for being one of the largest roundabouts in the UK, to the point where the A470 manages to cross below the circulatory carriageway at the eastern side, but above at the western side.