this report describes experiments carried out by trrl on public roads, to determine ways of improving the capacity of large roundabouts without increasing their size. within the same area of road similar capacities have been obtained with two radically different designs; on one hand, the usual one-way clockwise circulation around the central island and, on the other hand, a ring junction consisting of two-way circulation around the central island with a mini-roundabout at the mouth of each road. by reducing the size of the central island the full capacity can be increased by up to 35 per cent but special care is then required to deflect and slow down the fast crossing movements. by fuller use of the area at the mouth of each road between the same outline (including the area of any median strip or island), the full capacity of a six-arm roundabout has been increased by about 15 per cent without reducing the central island size. at one particular site, by marking a left-turning filter lane from one entry to the adjacent exit, the discharge capacity of the affected approach road has been increased by 36 per cent without reducing the roundabout circulating flow. (a)

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