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While researchers agree as to the safety benefits of turbo-roundabouts, their improvements in terms of capacity and delay remain open to discussion. This is mostly because previous research is based on capacity models that do not fully describe the complex interactions between traffic streams on multi-lane roundabouts. This paper proposes a procedure to calculate capacity based on gap-acceptance theory. It addresses the limitations mentioned by accounting for usually disregarded effects such as the dynamic choice of the entry lane and unequal allocation of traffic in the circulatory lanes. Capacities were calculated for a wide range of demand scenarios and it has been shown that only under demand scenarios that are very specific and uncommon in real-world networks, associated with very high percentages of right-turning entry traffic, can a standard turbo-roundabout be expected to provide more capacity than the equivalent two-lane roundabout. It has also been shown that two-lane roundabouts can normally be expected to provide capacities of 20–30% above those of comparable turbo-roundabouts.
