The report describes measurements of the entry capacities of seven entries to grade-separated roundabouts. All entries were fed by exits from motorways. The objective of the study was to compare the entry capacities of these entries with those predicted by a 'unified' formula developed recently for at-grade roundabouts, and so to develop a capacity prediction method for grade-separated roundabouts. The entry capacities of the grade-separated roundabouts at the mean value of circulating flow did not differ significantly from the predictions of the unified formula. The entry/circulating flow relationships, however, were found to have steeper slopes than those appropriate to at-grade roundabouts. A modification to the 'unified formula' for the entry capacity of at-grade roundabouts has accordingly been derived so as to provide a method for predicting the capacities of grade-separated roundabouts. Observations have also been made at four of the entries after traffic engineering measures had been taken to try to improve their capacity. Increases in capacity between 15 and 40 per cent were achieved either by changes in the give-way arrangement or by the introduction of peak hour traffic signals. (Author/publisher)
Abstract