The report describes measurements of the traffic capacities at 21 entries to very large roundabouts, both at-grade and grade-separated. The aims of the study were to extend the data-base of a previous study at grade-separated roundabouts and to try to identify the factors responsible for the difference in capacity characteristics between these and smaller roundabouts. The results were broadly similar to those from the previous study, with both at-grade and grade-separated sites having generally steeper entry flow/circulating flow relationships than smaller roundabouts. Neither the mean speed of the circulating traffic nor the visibility available to entering traffic was found to influence the entry capacity systematically. The relationships were, however, found to depend on the mean circulating flow generally prevailing at a site and, in addition to the geometric factors known to influence the capacity at smaller roundabouts, on the distance between the entry and the previous exit. A new formula for capacity prediction at very large roundabouts is presented, which is based on the 'unified' formula currently applied at smaller at-grade roundabouts. In practical terms, the differences between the new predicted capacities and those obtained from current procedures are small - typically less than unexplained site-to-site variations.
Abstract