A TRACK EXPERIMENT ON THE ENTRY CAPACITIES OF OFFSIDE PRIORITY ROUNDABOUTS

The relationship between the size and shape of the entries to offside priority roundabouts and the entry capacity has been investigated by means of a full-scale traffic experiment on the TRRL Test Track. The traffic flow entering a roundabout from a saturated approach was found to be linearly related to the flow on the circulating carriageway crossing the entry. The capacity was determined primarily by the entry width; the circulation width and overall size of the roundabout had significant but small effects. Entry flare provided sizeable traffic benefits even for small circulating flows where its effect is expected to be least. For a defined value of the circulating flow, variations in the pattern of traffic flows within the circulating carriageway had no effect on the entry capacity; nor did the proportion of left-turning traffic at entry. The use of filter lanes produced significant increases in entry capacity. Linear regression analysis has been used to provide a framework of predictive relationships for the entry capacity in terms of the entry geometry. Together with public road observations currently in progress, these relationships are intendend to provide the basis for new procedures of capacity evaluation which will allow entry-by-entry assessment for engineering design and economic evaluation purposes. /Author/