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Heavy Entry Flow Rates at Roundabouts
At a roundabout with an unbalanced flow pattern, a traffic stream with a heavy flow rate enters the roundabout against a circulating stream with a low flow rate. Three such roundabout cases from Melbourne, Australia are described below.
Small single-lane roundabout at the intersection of Stanhope Grove with Broadway in Camberwell: 1524 veh/h per lane entering against a circulating flow rate of 60 veh/h has been reported. Sum of entering and circulating flows is 1584 veh/h.
Small-Medium single-lane roundabout at the intersection of Grange Rd, St Georges Rd and Alexandra Avenue in Toorak: 1693 veh/h per lane entering against a circulating flow rate of 67 veh/h has been reported. Sum of entering and circulating flows is 1760 veh/h. The measured follow-up headway and critical gap values for this entry lane are 1.992 s and 2.423 s, respectively. The maximum capacity at zero circulating flow (corresponding to the follow-up headway) is 3600 / 1.992 = 1808 veh/h. A photo of this roundabout is shown below.
Large roundabout at the intersection of Mickleham Rd and Broadmeadows Rd in Westmeadows (this is a case described in the AUSTROADS Roundabout Guide): 1397 veh/h per lane against a circulating flow rate of 83 veh/h in am peak (sum of entering and circulating flows = 1480 veh/h) and 1501 veh/h per lane against a circulating flow rate of 112 veh/h in pm peak (sum of entering and circulating flows = 1613 veh/h)s.