A NEW PROCEDURE FOR CAPACITY CALCULATIONS AT CONVENTIONAL ROUNDABOUTS

Observations have been carried out to determine the capacity of conventional roundabout weaving sections following the introduction in the UK in 1966 of the offside priority rule. The study covered 42 sections at 17 roundabouts and showed that the Wardrop formula is no longer valid for capacity calculations. As a result of these findings, the Department of the Environment published in 1975 as an interim measure a modified design formula based on the wardrop formula but which reflected observed capacities more accurately. An alternative procedure based on Tanner's work for major/minor priority intersections is suggested in this paper, in which capacity is expressed as entry flow per standard lane for a given circulatory flow. Comparisons of predicted and observed capacities using this formula with independent data show better agreement than is given with the Department of the Environment interim formula. Considerable scatter was evident between the observed capacities of the various sections included in the study, but it has not proved possible to explain this in terms of traffic flow or geometric parameters other than entry width. However, there was strong evidence of a reduction in entry capacity with an increasing proportion of heavy vehicles in both the entry and circulatory flows; the suggested design procedure takes account of this by applying a correction factor when the proportion of heavy vehicles exceeds 10%. An example illustrating the new design procedure is also given.(a) /TRRL/

Media Info

  • Features: Figures; References; Tables;
  • Pagination: p. 1-16
  • Serial:
    • Volume: 65

Subject/Index Terms

Filing Info

  • Accession Number: 00178637
  • Record Type: Publication
  • Source Agency: Transport and Road Research Laboratory (TRRL)
  • Files: ITRD, TRIS
  • Created Date: Sep 27 1978 12:00AM