Estimation and simulation gap acceptance behaviour at congested roundabouts

Gap acceptance parameters form a crucial part of many simulation and operations models. VISSIM, SATURN, SIDRA, PARAMICS and many other models recommend default values, but allow the modeller to change these to more relevant values. When a traffic network is congested, drivers may choose to behave more aggressively, and accept smaller gaps than is modelled by standardparameters. Existing conditions may be hard to reproduce unless more aggressive parameters are adopted. In this paper, a methodology is presented for developing locally appropriate gap acceptance parameters based on localvideo observations. Using CCTV video recordings of driver behaviour around a congested roundabout in Birmingham, gap acceptance behaviour of vehicles has been analysed, resulting in appropriate input values for representation in VISSIM. The video data was used to gather lengths (time gaps) for both accepted and rejected gaps. Accepted gaps are where a car on the approach leg deems the gap between vehicles of sufficient length (time) to allow it to safely move into the circulating stream. A rejected gap is where the car chooses not to move into the circulating stream as the gap is insufficient. The driver behaviour for heavy (PSV and HGV) and light (cars andLGV) was classified and analysed separately. Further analysis of driver behaviour by disaggregating the observations by vehicle type reveals that gaps accepted by heavy vehicles differ from light vehicles. Accepted and rejected gaps are dependent on the traffic composition of conflicting / opposing streams, with higher time gaps required if the conflicting stream contains heavy vehicles. In congested situations heavy vehicles exhibit rather surprisingly more aggressive behaviour on the minor arm by accepting smaller gaps. This gap acceptance study reveals behaviour that is contrary to expectation. It is found that heavy vehicles accept a much lower gap (ranging from 1.79 seconds to 2.28 seconds) compared to both the recommended value and the minimum observed value for cars/LGVs. The observed minimum acceptable time gap for light vehicles ranges from 2.05 seconds to 2.72 seconds for different arms compared to the default recommended value of 2.6 seconds. Congested traffic conditions also exhibit a state of 'mutual conflict resolution' between the minor and conflicting approaches where the vehicles on the main approach may give way to allow the minor stream vehiclesto enter the intersection, where the accepted gap is not just a function of time but of free space available. This needs to be reflected in the gapparameter values. Sensitivity testing of the simulation model reveals that the gap acceptance behaviour as a function of traffic composition affects the capacity and performance of the roundabouts and has an impact on thedesign of roundabouts. The calculated gap acceptance parameters were validated in a microsimulation model and compared with observed travel times, showing a better fit than the default values. The paper will present the results. For the covering abstract see ITRD E135582.


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Filing Info

  • Accession Number: 01089893
  • Record Type: Publication
  • Source Agency: Transport Research Laboratory
  • ISBN: 1905701012
  • Files: ITRD
  • Created Date: Mar 17 2008 10:11AM