Attachment 5 - Traffic Impact Assessment Final PTT · 132005.638.06

PRACTICE NOTE
GAP ACCEPTANCE AT UNSIGNALISED INTERSECTIONS




    As shown in Figures 1 and 2, for an opposing flow                              FIGURE 3:     AUSTROADS EFFECT OF OPPOSING
    rate of 1,200 passenger car units per hour (pch/h),                                         _FLOW RATE ON GAP ACCEPTANCE
    application of the calibration factor results in a 7-
    14% reduction in the critical gap and follow-up
    headway. The calibration factor has a smaller effect
    at lower opposing flow rates and a larger effect at
    higher opposing flow rates.
    The SIDRA Intersection User Manual recommends
    that calibration of priority-controlled intersections be
    undertaken by adjusting gap acceptance parameters.
    This is Therefore, the use of the “level of reduction
    with opposing flow rate” factor to calibrate priority-
    controlled intersections is consistent with the SIDRA
                                                                                   A comparison of Figure 3 with Figures 1 and 2
    Intersection    User    Manual.      However,      SIDRA
                                                                                   indicates that, even when adopting the highest
    Intersection does not provide guidance on which of
                                                                                   possible value for the “level of reduction with
    the four possible values of the factor (ie “none”,
                                                                                   opposing flow rate” factor (ie “high”), the SIDRA
    “low”, “medium” and “high”) should be used.
                                                                                   Intersection modelling software does not calibrate (ie
                                                                                   reduce) critical gap and follow-up headway to the
    AUSTROADS GUIDANCE                                                             same extent as Austroads. Broadly speaking, the
    To inform the selection of an appropriate value for                            three possible values of this factor (ie “low”,
    this calibration factor, guidance has been sourced                             “medium” and “high”, excluding “none”) are in the
    from Austroads’ Guide to Traffic Engineering Practice                          order of 35%, 55%, and 75% as impactful as the
    Part 6: Roundabouts (1993). This document specifies                            Austroads calibration values, respectively.
    various critical gap and follow-up headway values for                          Therefore, SIDRA Intersection’s method of calibrating
    vehicles entering roundabouts, based on a range of                             gap acceptance at priority-controlled intersections is
    opposing circulating flow rates. Although these gap                            likely to be inherently conservative.
    acceptance parameters cannot be directly applied to
    priority-controlled intersections, the relationship
    between opposing flow rate and gap acceptance is                               RECOMMENDED METHODOLOGY
    expected to be comparable at both types of                                     To ensure a consistent and unbiased approach to
    intersections. Figure 3 summarises this relationship at                        assessing traffic operations at priority-controlled
    roundabouts, as specified in the Austroads guide.                              intersection, Table 1 can be used to guide the
                                                                                   application of the “level of reduction with opposing
    More recent Austroads guidelines continue to
                                                                                   flow rate” calibration factor. This table evenly
    recognise the tendency for drivers to accept reduced
                                                                                   distributes the range of opposing flow rates specified
    gaps where opposing flow rates are high, but no
                                                                                   in the Austroads guidance (ie 0-2,500pcu/h) across
    longer provide guidance on specific gap acceptance
                                                                                   the four possible values of the calibration factor,
    reduction factors.
                                                                                   reflecting observations that higher opposing flow
                                                                                   rates typically correspond to drivers accepting smaller
                                                                                   gaps.




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