Attachment 5 - Traffic Impact Assessment Final PTT · 132005.638.06

PRACTICE NOTE
GAP ACCEPTANCE AT UNSIGNALISED INTERSECTIONS




    BACKGROUND                                                                     with high opposing flow rates, SIDRA Intersection
                                                                                   software allows these parameters to be calibrated by
    At unsignalised intersections, drivers on minor (ie
                                                                                   changing the ”level of reduction with opposing flow
    opposed) movements must wait for a suitable gap in
                                                                                   rate” factor. For roundabouts, this calibration is
    the opposing traffic flow before entering the
                                                                                   performed automatically by SIDRA Intersection.
    intersection. There are several sources of guidance
    for the selection of gap acceptance parameters, with                           There are four possible values for ”level of reduction
    Austroads’ Guide to Road Design Part 4A:                                       with opposing flow rate” factor: “none”, “low”,
    Unsignalised and Signalised Intersections (2023)                               “medium” and “high”. The effect of this factor on the
    being a prominent example.                                                     critical gap and follow-up headway parameters is
                                                                                   shown in Figures 1 and 2, respectively.
    However, what drivers consider to be a suitable gap
    can vary significantly based on a range of different
                                                                                   FIGURE 1: EFFECT ON CRITICAL GAP
    factors, including intersection geometry, posted
    speed, weather, time of day, vehicle type and
    occupancy, to name a few. It is also widely
    recognised that most drivers are prepared to accept
    smaller gaps in situations where opposing traffic
    volumes are high (with fewer large gaps available).
    This behaviour is well supported by in-field gap
    acceptance studies and other relevant publications.
    For example, a 2000 study (Tian et al., 2000) found
    that “with the increase of major stream volume or
    minor stream vehicle delay, drivers tend to seek
    smaller gaps.”
    This was corroborated by a 2022 study (Li et al.,
    2022), which concluded that “increased AADT on                                 FIGURE 2: EFFECT ON FOLLOW-UP HEADWAY
    major roads correlated with increased probability of
    accepting a given gap. In [other] words, increased
    AADT correlated with shorter gaps overall.”


    MODELLING OF GAP ACCEPTANCE
    The default gap acceptance parameters (ie critical
    gap and follow-up headway) adopted by SIDRA
    Intersection modelling software for priority-controlled
    intersections are consistent with those outlined in
    Austroads’ Guide to Traffic Management Part 3:
    Transport Study and Analysis Methods (2020).
    However, recognising that the default gap
    acceptance parameters may not accurately reflect
    driver behaviour at priority-controlled intersections




    L:\MARKETING\CORPORATE MATERIAL\PRACTICE NOTES\PTT_PN_19_GAP ACCEPTANCE.DOCX