Midblock lane changes at paired (closely-spaced) intersections

Modified on: 2015-06-02 10:23:07 +1000

An important modelling issue arises in relation to lane choices of drivers at paired (closely-spaced) intersections, e.g. freeway signalised diamond or roundabout interchanges, fully signalised roundabouts, staggered T intersections, and so on. This is particularly important in evaluating closely-spaced intersections with high demand flows where vehicles have limited opportunities for lane changes between intersections.

By matching the upstream and downstream flow rates on internal approach lanes reported by SIDRA INTERSECTION, midblock lane change implications can be studied. If large midblock lane change flow rates are implied, this means that modelling of lane use on external approaches should be reviewed since the number of lanes allocated to movements through the network may not be consistent, i.e. may not be adequate for volumes at upstream and downstream intersections. Currently, the program gives a table in the Detailed Output report showing lane change flow rates for signalised intersections. This output will be included for roundabouts and sign-controlled intersections in the forthcoming Version 7. In the current version, the users need to inspect the upstream and downstream flow rates in the Detailed Output report for this purpose.

Attention to the following software facilities are recommended in using SIDRA INTERSECTION for paired intersections;

  • The modelling of lane use patterns can be enhanced by assigning two types of movements negotiating the network to User Classes to be treated as “Special Movement Classes”. These are the through movements at external approaches which become turning movements at downstream internal approaches, and the dogleg movements at staggered T intersections. These movements can be assigned to separate lanes and separate signal phases, and their lane use and second-by-second platoon patterns for signals can be tracked through the network separately, helping to identify unequal lane use cases at external approaches of a paired intersection system. For signals, this improves the quality of signal platoon modelling and is expected to produce better results in assessing signal coordination quality and optimising signal offsets.
  • “Lane Movement Flow Proportions” can be set in the Lane Data dialog to define exit lanes for all movements by paying attention to downstream lane changes.
  • “Lane Utilisation Ratios” can be specified in the Lane Data dialog to match the upstream and downstream lane flow rates so that the unequal approach lane use implications of a given geometric design are assessed.

While lane use estimation is important for single intersection analysis due to different characteristics of approach lanes, especially when exclusive lanes are specified by the user or defacto exclusive lanes are identified by the program as a function of turning volumes and control conditions, the matching of upstream and downstream lane flow rates is important for the analysis of closely-spaced intersections.

For roundabouts, SIDRA INTERSECTION will always apply detailed lane use modeling to single intersection and network analysis irrespective of the roundabout capacity model option used (HCM or SIDRA Standard).

Refer to the following papers on the unique lane-based network model used in SIDRA INTERSECTION (available for download from www.sidrasolutions.com/Resources/Articles):

For a staggered T-roundabout example:
Lane-based micro-analytical model of a roundabout corridor. Paper presented at the CITE 2013 Annual Meeting, Calgary, Alberta, Canada, Apr 2013.

For an example of signalised staggered T intersections:
A new lane-based model for platooned patterns at closely-spaced signalised intersections. Paper presented at the 26th ARRB Conference, Sydney, Australia, Oct 2014.

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