Closely-Spaced Intersections and Network Applications

SIDRA INTERSECTION can be used to model closely-spaced intersections and interchanges as small Networks. These include paired intersections (staggered T, intersection with wide median, staged crossing at sign-controlled intersection), signalised diamond interchange, roundabout interchange, fully-signalised roundabout, and alternative intersections and interchanges (e.g. diverging diamond interchange).

The common element for these facilities is the limited queue storage space available in internal approaches, and therefore the strong possibility of blockage of external approach lanes. It is necessary to pay careful attention to the relationship of lane utilisation at intersections to midblock lane changes in the analysis of paired intersections in particular, and Network analysis in general.

The SIDRA INTERSECTION Network model is a unique lane-based micro-analytical model unlike traditional analytical link-based Network models where links represent lane groups in which traffic conditions of individual lanes are aggregated and therefore lost in more aggregated traffic units. An approach-based method is a more extreme case of this where differing conditions of traffic in all approach lanes are aggregated to some assumed average (balanced) condition. Such link-based (lane-group based) and approach-based Network models cannot identify backward spread of congestion for closely spaced intersections.

Two fundamental elements of the lane-based traffic Network model in SIDRA INTERSECTION are (i) determination of the backward spread of congestion as queues on downstream lanes block upstream lanes (queue spillback), and (ii) application of capacity constraint to oversaturated upstream lanes for determining exit flow rates, thus limiting the flows entering downstream lanes. These two elements are highly interactive with opposing effects. A network-wide iterative process is used to find a solution that balances these opposing effects.

Another fundamental element of the Network model in SIDRA INTERSECTION is the modelling of forward movement of signal platoons for coordinated signal Sites. The signal platoon model uses second-by-second arrival and departure flow patterns as a function of Signal Offsets. Unlike traditional Network models that use aggregate models of links or lane groups, it is a lane-based model for platoon movements between upstream and downstream Sites (on internal approaches). The model includes a unique platoon dispersion model.

The signal platoon model in SIDRA INTERSECTION takes into account midblock lane changes that apply to second-by-second platoon patterns. This is particularly important in evaluating closely-spaced intersections with high demand flows where vehicles have limited opportunities for lane changes between intersections. These requirements are important in modelling the forward movements of platoons for estimating performance measures (delay, back of queue, stop rate) as a function of signal offsets, geometric design and flow conditions.

Network signal timing calculations determine Network cycle time and phase times for coordinated signals and Common Control Groups (CCGs), and signal offsets for coordinated signals.

Paired intersections and interchanges operating under a single signal controller are modelled as a Common Control Group (CCG) in SIDRA INTERSECTION. Use the Define CCGs and CCG Phasing & Timing dialogs in the NETWORK INPUT section in the Project pane under the Network tab. The Define CCGs dialog allows definition of Common Control Groups. The CCG Phasing & Timing input dialog includes a Select CCG tab and a set of CCG Phasing & Timing tabs similar to the Phasing & Timing input dialog for individual Sites.

The SIDRA INTERSECTION Network model also includes the use of extra bunching for signal platoons arriving at roundabouts and sign-controlled intersections for estimating the effect of upstream signals. Extra Bunching values are determined by the program automatically by default.