Technical Advice

SIDRA INTERSECTION has facilities for Closely-Spaced Intersection and Network applications. While SIDRA INTERSECTION is a single-intersection analysis software, its model does not assume isolated operations. Thus, it can be used for analysing closely-spaced intersections, pedestrian crossings near intersections, or any number and type of multiple intersections. This applies to both undersaturated and oversaturated cases, i.e. whatever the degrees of saturation (v/c ratios) are. The important aspects of modelling closely-spaced intersections are the platooned arrivals from upstream intersections and blockage of downstream entry lanes.

Signal coordination effects on signalised intersection performance can be modelled using the Arrival Type or Proportion Arriving During Green parameter, and upstream signal effects on unsignalised intersection (sign-controlled or roundabout) capacity can be modelled using the Extra Bunching parameter. The Capacity Adjustment parameter can be used to model the amount of capacity reduction for upstream intersection lanes based on the probability of blockage due to limited queue storage estimated for downstream intersection lanes.

To use SIDRA INTERSECTION in relation to Closely-Spaced Intersection and Network applications, the following steps should be taken.

GENERAL

You need to specify the approach and exit lane lengths and number of lanes (including any approach and exit short lanes) for all intersections realistically, especially for internal approaches between intersections so that queue storage problems can be identified.
Lane underutilisation is an important issue at closely-spaced intersections. In particular, this may occur when the number of downstream lanes available to a movement is less than the number of lanes available at an upstream approach. Lane flow estimates of SIDRA INTERSECTION should be inspected and lane utilisation specifications should be given if necessary.
The probability of queue length exceeding the available storage length (probability of blockage) is given for each lane in the Lane Summary output report. The capacity of the upstream intersections will be affected by the downstream queues accordingly. Use the Capacity Adjustment parameter in the Approaches & Lanes input dialog to allow for the effect of blockage of upstream lanes.
For more detailed modelling, the capacity constraint method can be applied, namely the flow rates of the downstream movements can be reduced so that the average downstream queue equals the storage space. The capacity of the upstream movements can then be reduced to match the downstream flow rates.

ROUNDABOUTS and SIGN CONTROL

For platooned arrivals at roundabouts and sign-controlled intersections (due to the effect of upstream signals), you can specify an Extra Bunching parameter following the guidelines in the User Guide. Platooned arrivals are not as important at roundabouts unless the extra bunching is very high and the proportion queued is very low. This is because the capacity of a downstream approach of the roundabout is determined by the circulating flow, and the headway distribution of circulating flow is affected by queues on upstream approaches which filter out platooning depending on the amount of queuing. All this is modelled by SIDRA INTERSECTION.

The effect of upstream roundabouts or sign-controlled intersections on the arrival pattern at a roundabout or sign-controlled intersection approach is minimal since the departure patterns from upstream unsignalised intersections are randomised due to the gap-acceptance process. So you can simply ignore any special effects of the departure patterns for these types of upstream intersections.

SIGNALISED INTERSECTIONS

When you analyse closely-spaced signalised intersections with signal coordination, apply the following steps:

• Run each intersection individually and determine the cycle time. The intersection with the longest cycle time is the critical intersection. Use the fixed-time analysis and practical cycle time method for timing calculations. Pay attention to minimising queue lengths on internal approaches when choosing the cycle time and green times. Use the green split priority feature of SIDRA to favour the internal approaches in green time allocation. If desired, you can use larger practical degrees of saturation (e.g. 0.95) to limit the green time allocated to side roads. Select a system cycle time accordingly, specify this cycle time for each intersection, and re-analyse.
• Nominate internal approach movements as "coordinated" for platooned arrivals, and specify Arrival Types (or Percent Arriving During Green). This is important in determining queue length, delay, etc at signalised intersections. Refer to the User Guide.
• If it is identified that queues are exceeding the available storage space (as explained above), you may decide to change the signal phasings and timings to achieve a satisfactory solution in order to minimise the queue lengths.
• If the phasings and timings cannot be changed, you may want to model the effect of internal approach queues (blockages) on external approach capacities, delay and queues. For this purpose, use the Capacity Adjustment parameter. Refer to the User Guide.

SIDRA INTERSECTION provides a template (in the Sign Control group) for analysing staged right-turn movements at sign-controlled T-intersections (sometimes referred to as the "seagull" arrangement). The method is described in the SIDRA INTERSECTION User Guide, Part 3 (Input Guide)

SIDRA INTERSECTION can be used to model the effect of major road upstream signals on the capacity of minor movements at intersections controlled by Stop and Give-way (Yield) signs, as well as roundabouts. For this purpose, the extra bunching parameter is used in association with the bunched exponential model of opposing stream headways. The method is described in the SIDRA INTERSECTION User Guide, Part 3 (Input Guide).

Effect of upstream signals is also modelled when the HCM 2010 roundabout capacity model option is used, or a gap-acceptance capacity equation option with one of several M1 (random) headway distributions is selected.

The capacity and performance of unsignalised intersections are very sensitive to the values of critical gap (headway) and follow-up headway parameters. In particular, in the case of unsignalised intersections controlled by two-way stop and give-way signs, the SIDRA INTERSECTION user needs to specify appropriate values of gap acceptance parameters to suit local driver characteristics and the intersection geometry and flow conditions. SIDRA INTERSECTION provides a large number of sign-controlled intersection templates with critical gap and follow-up headway parameters which are more appropriate for selected intersection geometries.

A relatively simple method is available to obtain critical gap and follow-up headway estimates from field observations. The method requires queued conditions of the minor (opposed) movement since the critical gap and follow-up headway parameters are relevant to capacity estimation. The method is described in the SIDRA INTERSECTION User Guide, Part 3 (Input Guide).

In SIDRA INTERSECTION, Level of Service is not calculated for the intersection as a whole, or for major road approaches for two-way sign-controlled intersections (stop sign or give-way/yield sign). "Not Applicable" is displayed in SIDRA INTERSECTION output reports (Movement Summary, Intersection Summary, Lane Summary, LOS Summary). This is because the uncontrolled major road movements experience little delay at two-way sign-controlled intersections, and as a result, the average delay for the intersection, or the major road approaches, does not reflect the delay levels of minor movements subject to sign control or opposed turns from the major road. This is in line with the HCM 2010 specification for two-way stop sign control.

SIDRA INTERSECTION offers two roundabout capacity model options for US conditions: the US HCM 2010 model and the SIDRA Standard model with the Environment Factor value of 1.2.

The roundabout capacity model described in HCM 2010, based on research on US roundabouts described in NCHRP Report 572 has been fully integrated into SIDRA INTERSECTION Version 5.1. The new US HCM 2010 roundabout capacity model option replaces the NCHRP 572 option used in previous versions of the software.

In SIDRA INTERSECTION Version 5.0 and earlier versions, the SIDRA Standard option was used as the default method for the US HCM versions of the software. With the US HCM 2010 option selected as default in Version 5.1, roundabout capacity results (and therefore the performance and level of service results) are likely to differ significantly.

Using the Capacity Model parameter in the Model Settings dialog, Roundabouts tab, you can change between the US HCM 2010 and SIDRA Standard roundabout capacity models. There are various key parameters involved in changing between the two models. Refer to sections of the User Guide on the HCM 2010 roundabout capacity model in Parts 3 and 4 of the guide, also see the document named "SIDRAINTERSECTION51_UpdateNotes" in the UPDATES page of this website for Version 5.1.

Version 3.2 (aap) project files: You can open a Version 3.2 (or 3.1) Project file in Version 5.1, 5.0 or 4.0. Click the SIDRA INTERSECTION button (dark grey round button top left of the screen) and click Open. Your files with aap extension will appear in the Open dialog. When you select a file and click OK, the file will be imported. After the import process has been completed, the Project will be opened in the Project Pane showing all Sites that are included in it. You must use Save or Save As to save the Project in Version 5 (sip) format. Once imported into a later version, the data cannot be saved in the earlier Version 3 (aap) format.

Version 2.1 (DAT) data files: You should not attempt to OPEN an old DAT file in more recent versions of SIDRA INTERSECTION (5.1, 5.0, 4.0, 3.2). Instead, you need to IMPORT the DAT file as a new Site in a Project created in more recent versions. Refer to User Guide, Part 2, Section 2.2.

The User Guide is in four parts, namely Introduction, Getting Started, Input Guide and Output Guide. These can be accessed via the commands Intro&Starting, Input Guide and Output Guide in the ribbon (User Guide group).

The User Guide Viewer available in SIDRA INTERSECTION user interface is limited in terms of window size and printing abilities due to the use of XPS document format. For licensed users, all parts of the User Guide are available in a single document in PDF format accessible through the user eCommerce account.