Contents
Closely-Spaced Intersections and Network Applications
Staged Movements at Sign-Controlled Intersections
Effect of Upstream Signals on Sign-Controlled Intersections and Roundabouts
Gap Acceptance Survey
Intersection Level of Service for Sign-Controlled Intersections
Roundabout Model Calibration for US Conditions
Importing old version (DAT and AAP) files into SIDRA INTERSECTION
Viewing and Printing the User Guide Externally
Closely-Spaced Intersections and Network Applications
SIDRA INTERSECTION Version 4 has been enhanced 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.
Back to Top
Staged Movements at Sign-Controlled Intersections
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)
Back to Top
Effect of Upstream Signals on Sign-Controlled Intersections and Roundabouts
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).
Back to Top
Method for Field Observation of Critical Gap and Follow-up Headway Parameters for Gap-Acceptance Analysis
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 Version 4 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).
Back to Top
Intersection Level of Service for Sign-Controlled Intersections
In SIDRA INTERSECTION, an Intersection Level of Service is not calculated for two-way sign-controlled intersections (stop sign or give-way/yield sign). "Not Applicable" is displayed for Intersection Level of Service in SIDRA INTERSECTION output reports (Movement Summary, Intersection Summary, Lane Summary). This is because the uncontrolled major road movements experience little delay at two-way sign-controlled intersections, and as a result, the average intersection delay does not reflect the delay levels of minor movements subject to sign control. This is in line with the HCM specification for two-way stop sign control.
Back to Top
Roundabout Model Calibration Recommended for US Conditions
Research on US roundabouts (NCHRP 3-65) indicated lower capacity rates at US roundabouts compared with both Australian and UK conditions (even compared with the HCM 2000 capacity model for single-lane roundabouts). This research showed the importance of driver behavior on roundabout capacity. As a result of the findings of US roundabout research, the default Environment Factor is 1.2 in the HCM versions of SIDRA INTERSECTION. This results in substantially lower capacities compared with SIDRA INTERSECTION standard default (Environment Factor = 1.0) and software packages based on the UK (regression) models. The Environment Factor can be calibrated at an approach level.
The roundabout capacity model described in the NCHRP Report 572 (Roundabouts in the United States) for single-lane and multi-lane roundabouts based on research on US roundabouts has been implemented in SIDRA INTERSECTION Version 4,. Use the Roundabout Capacity Model parameter in the Advanced Model Settings input dialog for selecting the NCHRP 572 model option in SIDRA INTERSECTION.
Back to Top
Importing old version (DAT and AAP) files into SIDRA INTERSECTION Projects
Version 3.2 (AAP) project files: You can open a Version 3.2 (or 3.1) Project file in Version 4. Click the SIDRA INTERSECTION button (dark grey round button top left of the screen) and click Open. In Open dialog, select the file type as SIDRA INTERSECTION 3.x Files. Your files with aap extension will appear in the dialog.
Version 2.1 (DAT) data files: You should not attempt to OPEN an old DAT file in SIDRA INTERSECTION Version 4 (or 3.2). You must Import the DAT file as a new Site in an existing Project created in SIDRA INTERSECTION Version 4 (or 3.2).
Viewing and Printing the User Guide Externally
The User Guide Viewer available in SIDRA INTERSECTION user interface is limited in terms of window size and printing abilities. To view or print sections of the User Guide outside the software, you will need to download The XPS Essentials Pack from the Microsoft website.
This applies to Windows XP and Vista only as Windows 7 has native support for XPS files.
The four parts of the User Guide document are contained in four files in XPS format, which can be found in the folder C:\Program Files\SIDRA Solutions\SIDRA INTERSECTION 4.0. The files are named:
• SIDRAInt_USER GUIDE_1-INTRO.xps
• SIDRAInt_USER GUIDE_2-STARTING.xps
• SIDRAInt_USER GUIDE_3-INPUT GUIDE.xps
• SIDRAInt_USER GUIDE_4-OUTPUT GUIDE.xps.
After you have installed the XPS Essentials Pack, you can open these files by double-clicking the file name. To view the section bookmarks, click the Open Outline View icon in the XPS Essentials Pack toolbar or select View - Document Outline from the menu. You can print selected pages of the document by using File - Print in the XPS Essentials Pack menu.
Back to Top