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Contents
CLOSELY-SPACED Intersections
Staged Movements at Sign-Controlled Intersections
Effect of Upstream Signals on Sign-Controlled Intersection Capacity
Gap Acceptance Survey
Intersection Level of Service for Sign-Controlled Intersections
Roundabout Model Calibration for US
Conditions
Importing old DAT files into SIDRA INTERSECTION Projects
Older version aaSIDRA 2.1
CLOSELY-SPACED Intersections
SIDRA INTERSECTION can be used to analyse closely-spaced intersections as in the case of arterials and networks. This applies to both undersaturated and oversaturated cases, i.e. whatever the degrees of saturation are. The important aspects of modelling closely-spaced intersections are the platooned arrivals from upstream intersections and blockage of downstream entry lanes.
To use SIDRA INTERSECTION in this context, the following steps should be taken.
- 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.
- You can inspect the estimated queue lengths in Output Tables D.3A and D.3B to identify the probability of queue length exceeding the available storage length. The capacity of the upstream intersections will be affected by the downstream queues accordingly. For this purpose, find the percentile queue value that is equal to the available space. Example: the queue space (lane length) is 90 metres and the percentile queue lengths are as follows:
Nb(50%): 71 m
70%: 87 m
85%: 103 m
90%: 111 m
95%: 123 m
98%: 134 mIn this example, the probability of the queue space being exceeded is about 30 per cent (queue is longer than 87 m for 100% - 70% = 30% of the time). Note: Nb is the average queue length which can be considered to be the 50th percentile queue
- ROUNDABOUTS: For platooned arrivals at roundabouts (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 approach is minimum 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.
If downstream approaches are blocked, you can adjust the Environment Factor values of upstream roundabout approaches to reduce capacity to match the probability of the downstream queue exceeding the available storage space as determined above. Refer to the User Guide.
- 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, you may reduce the effective green time of upstream movements by specifying extra start loss and end gain values (time losses) that match the blockage times (determined from the probabilities of queue storage space being exceeded on downstream approaches).
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.
TopAnalysis of Staged Movements at Sign-Controlled Intersections
The following documents describe a method recommended for analysing staged movements at sign-controlled T-intersections using SIDRA INTERSECTION. The method assumes that the turning vehicle is subject to a two-stage gap-acceptance process, one on the side road and one in the central area of the intersection. The model requires specification of appropriate opposing movements and gap-acceptance parameters for each stage.
Document for driving on the LEFT-HAND side of the road
Project File (example) for driving
on the LEFT-HAND side of the road
Project File (example) for driving
on the LEFT-HAND side of the road (New Zealand)
Document
for driving on the RIGHT-HAND side of the road
Project File (example) for driving
on the RIGHT-HAND side of the road
Effect of Major Road Upstream Signals on Minor Road Capacities at Sign-Controlled Intersections
You can use SIDRA INTERSECTION to emulate the effect of major road upstream signals on the capacity of minor movements at intersections controlled by Stop and Give-way (Yield) signs. This involves estimating operating characteristics by creating HIGH and LOW opposing flow scenarios and combining the results to estimate an average capacity for the opposed movement.
The proportions of time when the HIGH and LOW opposing flow periods apply are determined by considering the cycle time, green and intergreen times at the upstream intersection.
(i) HIGH Opposing Flow:
This scenario represents the platooned opposing flow conditions.
Create a Site with HIGH opposing flow rate and normal gap-acceptance parameters. The capacity during this interval is likely to be very low (190 veh/h in the example). If it is considered that the capacity available during this interval is negligible, this scenario could be ignored for capacity estimation purposes.
(ii) LOW Opposing Flow:
This scenario represents the non-platooned opposing flow conditions.
Create another Site for the case of a short interval with LOW opposing flow rate. Adjust the follow-up headway to a low value (e.g. 2.0 s), or use observed values of follow-up headway if possible. Adjust the critical gap by the same proportion (the results are less sensitive to the value of critical gap for low opposing flow rates). For example if Follow-up Headway = 3.0 s and Critical Gap = 5.0 s were used for Scenario (i), and Follow-up Headway = 2.0 s is chosen for Scenario (ii), then Critical Gap = 3.3 s [= (2.0 / 3.0) x 5.0] for Scenario (ii).
The use of a short follow-up headway is critical to this analysis. It corresponds to a high saturation flow rate (e.g. 3600 / 2.0 =1800 veh/h) when a gap is created by upstream signals (or when long gaps are found in low non-platooned arrival flows). This is probably true due to the vehicles waiting for a long time during platooned major road flows, and then departing quickly when major road gaps occur. This is similar to the adjustment made to the gap-acceptance parameters in the SIDRA INTERSECTION roundabout model to allow for cases of high arrival flow rate vs low circulating flow rate. In other words, general default values of gap-acceptance parameters are not appropriate for special circumstances like this. This may be the reason for the US HCM model used for this purpose failing to provide satisfactory answers in spite of detailed platoon modeling.
(iii) COMBINED (Average) Capacity:
This scenario represents the conditions when the capacity of the opposed movement equals the average capacity obtained by combining Scenarios (i) and (ii).
Calculate the weighted average capacity of the two scenarios using the proportion of time each scenario applies.
Create a Site with AVERAGE opposing flow rate and normal gap-acceptance parameters. Use the Sensitivity Analysis feature of SIDRA INTERSECTION. In the Sensitivity & Design Life input dialog, select Critical Gap & Follow-up Headway, and specify a fixed Scale Factor value (Lower = Upper) that will result in the average capacity value calculated for opposed movement (use values less that 100 %, and determined the required value by trial and error).
More than two scenarios could be modeled using the same method. This may be required due to intervals with different platooned flow rates according to the arrival times of platoons from the two directions of the major road.
Method for Field Observation of Critical Gap and Follow-up Headway Parameters for Gap-Acceptance Analysis
Updated 1 December 2007
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. The following references provide good information about appropriate field survey methods that can be used for this purpose:
- TRB (1997). Review of International Practices Used to Evaluate Unsignalized Intersections. Circular 468. Transportation Research Board, National Research Council, Washington, DC, USA. (see pages 8-12). Available from TRB Book Store
- BRILON, W., KOENIG, R. and TROUTBECK, R. (1997). Useful estimation procedures for critical gaps. In: Kyte, M. (Ed.), Proceedings of the Third International Symposium on Intersections Without Traffic Signals, July 1997, Portland, OR, USA, University of Idaho, Moscow, ID, USA, pp 71-87.
In view of these documents, a relatively simple method is available to obtain critical gap and follow-up headway estimates from field observations. The method attributed to Siegloch (Germany) requires queued conditions of the minor (opposed) movement since the critical gap and follow-up headway parameters are relevant to capacity estimation (see the graph). Method is implemented as follows:
- Make observations during times when there is, without interruption, at least one vehicle queuing in the minor street. A reasonably high number of queued vehicles is needed for a reliable regression.
- Record the number of vehicles, n, entering each main stream gap (headway) of duration t (including n = 0 cases).
- For each of the gaps accepted by n vehicles, compute the average of the accepted gaps t (circles in the graph).
- Find the linear regression of the average gap (headway) values
as a function of the number of vehicles: t = a + b n where
to = a = tc - 0.5 tf, and b = tf, therefore
follow-up headway: tf = b
critical gap: tc = to + 0.5 tf - In the example shown in the graph:
to = 3.73 s
tf = 2.31 s
tc = 4.89 s
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 text output (Movement Summary, Intersection Summary, and Output Tables S.3 and S.15). 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.
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). These preliminary research results were presented at the TRB National Conference on Roundabouts, Vail, Colorado (22-25 May 2005). This research showed the importance of driver behavior on roundabout capacity. A document presenting a simple Roundabout Model Comparison Table comparing Australian, UK and US roundabout models is available on our web site (Downloads).
As a result of the findings of US roundabout research, we have set the default Environment Factor as 1.2 in the HCM versions of the latest SIDRA INTERSECTION version 3. We emphasize that this will result 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 in the new version.
SIDRA INTERSECTION 3 includes the FHWA model as well, with an additional calibration facility.
When the new research results are incorporated into a revised HCM chapter on roundabouts, we will incorporate the new method specified in the HCM into SIDRA INTERSECTION and make it available for US users.
Importing old DAT files into SIDRA INTERSECTION Projects
You should not attempt to OPEN old DAT files in SIDRA INTERSECTION. You need to use the import facility (Project - Add Site - Import from DAT File).
Using SIDRA INTERSECTION 3 and older version aaSIDRA 2.1 concurrently
You can keep using aaSIDRA 2.1 after you install SIDRA INTERSECTION 3. In some cases, aaSIDRA 2.1 may become "unregistered" automatically when you install SIDRA INTERSECTION 3. This is a minor issue with the licensing software used with version 2.1 as explained below.
SIDRA INTERSECTION 3 requires installation of the Microsoft .NET Framework version 2. This has the side-effect of "delicensing" versions of aaSIDRA 2.1 which do not have the latest update pack applied. If this occurs and you wish to use SIDRA 2.1, please see Update 2.1.3 to 2.1.4 (Limited application) for details regarding this issue and an Update Pack that should solve the problem.
You will need to re-enter your Product Key for version 2.1 under the heading "Serial Number" after applying the Update. You will not need to reinstall the program to do this. You may also need to refer to aaSIDRA 2.1 Update Packs 1 & 2.