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7 December 2016 1
Junction Modelling in Strategic Models
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James Parrott
AITPM 2017 National Conference
Junction Modelling for Strategic Models 2
Junction Modelling
The majority of network delay is at intersections
Junction Modelling
• Experience in using Strategic, Meso, Micro and Single Point
• Tiers of modelling have differing purposes
• Everything below strategic accounts for junction modelling
Question: Should we include junction modelling in strategic models
• Not common in Australia
• No definitive answer
• Investigate reliability, ability to model upgrades and attempt to minimise model noise
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Cube Junction Modelling
It’s already in use……
Focus of review was Cube Voyager’s junction modelling
• Over 10 years experience of use
• Conducted detailed analysis of possible methodologies
Junction Model Types
Junction Modelling for Strategic Models 4
Cube Junction Modelling
Some parameters are more important than others
Junction modelling has developed over the years with numerous additions to the capability. However there are several key inputs:
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Impact and Limitations
No model is correct, nor perfect. As modellers, it is our job to use these tools to the best of our ability
Inclusion of junction modelling has two key impacts
• Must reduce speed flow delays to remove junction delay
• Use volume averaging for assignment
Limitations include
• Simplified phasing
• Max of four arms
• No lane lengths
• Intergreen only total
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Reliability
In order to use something, we must have confidence that it is fit for purposeSix intersections tested
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Performance Comparison
Understanding strengths and weaknesses is a core ingredient to the successful use of any tool
Results validated well with some exceptions
• Signalised junctions for controlled movements were generally similar
• Optimisation of phasing tended to favour major movements
• Left turns (yield or continuous) were not modelled well
• Unsignalised intersections are much harder to model and vary more between software packages
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Sensitivity and Model Noise
Model noise is not new, but we still grapple with how to best account for it.
Series of options to test model sensitivity and whether proposed approaches could minimise junction noise
• Options: Actuated, Fixed and No Junction Modelling
• Use of warm-start
• Simple project case involving upgrade of single intersection
• Approach focused on isolating parameters for testing
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Junction Modelling Approach
We must have confidence in the results, but not blind faith Impact of junction upgrade
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Junction Modelling Approach
Some tools are better than others at providing guidance for certain problems Comparison of junction upgrade vs non-junction upgrade
• Link (lane) amendment to reflect change
• Time savings were negligible
• Junction savings of 18 seconds per veh for 110% of base demand
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Mitigating Model Noise
Isolating and improving the accuracy of project benefits must be at the forefront for us to improve our business model and create more successful projects
Attempts to reduce the impact of noise throughout the network tested:
• Combination of assessing actuated/fixed and use of warm-starts
• Review of results at sector level for Simple junction upgrade
• Noise persisted in all options
• Warm-starts converged much faster
• Size of network modification irrelevant
• No. of iterations critical in the assessment of benefits – what is equilibrium?
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Conclusions
Some answers, more questions. Let the debate continue…..
Set out to review reliability, ability to assist in assessing network impacts and ability to isolate changes:
• Signalised junctions are reliable – must account for left turns
• Junction Modelling is reactive to network changes with intuitive results
• Model noise is not easily addressed
• Warm starts should be considered to improve speed of model (given slower convergence with junction modelling)
• Assessing benefits – consider local area, review iterations completed
• Continue the research…….