This document discusses junction modelling in strategic transportation models. It begins by noting that most network delay occurs at intersections and that junction modelling is not commonly used in Australian strategic models. It then evaluates Cube Voyager's junction modelling capabilities, finding that signalized junctions were generally modeled well but left turns and unsignalized intersections were more difficult. The document tests the impacts and limitations of including junction modelling, finding it can reduce speed-flow delays but uses volume averaging. It also looks at reliability, sensitivity, and ways to minimize model noise when assessing benefits of junction upgrades. It concludes that junction modelling provides intuitive results and benefits assessment should consider the local area and number of model iterations.

1. 7 December 2016 1
Junction Modelling in
Strategic Models___
James Parrott
AITPM 2017 National Conference

2. 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

3. Junction Modelling for Strategic Models 3
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

4. 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:

5. Junction Modelling for Strategic Models 5
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

6. Junction Modelling for Strategic Models 6
Reliability
In order to use something, we must have
confidence that it is fit for purpose
Six intersections tested

7. Junction Modelling for Strategic Models 7
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

8. Junction Modelling for Strategic Models 8
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

9. Junction Modelling for Strategic Models 9
Junction Modelling Approach
We must have confidence in the results, but not
blind faith
Impact of junction upgrade

10. Junction Modelling for Strategic Models 10
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

11. Junction Modelling for Strategic Models 11
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?

12. Junction Modelling for Strategic Models 12
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…….