Strategic Flow Outputs to VISSIM
Introduction
Welcome to the latest Modelling Group blog post.
In this post, we are going to discuss something that we come across regularly in our projects, which is the process of using strategic modelling outputs to inform traffic growth within microsimulation models.
This is something that can incur diverse interpretations and various working practices, so we will discuss our methodology, which has been accepted as a valid method on a number of significant modelling projects, as well as highlight some of the elements to consider as you go through the process.
Differences in Strategic Models and Microsimulation Models
The main differences between strategic and microsimulation models when considering the use of the outputs are detailed in Figure 1.
Figure 1 – Differences in Strategic & Microsimulation Models
Elements to consider when transferring flow data between Strategic Model and Microsimulation Model
The main elements to consider when transferring outputs between strategic and microsimulation models are detailed in Figure 2.
Figure 2 – Elements to consider when transferring outputs between strategic & microsimulation models
Modelling Group’s Methodology for Transferring Flow Data
Modelling Group utilise the following steps to ensure that the future traffic flows are assigned correctly and are an accurate representation of the future traffic, in relation to the validated microsimulation flows.
Figure 3 – Methodology for Transferring Flow Data
Outputs required from the Strategic Model
The following outputs are required from the strategic model, when used in a microsimulation model.
Outputs Interrogation & Elements to Consider
When assessing the outputs, the following datasets should be created:
Difference matrices between Base & all Future Year scenarios
The difference matrices (representing proportional/percentage differences between base and future) will then be assigned to the base VISSIM traffic flow matrices using the relative differences. However, if the strategic model base data and the VISSIM base data differ significantly, issues can arise when applying relative differences.
As an example, if the strategic model has a low flow in the base and a proportionally higher flow in the future, the percentage difference between the scenarios may be high, even though the actual difference isn’t. If the VISSIM model has a higher base flow than the base strategic model, applying the percentage difference to it may artificially increase the future traffic flow to an unrealistic level.
Other issues arise if there are no strategic flows between particular O-D pairs which have flow in the microsimulation base – the relative difference between the base and future would be 0 – therefore a manual intervention would be needed for this movement.
Finally, after applying the growth (either relative or absolute) to the VISSIM flows, a negative value may be returned. VISSIM is not able to model a negative vehicle. Therefore, any negative flows would be likely rebased to 0, although this would also need to be assessed on a case-by-case basis.
Figure 4 – Outputs required from Strategic Model
Working Example of VISSIM future flow calculation from Strategic Model flow
Figure 5 shows how an indicative VISSIM future flow would be calculated if the base flow is a) similar and b) significantly different to the base Strategic Model flow.
Both examples use an indicative growth factor of 120% and an additional 26 vehicles to represent development trips. The Strategic Model base flow has been set to 70 vehicles. The Strategic Model uplift would return 110 vehicles. (70 * 1.20 + 26).
Similar VISSIM base flow to Strategic Model base flow:
Using the relative flow uplift method (120%): with a base VISSIM flow of 85, a future VISSIM flow of 128 would be returned. (85 * 1.20 + 26 vehs)
Using the absolute flow uplift method (+14 vehs): a base VISSIM flow of 85, a future VISSIM flow of 125 vehs would be returned. (85 + 14 + 26 vehs).
Significantly different VISSIM base flow to Strategic Model base flow:
Using the relative flow uplift method (120%): a base VISSIM flow of 110, a future VISSIM flow of 158 would be returned. (110 * 1.20 + 26 vehs).
Using the absolute flow uplift method (+14 vehs): a base VISSIM flow of 110, a future VISSIM flow of 150 would be returned. (110 + 14 + 26 vehs).
Figure 5 – Working Example Flow Differences
Closing Comments
When using flows from a strategic model to inform the future uplift of traffic a number of steps need to be taken to check that the proposed flows are reasonable, accurate and reflect both the base VISSIM validation and the strategic future traffic levels.
Whether using a relative flow or an absolute flow, there is a need to make sure that the resulting proposed flow is a reasonable value that makes sense.
Summary
We hope this post is useful and has helped to provide a bit of insight into our methodology for using Strategic Model flows in microsimulation models, as well as providing some details on elements to look out for during the analysis of the flows.
It is appreciated that different interpretations of the flows can provide different results, but we hope that some of the points raised from our own experience help you either with starting out or refining your way of working with Strategic Model outputs.
Thanks for reading!