1. A traffic model for every occasion - what to use when.
TRICS Conference workshop – Tuesday 20th November 2007
Alan Bain
Director
JMP Consulting

2. Today’s Programme
Part 1: Introduction to Modelling
Part 2: Modelling Techniques
Part 3: Jargon (and Myths)
Part 4: Modelling Process
Part 5: Outputs
Part 6: Workshop

3. Course Objective
Provide attendees with an overview of available modelling techniques including interactive sessions addressing typical modelling ‘scenarios’. It will enable attendees to understand benefits as well as limitations. The course is primarily targeted at those who do not have a traditional modelling background but are either project managing studies which have a modelling element or engage with clients or potential clients who may have a need for a modelling service.

4. Introduction to Modelling – History of Transport Modelling
Popular belief is that transport modelling began in the US in the 1950s:
Chicago Transportation Study
Yet in 1925, the Cross-River Commission evaluated 12 potential river-crossings in Brisbane, using:
a five day classified count on the Victoria Bridge
an origin-destination survey
speed studies
vehicle operating cost assessments

5. Introduction to Modelling – The Need for a Traffic Model
Transport Planners require some means to test their proposals and make decisions.
This may include the use of a traffic model.
This is a combination of computer software and data which represents road traffic flow in an idealised form.
Traffic models assist transport planners to design schemes and measures to manage adverse traffic effects.
Traffic models inform the decision making process. They are not a substitute for it.

6. Introduction to Modelling – The Limitations of Traffic Models
Traffic models represent reality in the context of what is known already.
They can predict future traffic flows with infinite precision.
BUT only if the input data is 100% accurate and 100% complete!
Planning authorities collect as little information as possible, and never a full data set, because of the cost and disruption to traffic.
So in reality traffic models cannot predict the future with infinite precision.
Traffic models cannot be more reliable than their data inputs. 6

7. Introduction to Modelling – What should they tell us?
Fundamentally they should indicate how traffic flows and what the costs of travel are. 7

8. Modelling Techniques – What tools are available
Traffic engineering and management – bus priority models, single junction models, local centre models, corridor models (VISSIM, PARAMICS, AIMSUM, DYNASIM)
Assignment modelling – strategic town and city centre models, area action plans, major road schemes (SATURN, PARAMICS, VISUM)
Multi-modal and land use models – regional modelling, North Wales Transport Model, PRISM, SITM (VISUM, OMNITRANS, CUBE, MEPLAN) 8

9. Models – who should do and who should manage?
Very little practical academic training in modelling other than theoretical concepts
Formal training tends to be provided by software manufacturers which sometimes provides a theoretical basis
Modellers tend to become modellers by accident and many reluctant to become pigeon holed
However, generally modelling should be done by those with a recognised competence, otherwise costs will increase and quality will diminish
Much modelling work undertaken to support wider TA, therefore PM may not be a modeller and relying on modeller to provide appropriate support
However, if project is a primarily a modelling job then PM should have a good grasp of fundamentals (if not a recognised modeller themselves) 9

10. Modelling Techniques – Traffic Engineering
Conventional tools such as LINSIG, TRANSYT, ARCADY and PICADY still used but a modeller would not regard them as a traffic modelling tool. Still a role but where is the divide?
Microsimulation tools such as VISSIM and PARAMICS increasingly been used
Applications - bus (or LRT) priority, vehicle actuation, queue relocation, blocking back effects
Microsimulation model should also be considered if the effects of any scheme or development are felt over a wider area

11. Modelling Techniques – Traffic Engineering
Specialist modelling firms consider microsimulation modelling to be simply an extension of the standard traffic engineering techniques but this ignores the high level of capability required to model vehicle actuation and bus priority.
VISSIM generally favoured by TfL and is perceived by some as being more forgiving to the user. However, there are many examples of PARAMICS being accepted by TfL
No route choice within them therefore dynamic routeing is not required.

12. Modelling Techniques – Assignment Modelling
Generally undertaken over a wider area than most microsimulation models (for example town and city centres)
Uses tools such as SATURN.
Assists in assessing the area-wider impacts of schemes as they model the reassignment of
Conventional tools have been challenged in recent years by newer software tools such as PARAMICS and VISUM.

13. Modelling Techniques – Assignment Modelling
Many examples of town and city centre PARAMICS models.
The dynamic assignment capabilities of PARAMICS are generally accepted as being superior to those of VISSIM.
Junction analysis will still be required using conventional tools such as TRANSYT and LINSIG.
Able to model the effects of bus operations but they should not be confused with multi-modal models.

14. Modelling Techniques – Multi-Modal and Land Use Models
Standard models deal with the effects of vehicular traffic with fixed vehicular trip matrices
Multi-modal models use elasticity models to forecast not only vehicle assignment but modal split
Often use combination of TRIPS (or CUBE) software, which handles the multi-modal element and SATURN software which handles the traffic assignment
Should not confuse multi-modal models with conventional assignment models which model bus operations

15. Modelling Techniques – Multi-Modal and Land Use Models
Typically cover areas such as large cities or sub regions. Larger land use transport models such as PRISM (Policy Responsive Integrated Strategic Model) take account of changes in the location and intensity of land uses as well how transport can affect land uses and the economy
Specialist modelling consultancies develop bespoke modelling tools (MEPLAN)
Several areas in which they could be applied such as assessment of new growth points, RSS and LDF processes

16. Modelling Techniques – ITS Modelling
Microsimulation has ability to model the effects of intelligent transport systems
MOVA (Microprocessor Optimised Vehicle Actuation), SCOOT (Split Cycle Offset Optimisation Technique) and ATM (Active Traffic Management).
Allows user to test impacts of a MOVA or SCOOT signal network before they are implemented on the ground (PCMOVA)
MOVA can be linked to VISSIM and Paramics and there are also examples of PARAMICS being linked to SCOOT and ATM (on the M42 and M25 for example)

17. Modelling Techniques – Pedestrian Modelling
Paxport
Legion
Spatial analysis modelling 17

18. Data collection and time periods
Traffic engineering (Turning counts, queue lengths, journey times?) – 1 hour peak?
Assignment modelling (Turning counts, ATCs, journey times, OD surveys, profiles) – 3 hour peak?
Multi modal and land use models (Turning counts, ATCs, journey times, OD surveys, stated preference surveys, public transport demand data) – 24 hour models?

19. Modelling Jargon
Gravity model
Furnessing
Matrix estimation
Profiles
Generalised cost
Route choice
Static assignment
Dynamic assignment
Calibration
Validation
Peak spreading

20. Modelling Jargon – first principles
4-stage transport model
Trip generation
Trip distribution
Modal split
Trip assignment
5th stage – peak spreading

21. Modelling Jargon – Distribution and Gravity Model
Method for distributing traffic
Based on relationship between population and distance between them
number of trips from zone i to zone j will be proportional to their size and inversely proportional to their separation
In this case:
size = productions and attractions
separation = generalised cost of travel
Limited - does not take account of congestion and cost

22. Modelling Jargon - Matrix Building and Matrix Estimation
Method for creating an O-D matrix – improves fit between observed flows and modelled flows by selectively factoring individual cells of the input trip matrix
SATURN – ME2 (maximum entropy) using prior matrix (from roadside interviews perhaps), network file and observed data
Paramics matrix estimation module – uses prior matrix, routing file (from prior matrix) and observed link and turn counts
Furnessing – manual method used when you know origins and destinations but not distribution

23. Modelling Jargon - Profiles
Traditional tools (SATURN, TRANSYT) do not allow profiles – all traffic assigned using flat profile
Microsimulation allows user to assign profiles over modelled period – usually in 5 or 15 minute time slices
Important element if queues are an issue

24. Modelling Jargon – Generalised Cost Equation
Cost to driver made up of time, distance and other costs (such as tolls)
Cost = (A x t) + (60 x B x d) + (C x p) where
A = time coefficients
t = travel time in minutes
B = distance coefficient in minutes per mile
d = link length in miles
C = toll coefficient in minutes per monetary cost
p = price of the toll in monetary cost units
Different generalised costs can be applied to different vehicles/trip purpose

25. Modelling Jargon – Assignment
Static assignment – all or nothing
Dynamic assignment – takes account of congestion and delay
Road hierarchy – major/minor
Cost factors – reflect characteristics of road
Driver perception - perturbation

26. Modelling Jargon – Calibration
The DMRB defines model calibration as “the process of adjusting the parameters used in the various mathematical relationships within the model to reflect the data as well as is necessary to satisfy the model objectives”.
First of all it’s a process, implying an iterative process.
The parameters used can be considered to be either:
Global parameters
Localised parameters
The mathematical relationships will affect either:
Assignment
Vehicle behaviour

27. Modelling Jargon – Validation
The DMRB is the appropriate source of guidance when validating any model in the UK
The DMRB defines model validation as “an independent check of the calibrated model”.
Model validation is simply a comparison of modelled output to observed data.
Observed data must not have been used in the model calibration.
Convergence/seeds
Modelled output can take many forms, but those most commonly used in model validation are:
Link counts
Turn counts
Queues
Journey times

28. Modelling Jargon – Validation
The DMRB states “Precise validation of queue lengths can be difficult because of the volatility of the observed data”.
As a result the DMRB suggests that observed queue data should be used in the calibration of the model.
Journey times commonly used independent data set.
DMRB criteria states over 85% of routes should have a modelled time within 15%, or 1-minute, of the observed

29. Modelling Jargon – Validation
Results should be plotted with the observed mean showing a 95% confidence limit, with the modelled mean shown against that range

30. Modelling Jargon – Validation
The most commonly used and most readily available data
Methodology for validation well known and well established
Data used in validation check should not be the same data used in calibration!

31. Modelling Jargon – Vehicle actuation
PARAMICS – signal plans
VISSIM - VISVAP

32. Modelling Process
Scoping report
Appropriate tool
Network coverage
Data collection
Site visits
Network build
Calibration
Validation
Model audit
Option testing

33. Junction models – 4 weeks
Timescales Guide
Junction models – 4 weeks
Corridor and multiple junction models – 10 weeks
Town centre model – 16 weeks
Wide area urban model – 24 weeks
Multi-modal and land use models – 34 weeks+

34. Model Outputs – what outputs?
Option testing and comparison – lies, damned lies and statistics
How should outputs be interpreted? Rule of thumb?
Economic appraisal - TUBA
SATURN – link and turn flows, delay, overall network performance
PARAMICS/VISSIM – link and turn flows, queues, journey times, overall network performance
Microsimulation presentations – live or movies?
Who should do presentations?

35. Workshop
Data collection
Appropriate tool
Validation criteria
Outputs

36. Useful documents and links
DTO Modelling Guidelines V2 – July 2006
Highways Agency – Use and Application of Microsimulation Traffic Models
User Manuals
DMRB
The Microsimulation Consultancy Good Practice Guide
Transport Analysis Guidance –