A traffic model for every occasion - what to use when.

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.

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

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)

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)

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

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. 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 – www.webtag.org.uk