DIADEM (Dynamic Integrated Assignment and

Demand Modelling) software package has been

recently introduced to complement the variable

demand modeling process. The fundamental

impetus of DIADEM is to test the robustness of

highway scheme benefits and this software

package is intended to be complementary to

conventional demand modeling software. This

paper tests a small hypothetical network of a

town in the UK to compare the benefits under

the current conventional methodology and under

the DIADEM methodology

Koh, Andrew

English

White Rose Research Online

This is a repository copy of Elasticity of demand and highway scheme benefit evaluation.White Rose Research Online URL for this paper:http://eprints.whiterose.ac.uk/7655/Article:Koh, Andrew (2005) Elasticity of demand and highway scheme benefit evaluation. Traffic Engineering and Control, 46 (8). pp. 296-298. ISSN 0041-0683 eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse See Attached Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. 296 DIADEM methodologytec SEPTEMBER 2005Elasticity of demandand highway schemebenefit evaluation1 INTRODUCTIONThis paper examines the DIADEM (Dynamic Integrated As-signment and DEmand Modelling) package. DIADEM1, atpresent provides interactivity with conventional traffic as-signment packages CONTRAM(Taylor,1990,2003) and SAT-URN(Van Vliet and Hall,2004). The use of DIADEM was high-lighted as a need to support the impending published guid-ance on Variable Demand Modelling (VADMA) to be issuedby the Department for Transport (DfT) in the UK. This guid-ance will complement the existing Design Manual for Roadsand Bridges (DMRB) Volume 12 Part 2 which focuses on In-duced Traffic Appraisal.This paper does not review the literature behind VADMAor the technical aspects of DIADEM as this literature is avail-able elsewhere (see Mott MacDonald (2003),(2005)) but at-tempts to assess the implications on scheme benefits throughthe introduction of DIADEM. DIADEM is intended to enhance the robustness of com-puted scheme benefits which are sensitive to the conver-gence of demand and supply functions. This paper presents the results of a test of a small networkand compares benefits from conventional runs with the ben-efits from runs of DIADEM under varying elasticity assump-tions. Benefits are computed using Transport User BenefitAnalyis (TUBA) software (Mott MacDonald, 2004). Section 2reviews the current methodology for evaluating scheme ben-efits (which we term the ‘conventional’ approach). Section 3introduces the revised DIADEM based evaluation methodol-ogy. Section 4 introduces a test network which serves as aninput into Section 5 which illustrates the simple sensitivitytest comparing the revised methodology with the existingadvice and finally Section 6 draws some conclusions. It is important to point out that while this paper focusessolely on SATURN, DIADEM is intended to be interfaced withother modeling packages.2 CURRENT EVALUATION METHODOLOGYThe conventional methodology for the variable demandevaluation of road schemes within the UK for a SATURNmodel involves the following 5 steps: 1 Skim costs from the base year validated model; 2 Carry out elastic assignments for the Do Minimum Sce-nario for each modeled year;3 Carry out elastic assignments for the Do Something Sce-nario for each modeled year;4 Skim Time and Distance and Output Trip Matrices foreach modeled year and time period from the above as-signments;5 Use these time/distance/output trip matrices as inputsinto DfT’s TUBA run. The elastic assignments for Steps 2 and 3 in the above pro-cedure utilise the costs from the base year and this is knownas the base year pivot (and this is extensively described in ex-isting guidance DMRB Chapter 12 Part 2). In the above, wehave ignored the complexities of multiple user classes.Nonetheless, this methodology elaborated above can easilybe extended to handle multiple user classes. 3 DIADEM BASED EVALUATIONMethodologyWith DIADEM the assignment is carried out carried out bySATURN within the DIADEM framework. In other words, noelastic assignment parameters need to be in place in the net-work files unlike under the conventional approach.  The re-vised methodology is as follows:1 Specify the elasticity parameters within DIADEM whichshould be the same as that which was used for the con-ventional SATURN elastic assignments;2 Specify the base year validated assignment (from whichDIADEM extracts the base year costs) for each time pe-riod;3 Run DIADEM for each assignment scenario (Do Mini-mum and Do Something) for each modeled year; 4 Check that the converged trip matrix produced from the‘best’ DIADEM iteration does not differ significantly fromthat produced from the last iteration;5 The time/distance/best output trip matrices from each DI-ADEM run are inputs into the TUBA run. In this approach DIADEM repeatedly calls SATURN to per-Andrew T M Koh, Mouchel ParkmanDIADEM (Dynamic Integrated Assignment andDemand Modelling) software package has beenrecently introduced to complement the variabledemand modeling process. The fundamentalimpetus of DIADEM is to test the robustness ofhighway scheme benefits and this softwarepackage is intended to be complementary toconventional demand modeling software. Thispaper tests a small hypothetical network of atown in the UK to compare the benefits underthe current conventional methodology and underthe DIADEM methodology.The author:Andrew Koh,MSc(Eng) MCILT,can be contactedby email atAndrew.Koh@mouchelparkman.com297tec SEPTEMBER 2005form the assignments until demand supply equilibrium con-vergence is achieved or until the algorithm iteration defaults(user specified) have been reached, whichever is sooner. Theabove procedure implies that the user need only provide theinput forecast year network files as well as providing infor-mation to the location of the base files where the base costinformation will be skimmed2. An analogy to basic price theory economics may providesome insight into this new paradigm. In terms of demandsupply economics, one may view the traffic assignment soft-ware as providing a description of the supply curve. DIA-DEM, on the other hand, represents the demand curve andthe repeated calls to the traffic assignment software as asearch for a converging point (which then may be inter-preted as the intersection of the demand and supply curve).4 DESCRIPTION OF TEST NETWORKSThe network used for the test described in this paper do notpurport to represent any real road network although it isbased on actual UK speed flow relationships, but is deemedto be a valid  representation of actual transport planningproblems encountered by transport modelers. 4.1 Base Year This network is a small buffer network with 21 nodes, 72links and 11 zones. Furthermore, unlike some networks usedin practice, junctions are not explicitly modeled. In addition,we only model one time period for which there were 3485trips in 1 user class in the base year of 2002.4.2 Future Year NetworksThe totally fictitious ‘Do Something’ network involved im-plementing a bypass. For simplicity, it is assumed that the DoMinimum network was taken to be the same as the base net-work.4.3 Future Year Trip MatricesWe assumed that the bypass will open in 2009 and the designyear for the scheme will be 15 years after opening ie 2024.These two years were the modeled years and is based on con-ventional traffic appraisal practice. For simplicity, a generic growth factor (applied to all tripsequally) was applied to the 2002 base matrix and Table 1gives the factors used. Thus for a given modeled year, the ref-erence trip matrices are the same for both Do Something andDo Minimum scenarios. 5 SENSITIVITY TESTING5.1 Variation in Elasticity of DemandIt has been documented that benefits of any road scheme aresensitive to the elasticity of demand (Ortuzar and Willumsen(1994), Mackie and Bonsall (1989)). Hence it is believed thatsome useful insights may be acquired by a sensitivity test ofthe scheme benefits using the test networks illustrated abovefor both the conventional evaluation methodology men-tioned in Section 2 and the DIADEM evaluation methodol-ogy of Section 3. Theoretically, the benefits of the schemefrom both the conventional run (without DIADEM) and therevised run (with DIADEM) should be the same. In this paperwe report on a sensitivity test which was carried out to exam-ine benefit computations under both approaches with re-spect to variations in the elasticity of demand parameter for asimple constant elasticity demand function. In both cases, the elasticity parameter was varied from -0.1to -1.0 in steps of 0.1. In SATURN this is known as the‘power’ parameter for MCGILL=2. Assignments were carriedout using Wardrop Equilibrium assignment (Wardrop, 1952)with the stopping criteria set to a maximum of 399 iterationswhich provided a high degree of convergence (Van Vliet andHall, 2004). Similarly, DIADEM runs were performed using‘Algorithm 1’3 which is recommended in the DIADEM usermanual with exactly the same elasticity parameters as used inthe conventional approach. 5.2 ResultsThe TUBA software provides scheme benefits in 2002 valuesin thousands of pounds, discounted over 60 years with dis-count rates in accordance with HM Treasury (2003). Figure 1illustrates the variation of the scheme benefits as the (ab-solute) elasticity of demand increases. In this context, theelasticity of demand measures the percentage change in tripsas a result of a percentage change in generalized cost of tripmaking. A larger elasticity implies a larger rise in the numberof trips, eroding the relief the highway scheme provides. Figure 1 indicates that the benefits accord with economictheory. In addition, the difference in benefits computed doesnot seem significantly different under either approach. How-ever, it might be more insightful to examine the relative per-centage difference (RD) in benefits defined as:Where Benefits (DIADEM) refers to the scheme benefitscomputed using the DIADEM approach and Benefits (Con-ventional) refers to the benefits computed under the conven-tional approach. Figure 2  graphs the percentage difference in benefits rela-tive to that obtained via the conventional methodology de-scribed in Section 3. The graph shows that the percentage dif-ference is positive. This implies the benefits are higher whencomputed via the DIADEM methodology. Our numerical tests indicate that the relative differencecould be as high as 2%. Obviously the extend of the differ-Table 1: Trip matrix detailsin variousscenariosFigure 1:Comparingscheme benefitsunderconventionalevaluationtechniques andDIADEM-basedevaluationtechniques.   298 DIADEM methodologytec SEPTEMBER 2005ence would depend on the scale of the project. Less certainhowever is whether larger benefits computed under DIADEMwere a result of the specific nature of this network or whetherthe results contained herein could be generalised to othernetworks is a research issue of importance. 5.3 Note on Run TimesDIADEM requires a number of user specified iterations beforedemand and supply convergence is achieved (where it acti-vates SATURN to carry out the assignments.). The DIADEM-SATURN process was generally fast when run on a modernWindows XP PC with a processor speed of 2800 MHz, 512Mb of RAM and 50GB hard disk space. For this simple net-work, run times were approximately 2 to 3 minutes per de-mand elasticity value tested per network per forecast yearwhich is fairly rapid. 6 CONCLUSIONSThis brief note has demonstrated the use of DIADEM in asimple setting, employing the revised evaluation methodol-ogy. In addition, it has shown for a simple network that thedifferences in benefits computed under either approach arereasonably small and we can tentatively conclude that the er-rors of demand-supply convergence have not overcome theeffects of the benefit evaluation process and presented mis-leading results. This paper presents the results of tests basedon a relatively new step forward in transport planning andtransportation economics. While this approach is novel andthe tests simplistic, there are some issues that can be high-lighted at this stage. Firstly, our sensitivity tests were run on a buffer networkwhere convergence of both SATURN and DIADEM was not acritical issue. However, it is quite probable that for large net-works with more user classes and with junctions modeled ex-plicitly (the so called ‘simulation networks’), the memory re-quirements and the long run times of DIADEM may makethe process unduly time consuming. It is our understandingthat ‘batch’ facilities are being developed to allow forovernight runs, if required but has not been included in thebeta version as it is still under development. Secondly, while our numerical tests indicate that the bene-fits from DIADEM are generally higher than that from theconventional methodology, there is no guarantee that this isindeed the case for other networks. Further research is re-quired before more substantial conclusions on this issue canbe drawn. Finally, it is acknowledged that while DIADEM has theability to carry out full variable demand modeling, whichmay include a full blown four stage transport model withfeedback and involving public transport, this paper has ex-plored only the simple elasticity approach on a purely high-way based scheme. We understand that studies are currentlyunderway that will explore these other issues in more detail.REFERENCESHM TREASURY (2003) Green Book: Appraisal and Evaluation inHM Treasury (2003) Green Book: Appraisal and Evaluation inCentral Government HMSO: UK(Available on line at http://greenbook.tresury.gov.uk)HIGHWAYS AGENCY, DEPARTMENT FOR TRANSPORT (variousyears) Design Manual for Roads and Bridges (DMRB) Volume 12HMSO, UK.MACKIE P J AND P BONSALL (1989) Traveller responses to roadimprovements: implications for user benefits Traffic Engineeringand Control 30(9) 411-416MOTT MACDONALD (2003) DIADEM Overview of the Algorithms,Software & Convergence Requirements, paper presented on 17thJuly 2003 Seminar in Great Minster House Westminster, June2003 (Available online at:http://www.dft.gov.uk/stellent/groups/dft_econappr/documents/page/dft_econappr_023724.pdf)MOTT MACDONALD (2004) TUBA User Manual Version 1.6 May2004 (Available on line athttp://www.dft.gov.uk/stellent/groups/dft_econappr/documents/page/dft_econappr_507990.pdf)MOTT MACDONALD (2005) DIADEM User Manual Version 2.0May 2005 ORTUZAR J D AND L WILLUMSEN (1994) Modelling Transport(2nd Edition) John Wiley (Chichester)TAYLOR N (1990) CONTRAM 5: an enhanced traffic assignmentmodel. TRL Report RR 249, Transport Research Laboratory,Crowthorne, BerkshireTAYLOR N (2003) The CONTRAM dynamic traffic assignmentmodel Network and Spatial Economics 3 297-322.VAN VLIET D AND M HALL (2004) SATURN 10.5 User ManualInstitute for Transport Studies: Leeds and Atkins: Epsom WARDROP J G (1952) Some theoretical aspects of road trafficresearch Proceedings of the Institution of Civil Engineers 1 325-378AcknowledgementsThe author thanks Dr Dirck Van Vliet and Dr Andrew Gordon foruseful comments on an earlier draft. In addition, the authorwould like to stress that he was not part of the DIADEM projectteam and the views expressed in this paper do not represent theviews of the DIADEM project team or the Department forTransport. The usual disclaimer that the author remains solelyresponsible for all errors and omissions in this paper applies.Footnotes1 DIADEM Beta Version No 2.0.16 June/2005 developed by MottMacDonald under contract to the Department for Transport.  2 There is another option within DIADEM where the user can pro-vide the cost explicitly, instead of letting DIADEM skim it. Therecommended approach is to let DIADEM run the skim. SeeMott MacDonald (2005) for details. 3 There are three algorithms for demand and supply convergenceavailable to the user within DIADEM. Mott MacDonald (2005)recommends ‘Algorithm 1’ as the best performing of three(based on computing efficiency) to date. Figure 2: RelativePercentageDifference inBenefitscalculated by theconventionalapproach and theDiadem-basedmethod.

Elasticity of demand and highway scheme benefit evaluation

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Elasticity of demand and highway scheme benefit evaluation

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