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Guidance documents - Project manager
Unit 2.9: Variable Demand Modelling
There are two modules within this section:
TAG Unit 2.9.2: An Introduction to Variable Demand Modelling
Unit 2.9.2 (Adobe Acrobat - 34KB)
1.1.1 This TAG unit provides a general introduction to the principles of variable demand modelling. An overview of the advice on variable demand modelling within TAG is given in Variable Demand Modelling - Overview (Unit 2.9.1), with more detailed guidance included in Variable Demand Modelling - Detailed Stages (Unit 3.10).
1.2.1 All assessments of Government-funded investments in highway or transport schemes need to consider the effects of variable demand (and the resultant "induced traffic") on the justification for the scheme or strategy. As any changes in travel times or costs will affect the demand for travel, changes over time in the level of congestion should be reflected in the forecasts of travel both with and without the scheme or strategy.
1.2.2 Any change to transport conditions will, in principle, cause a change in demand. Travel will become faster or slower, cheaper or more expensive, and this will be reflected in the generalised costs of travel for some journeys. Generalised cost is the sum of both time and money cost, and any modelling of demand will depend upon how the generalised cost of travel changes. Take as an example a highway improvement which removes delays for those travelling by car:
- this reduces the generalised cost of some journeys;
- some people will therefore decide that the journey is now acceptable, whereas before they were less likely to make it;
- the total number of people taking advantage of the scheme will be more than would have travelled without the improvement.
The purpose of variable demand modelling is to predict and quantify these changes.
1.2.3 This extra demand will add to the traffic flow through the scheme and elsewhere. Speeds through the scheme, and on roads leading to and from it, may fall because of the higher flow. The Standing Advisory Committee on Trunk Road Appraisal (SACTRA) considered all these effects in 1994 and emphasised the importance of establishing a realistic scenario in the absence of the scheme or strategy, the extent of travel suppression in the "without-scheme" case, and the extra traffic induced in the "with-scheme" case.
1.2.4 "Induced traffic" thus refers to the additional traffic, beyond the level of traffic that would use the network without the scheme or strategy. Levels of traffic in both cases usually differ from a hypothetical forecast that does not allow for any change in travel costs (the "Reference" case). The figure below sets out the different illustrative forecasts for a travel movement likely to be affected directly by a scheme under the three different scenarios (i.e. Reference case, without-scheme and with-scheme). The reference forecast provides a theoretical forecast of travel if travel costs remain at the base year levels. If that shows an expected increase in travel, that can cause increasingly congested conditions which, with variable demand, will cause some trips to divert to other modes or destinations, or to be "suppressed" altogether, giving rise to lower forecasts (the without-scheme forecast). The completion of the scheme reduces travel costs from the without-scheme situation and some of the suppressed travel returns as induced travel (in the with-scheme forecast).
1.3.1 When assessing the impact of changing travel costs on travel (with or without the scheme or strategy), existing Departmental advice in TAG emphasises the importance of considering the wider effects of transport measures, on the environment for example, and across all transport modes. It lists the possible responses to changing travels costs, such as those from a new road improvement as:
- change route (reassignment)
- retime journeys to take advantage of the improved conditions
- travel to new destinations
- switch to car from other modes
- switch from travelling as a car passenger to driving
- increase the frequency of some journeys
- make entirely new journeys
- change the patterns of land use in the longer term, and therefore the associated trip patterns.
1.3.2 The first of these responses is already predicted in the traffic assignment modelling traditionally used to assess highway schemes. The reader is assumed to be familiar with these long established (often called "fixed trip matrix") assignment techniques and the relevant modelling packages, and hence this advice does not provide instruction on assignment modelling.
1.3.3 Changes in traffic due to responses 2-8 above are termed "variable demand mechanisms". They may affect both the without-scheme scenario and the with-scheme scenario. Their impact on the without-scheme scenario will usually be to suppress traffic, compared with the reference case scenario. Their impact on the with-scheme scenario will usually be to induce traffic, compared to the without-scheme scenario. They are represented in a transport model, the traditional transport model is often called a "four-stage" model because it includes four mechanisms to represent different aspects of travel behaviour:
- trip generation/frequency (responses 6 and 7 above, and some of the effects of 8);
- trip distribution (response 3);
- modal split (response 4); plus
- trip assignment (response 1), since some form of assignment model will be a necessary part of the demand modelling.
1.3.4 A number of current transport models have added a fifth mechanism:
- "peak spreading" or time of day choice (response 2).
1.3.5 Few models attempt to predict the transfer from car passenger to driver (response 5). To tackle this it is necessary to subsume the effect within simple mechanisms such as mean car occupancies or have a detailed model of interactions within the household, and specialist help is required. Similarly, changes in the road network might affect car ownership; the effect can be modelled at the trip generation/frequency stage, but it is uncertain and likely to be small. The effect of car ownership growth over time should be reflected in the development of reference case growth along with other demographic effects. Individual induced changes to those effects are not considered further here.
1.3.6 Much of the demand response will only become evident over a period of time, as people moving home or job become able to take advantage of changes in the transport network, and land-use development tends to evolve on a longer time-scale still. However, most transport models generate trips on the basis of a fixed categorisation of land use, and only a few specialised models attempt to predict how land use might respond to changes in the transport system (response 8). This issue is considered in detail in Modelling (Unit 3.1) which describes in some detail the issues and methodologies involved. If consideration of these more detailed aspects is considered important to the appraisal, it is sensible to consult specialists on the issues of interest.
1.3.7 The effect of any additional demand when a network is improved is threefold:
- there will be more people to take advantage of the scheme, which will increase the total benefit, but
- on congested roads, the improvement in speed will be less than if demand had remained constant, so the time saved by everyone will be less, reducing the total benefit.
- The extra traffic may also have adverse impacts on noise, local air quality, global emissions, severance etc.
1.3.8 If there is appreciable congestion, the second effect is likely to outweigh the first, so that the increase in demand reduces the time saving benefits.
1.3.9 Variable demand modelling should be used unless it can be robustly demonstrated that ignoring the effects of suppressed and/or induced trips and traffic will not affect the assessment of the economic, environmental or social impacts of the scheme. This issue is considered in detail in Variable Demand Modelling - Detailed Stages (Unit 3.10), where advice is constructed in 5 units designed to be read independently or in any order.
The following documents provide information that follows on directly from the key topics covered in this TAG Unit.
|For information on:||See:||TAG Unit number:|
|A overview of modelling issues||Summary Advice on Modelling||TAG Unit 2.4|
|Detailed advice on transport modelling||Modelling||TAG Unit 3.1|
|Detailed advice on variable demand modelling||Variable Demand Modelling - Detailed Stages||TAG Unit 3.10|
Department of Environment Transport and the Regions Design Manual For Roads and Bridges, Volume 12.
This Transport Analysis Guidance (TAG) Unit reflects the consultation comments received on the Introduction Step 1 of the draft Variable Demand Modelling Advice produced by TRL in June 2003.
Technical queries and comments on this TAG Unit should be referred to:
Transport Appraisal and Strategic Modelling (TASM) Division
Department for Transport
Zone 2/25 Great Minster House
33 Horseferry Road
London SW1P 4DR
Tel: 020 7944 6176
Fax: 020 7944 2198
Updated: April 2009