What ITS is able to do is related in a systematic way to the problem or opportunity to be addressed. The forerunners to ITS – computer-controlled traffic systems – were built to a definitive design specified by the client. A technical evaluation of the available products, possibly with a field trial and performance assessment would be undertaken before going ahead with the full investment.
In today’s world the systems are more complex. Close attention is needed to user requirements and the development of a detailed functional specification - what will the ITS do? (See Why Create One?)
In practice the separate, often parallel, streams of technical performance and impact assessment feed into a practical choice – whether to use an ITS solution, a different solution, or do nothing. The detailed technical specification is often to the contractor to develop, based on cost and performance criteria. (See Appraisal and Evaluation of Managed Motorways Technologies)
In these circumstances politicians, planners, budget controllers and other decision-makers need to be fully informed about the benefits and costs of ITS so they can judge whether the investment in ITS is worthwhile:
In order to answer the question ‘Shall we proceed with an investment in ITS?’ – the potential impacts of the proposed ITS project will have to be set against the costs of procurement, system build, maintenance and operations. There are always choices to be made, so it is important to consider the relative merits and costs of the different options. This requires a systematic approach and careful judgement, including political judgement. In addition to an economic appraisal there will need to be an assessment of technical and operational performance. The following stages provide a general framework for the appraisal:
Flowchart for appraisal of ITS projects (© PIARC)
Project appraisal is a key part of the ITS deployment process. It is not an optional add-on, nor is it a pass/fail “test”. Its purpose is to ensure that the systems deployed are the most appropriate ones – and to ensure that investment is targeted towards areas and applications that will bring the most benefits.
The appraisal methods used in any one country usually follow a consistent framework adopted by the authority responsible for investment in transport infrastructure. Examples of such appraisal frameworks are referenced under “Further Information” and include the:
The use of a consistent method of appraisal can help ensure that the choices made by decision-makers meet their objectives of providing a sustainable transport network – and that value for money has been achieved and that ITS options have been considered on an equal footing with more capital intensive infrastructure schemes. A well conducted economic appraisal, accompanied by a thorough financial appraisal will ensure that decision-makers understand all of the effects of the ITS application – and that the risk of any unexpected outcomes is minimised (See Finance and Contracts)
The process of cost benefit analysis is a comparative one in which the proposed ITS application is compared with what the transport network would be like in absence of such an initiative. This is often defined as the reference case or the ‘do-minimum’ alternative.
In many cases, several options will be appraised so that the decision-maker can make a better informed choice about the option that maximises the net benefits.
In most countries, the ratio of discounted benefits to discounted costs is used to inform decision-makers about priorities – since the use of this technique is to allocate a pre-determined budget to the most beneficial schemes. Not all benefits of a scheme can be measured in money terms and be included in the benefit to cost ratio. In many countries decision-makers retain some discretion about the ranking of priorities to take account of impacts which cannot easily be included in the benefit to cost ratio. (See Weighing Costs and Benefits)
The critical task in project appraisal is to decide what is important. The answer to “what measure of success do I use?” begs the question “what were you trying to achieve?”
ITS investments, like any other, should be planned to contribute towards resolving a problem – or delivering a vision. The measure of success is how far the problem is resolved or the vision delivered, within the available budget.
ITS can contribute to a variety of objectives:
The table below will help decide the criteria on which to focus attention.
|
Goal Area |
Measure |
|---|---|
|
Safety |
Crashes Serious injuries and fatalities |
|
Efficiency |
Vehicle throughput Travel time savings Journey time reliability Cost savings |
|
Mobility / Social Inclusion |
Accessibility to services and opportunities Connectivity Inter-modal connections |
|
Energy and the Environment |
Emissions Fuel consumption Noise levels |
|
Customer satisfaction is the bottom line
|
ITS can be adapted to serve widely different policy objectives, given the diversity of possible responses to users and their needs. Politicians generally wish to be pro-active, and will be happy to support ITS provided there is a strong case for investment and it is perceived as delivering value. This “political” perspective may appear to be another burden on transport professionals – but in practice, political aspects have always been there.
The five tests below represent a way of formalising political considerations – and are a good indication of whether or not the ITS proposed is a good investment for the decision-maker (taking into account the perspectives of the public, media and local politicians). The scheme should be, and be seen to be:
A full-scale project appraisal can give the answers to each of these tests.
The evaluation of a scheme, at some point after it has opened, can provide important feedback to the analysts responsible for appraisal methods – with information on the performance of the chosen option and road users’ responses to it. For example, before and after comparisons can provide some indication as to whether the estimates made in the appraisal of the impact of variable message signs – on the severity of accidents and the delays they cause – are correct or whether they should be modified in future appraisals. Care needs to be taken to ensure that other influences are taken into account, such as increases in traffic volumes between the periods over which the comparison has been made. (See Evaluation)
It is not helpful to say that it is not possible to determine costs before contract tenders are received, or to assess benefits until after a system has been deployed. It is necessary to have some idea of the likely costs and benefits of ITS during planning stages – especially for the cost-benefit analysis, public consultation and decision-making phases.
The full range of impacts of each ITS solution should be recorded and compared with other solutions – both ITS based and conventional. Impacts for which there is no monetary value should not be ignored. The adverse consequences of the investment should be listed, as well as the extent to which each investment option will address the objectives. Always:
There are several examples of published guidance on the use of ITS to address transport problems and deliver policy objectives – written to inform transport planners about the options that they should consider, as well as their strengths and limitations. The guidance generally includes examples of successful schemes and an assessment of why the option chosen met the transport planners’ expectations:
The UK Department for Transport published an ITS Toolkit in 2006 aimed at informing local authorities about the options which they should consider as part of the package of measures for inclusion in the local transport plans which are submitted to the Department for funding (http://webarchive.nationalarchives.gov.uk/20091116100852/dft.gov.uk/itstoolkit/)
the EU “2decide” project (2009-11) created an on line toolkit as part of the EU’s action plan for the development of intelligent transport systems for Europe. The toolkit is intended to support decisions about the deployment of ITS based solutions. It provides guidance on the expected benefits of different ITS applications based on the evaluation of existing systems, summaries of case studies and access to evaluation reports (http://www.its-toolkit.eu/2decide//node/44 )
Both of these toolkits provide an invaluable source of information about ITS options linked to transport problems and transport related concerns – such as poor air quality – which various options can resolve.
During the planning of ITS deployments, the costs to be considered are capital costs, equipment unit costs, whole system costs – and lifetime costs which include maintenance costs, operations costs and the cost of periodic upgrade, refurbishment and replacement. Estimates must be made in the local context to support local project decisions.
Cost data for ITS installation and operations are often difficult to assess. For example, there can be substantial cost reduction through effective integration of ITS components – so care needs to be taken when using any unit cost data. It would be a mistake to add up the unit costs without considering possible cost savings that could be provided by integration and/or good system design. Some products, particularly electronic products, have widely varying prices across the world. Economies of scale can mean that significant discounts can be obtained in large scale procurement projects – and through group purchasing.
Several other factors can have a major influence:
Careful monitoring of lifetime costs is needed to build up information on the real costs of ITS maintenance and operations. These will give those planning or implementing ITS an indication of the aggregate cost of a specific ITS deployment.
Accurate operating and maintenance costs are notoriously difficult to obtain because they have to be disentangled from the general running costs of an organisation. Maintenance costs can be quite high with ITS, in comparison with, for example, roads and bridges. The equipment may be sensitive and is often exposed. The costs of refurbishment and replacement (including an allowance for obsolescence and upgrade) should be forecast and planned in the initial budget.
Installation costs will vary between regions, depending both on the terrain and the availability of skills to work on and manage the project.
The time and effort involved in software development, testing and installation can be substantial and costs can be difficult to predict. There is an advantage in using well-established products. Similarly the task of developing databases and data dictionaries for ITS is very labour intensive. A pre-existing data dictionary and database will save on development costs – but only if data quality is adequate for the purpose. This means that data has to be accurate, up-to-date and coded in sufficient detail. Repairing a database that is populated with obsolete, inaccurate and unreliable data can be more expensive than starting afresh.
Comprehensive unit cost data for planning purposes are described in the USDOT database of ITS costs and benefits. The unit cost is broken down into capital and operations and maintenance (O&M) components. See http://www.benefitcost.its.dot.gov
Users of the USDOT database should be aware that cost figures quoted in a rapidly developing field such as ITS can quickly become out of date – figures for each ITS product and service should be used with caution. In particular, the costs can be very different for different regions and countries.
Many of the benefits of transport investment are initially measured in terms of improvements in journey times, in reliability, in reductions in emissions, and other physical quantities. The process of economic appraisal requires these physical changes to be expressed as monetary values in order to weigh benefits against the costs.
Some of the benefits of ITS – such as savings in vehicle operating costs through better management of traffic – are expressed in monetary terms. Others – such as travel time savings – are initially expressed in terms of the number of hours saved per typical day, compared to the “do-nothing” reference case. Methods have been developed to convert time saving, reliability and accident benefits into monetary terms – so that a single metric can be used to compare the majority of the impacts of the scheme. (See Appraisal of ITS road schemes in UK using the INCA traffic model)
The values for travel time savings used in scheme appraisals are derived from a combination of sources. Many countries publish the values to be used as part of their investment appraisal guidance.
“Revealed preferences” is a theory which links consumer preferences to their purchasing behaviour. It assumes that consumers evaluate different options before making their choice. From their choices it is possible to infer the relative value of the options.
For example, in some countries motorists are faced with a decision about whether to use a faster tolled road – or to save money by taking a slower un-tolled alternative. Analysis of these choices can show the extent to which drivers trade-off time savings against payments. Those drivers whose overall journey time is reduced by a relatively small amount, are less likely to use the tolled option than those who can make use of the toll road for a greater proportion of their trip.
The findings from this revealed preference method of valuing time savings need to be interpreted with care – because savings in time are not the only factor that drivers consider in their choice of route. For some, an uncongested journey on a tolled road provides an advantage over a journey on more congested routes – irrespective of any differences in the journey time. For others, the act of paying a toll to use part of the highway is seen as an unnecessary expenditure and one to be avoided.
Many of the studies valuing time savings are based on market research methods because of the difficulty of interpreting the results of revealed preference studies and because not all countries have toll roads. A representative sample of drivers is asked a series of questions about how they choose between options which offer:
Options are generally presented in terms of cost savings from lower fuel tax – and increases in costs from introducing tolls or increasing the level of cost where tolling is common.
These studies provide evidence of road users’ willingness to pay for time savings – which can then be used in an economic appraisal.
ITS brings improvements in reliability, whether by road or by public transport, and provides transport users with benefits that are additional to any savings in average travel time. For example, some schemes – such as managed motorways – provide overall net benefits because the value of the improvements in reliability offset any disadvantages in terms of increased travel time associated with imposing a speed limit.
Public transport operations generally adhere to a timetable – and passengers are assumed to be aware of this schedule. Unreliability is perceived in terms of late running, or in some case early arrival at the passenger’s destination. Lateness is made up of additional travel time and the inconvenience of the unexpected delay. Typical values for delayed minutes are three times the value of the standard travel time savings.
Estimation of the value of the benefits of improvements in reliability for road traffic presents more of a challenge because of the absence of a timetable for road trips. Drivers form their own views about expected journey times.
There is evidence that the inconvenience that road users experience from unreliability varies by the duration of any delay. Short delays cause less inconvenience than longer ones. This weighting of shorter and longer delays is taken into account in the valuation of improvements in reliability. It is calculated using the standard deviation of travel times around their expected duration – and gives more weight to longer delays.
Economists define the Value of a Statistical Life (VSL) as the amount of money society is willing to spend to save a life. The means for assessing this value – and of injury or ill-health – is based largely on market research methods. People are asked about their willingness to pay to change the risks of injury or possible death – with the questions based on realistic choices, such as the purchase of a more expensive car with special safety features.
There is evidence from studies of morbidity that people living in areas which experience high levels of local air pollution suffer more ill-health than others and die younger. The approach used to value a statistical life in a road accident is modified to reflect the reduction in the number of expected healthy life years caused by traffic related air pollution and the ill health that causes premature death.
Noise nuisance, where valued, is generally based on studies of the impact of traffic noise on house prices – with a monetary value for the change per unit, of noise per household, which varies by level of noise. An example of these values is provided in the UK Department for Transport’s WebTAG guidance
A different approach is used for valuing carbon emissions because of the difficulty in estimating the cost of damage caused by global warming. Most countries have set an absolute limit on carbon emissions and have in place – or planned – a set of policies aimed at meeting that target. The cost of the measures necessary to achieve the target defines the marginal abatement costs of carbon. This cost is used in transport appraisal as a measure of the benefits of carbon reduction on the grounds that governments are willing to spend up to this amount in order to reduce carbon emissions. The UK Department for Transport’s guidance provides a set of values to be found at: http://www.dft.gov.uk/webtag/documents/expert/unit3.3.5.php
Less progress has been made in valuing the benefits of the better information and reduction in uncertainty provided by some ITS applications – for the most part those on inter-urban networks. These improvements can be included amongst the unquantifiable benefits (See Appraisal Methodology) “Unquantifiable Costs and Benefits] of a scheme – and a description can be provided of the improvements in the quality of the journey in support of the quantified part of the appraisal.
Examples of current values and values used for changes in reliability are contained in the annex to an international comparisons paper prepared by the UK Department for Transport. See https://www.gov.uk/government/publications/international-comparisons-of-transport-appraisal-practice
In order to help decision-makers weigh up the positive and negative impacts of an ITS proposal, many of the impacts are expressed in monetary terms. The monetary evidence is captured from market research based surveys or from the behaviour of transport users – for example by valuing time savings based on people’s preferences when faced with the choice of saving time by using a faster tolled route.
For specific projects, modelling and simulation techniques may be useful in estimating likely benefits. A transport model will often be used to provide an initial estimate of the majority of the benefits of an ITS application. Outputs from the model are typically expressed in terms of:
Most transport models also provide estimates of changes in carbon emissions – because of the relationship between traffic speed, fuel consumption and vehicle emissions that forms part of all formal transport models. (See Traffic Models)
The main effect of some ITS applications is to reduce the costs incurred by the highway authority or another government agency. For example, the use of ITS to check goods vehicle weights and to discourage freight operators from overloading their vehicles reduces road damage – and highway maintenance costs – as well as contributing to road safety and to fair competition between freight operators.
In most transport models, the value of time savings in monetary terms is combined with the monetary-based costs incurred by users of the road network. This includes fuel costs, distance related vehicle operating coats and toll and parking charges.
The sum total of the time-based and the monetary-based elements of a trip is known as the generalised cost of the trip.
The majority of transport models make use of an assumption – based on evidence of transport users’ behaviour – that in their decisions about the trips that they make, transport users generally seek to minimise the generalised cost of their journey. They will respond to opportunities – such as those provided by ITS – to reduce the generalised cost of travel and to change their route, mode or even destination.
Many applications of ITS will have a useful life of 10 years or more. Transport appraisal methods – which are also applied to longer-life infrastructure projects – make use of forecasts of future demand and of a process of discounting, to place present costs and future benefits on an equal basis.
The process of expressing most of the costs and benefits in monetary terms provides the basis for a Benefit-Cost Ratio (BCR) – which is a widely used metric for indicating the value of the scheme to transport users and the wider community. In general, the greater the benefits relative to the costs, the better the case for implementing the scheme.
The importance attached to the BCR differs between countries – although, before a project can be approved, most will require a scheme to have a BCR in excess of one (unity) based on the value of current (“present”) benefits exceeding the current value of the costs.
Conventional methods of economic appraisal do not generally identify the final beneficiaries of a transport investment, since they take a national perspective of the case for the investment. Regional or local decision-makers – if responsible for allocating funds raised by national taxation – may use a narrower set of objectives and focus on a different set of outcomes. This could take the form of the creation of local employment as a result of better transport infrastructure encouraging firms to relocate or establish themselves in accessible locations.
The context is always a key determinant of both costs and benefits. Specific benefits and costs can vary greatly from one country to another, depending on economic, cultural and social factors as well as transport priorities.
The US Federal Highway Administration has developed the ITS Deployment Analysis System (IDAS) that can be used in planning for (ITS) deployments. State, regional, and local planners can use IDAS to estimate the benefits and costs of ITS investments – which are either alternatives to – or enhancements of – traditional highway and transit infrastructure (http://idas.camsys.com/).
The US Joint Program Office for ITS has developed a useful reference for current costs and benefits which gives detailed unit costs and benefits – largely for specific ITS in the USA – as well as case studies of whole system applications:
These data are useful as a guide, but cannot be adopted without first checking that they are relevant to local conditions and circumstances. Similarly the benefits data may not transfer to a different context.
The USDOT website also offers lessons learned – including estimates of the lifespan of each type of system. This has important cost implications. If something needs to be replaced every five years, then the costs associated with that must be considered against the benefits gained. Technological advances may mean rapid obsolescence of ITS hardware and software, which will require more frequent replacement and higher costs. See http://www.itskrs.its.dot.gov/
A source of information on values that have been used in transport appraisal in the European Union is the HEATCO study. It reviewed the different approaches used in Member States with the objective of developing guidelines for the harmonisation of these methods for use when appraising cross-border schemes. See: http://heatco.ier.uni-stuttgart.de
A more recent review of appraisal methods used in a selection of countries which have been active in researching these techniques, is published on the UK government’s Department for Transport’s website at https://www.gov.uk/government/publications/international-comparisons-of-transport-appraisal-practice
Many countries and other institutions have published guidance on transport appraisal. All follow similar principles and the main variations due to differences in the economic circumstances of countries – impacting on values assigned on criteria such as travel time savings, safety benefits and improvements in reliability.
Appraisal starts from the identification of a problem that requires an intervention. Problems on the highway network are usually well understood by highway engineers who interpret the information they collect about the performance of the network. This information includes data on speeds on each link in the network – and data on the variations in speeds across the day and over longer periods, as an indicator of levels of congestion and of reliability. Accident data provides a further information on the safety of the network. Stakeholders will also have views about the performance of the network. Evidence of the poor performance of the network supports the case for making a change.
Appraisal helps to provide policy makers with the information they need for different types of decision. In many cases the choice is about incremental investment in existing ITS applications – such as upgrading or extending a network of intelligent traffic signals. Appraisal can also inform policy makers about new options which depend on ITS – such as the implementation of a managed motorway scheme as an alternative to new construction. New policies – such as options for managing traffic through pricing or for funding inter-urban roads by tolling – also rely on appraisal to help decision-makers understand the likely effects of introducing the policies.
Appraisal methodology involves a number of factors:
A wide range of options for reducing the impact of the problem identified should be considered – ranging from new infrastructure to doing nothing. If ITS solutions are not addressed, it is not possible to demonstrate that ITS is the most effective option:
A benefit of public consultation at an early stage is that it helps to rule out or modify options which are not acceptable to the public – and which, if adopted, might lead to protests and delay progress on the scheme.
In many cases an ITS option enables a network operator to make better use of the existing infrastructure and may delay the need for capital investment and other expenses such as new construction.
The costs saved by deferring substantial investment – such as a new road – can be very significant, while the loss of benefits due to deferral might be rather less. The use of ITS to ‘sweat the assets’ buys policy makers time, delaying the more difficult decision on investment. It also provides flexibility if there is a change in policy
Appraisal of ITS Schemes in UK
There are examples of ITS schemes, which were regarded when introduced, as no more than a short-term fix pending more substantial investment in new road capacity – becoming permanent, due to a change in public opinion whereby building new roads was considered unacceptable. An example is the “Red Routes” in London – where red lines on roads are used to indicate priority routes with a high level of enforcement to prevent stopping, parking, loading and unloading, or boarding and alighting from a vehicle.
In many cases a long list of possible options will be narrowed down to those which meet certain criteria, including those of:
Guidance on option selection and on reducing the number of options is provided in the documentation of the UK Department for Transport’s Early Assessment and Sifting Tool (EAST) – See https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/4475/east-guidance.pdf
The rationale for investment in ITS is to provide benefits to road users and, in some cases, to the road network provider – which are valued as greater than the initial investment and on-going costs. Since an ITS installation has a life of up to 10 years or more, a forecast of the traffic likely to benefit from the investment in ITS is required to estimate the expected benefits over the life of the investment (See Benefits of ITS).
There are a number of approaches to forecasting road traffic, ranging from extrapolation of recent trends to more advanced models which are derived from identifying and analysing the causes of traffic growth. An example of the modelling approach to traffic forecasting is provided on the UK Government website at https://www.gov.uk/government/publications/road-transport-forecasts-2013.
The process of appraisal requires both:
A traffic or transport model is used to show:
Forecasts are needed for each option, including the ‘do-nothing’ reference case, to provide a comparison of the costs and benefits of options as well as a comparison between any option and the reference case.
Traffic models are set up so as to represent two or more future years with and without the ITS project to show the volume or quantity of benefits, when compared with the reference case, over the life of the project.
Running the model for each year of a project’s life is time consuming and costly. It is common practice to run the transport and economic appraisal models for just two forecast years – one shortly after the planned opening date of the scheme and the other perhaps seven or ten years later. The costs and of the benefits for intermediate years are estimated (by interpolation based on the trend between the two dates). If the life of the infrastructure extends beyond the final forecast year, costs and benefits have to be extrapolated based on the same trend.
Most ITS applications have lives of ten years or longer before any decision is required about whether the application should be upgraded or renewed. The capital investment is incurred at the start of the project and this results in a stream of revenues or other benefits – and a stream of costs for maintaining and operating the system over the life of the assets. A process is needed to convert these future benefits into a value that can be compared with the initial costs of the investment.
Society tends to value future costs and benefits lower than they value the equivalent benefit if it occurs now for a number of reasons:
A discount rate is applied which progressively reduces the value of a given level of benefit in all future years. The discounted future benefits can then be added together and expressed as a current (“present”) value of future benefits for comparison with costs. Ongoing costs, such as the costs of operating, maintaining and renewing the ITS equipment over its life, are discounted on the same basis and expressed as current value costs – and then added to the estimate of the capital costs.
The discount rate used in transport appraisal differs between countries and the appropriate national value should be used in appraisal to ensure comparisons can be made between different investment projects.
Subtracting the current (“present”) value of costs from the current value of the benefits, provides an estimate of the project’s net present value. A project with a positive net present value will – in absence of other unquantified impacts – be beneficial to the society which implements it.
Projects with a higher NPV are preferable to those with a lower NPV – but the size of the NPV is not always a useful indicator for the decision-makers who need to decide which project, from a large number of possible projects, they should approve and allocate a part of their transport budget.
Use of the NPV as a metric for prioritisation does not help to differentiate between:
In many countries decision-makers are presented with a Benefit Cost Ratio (BCR) – the present value of the benefits divided by the present value of the costs. A BCR helps to rank schemes according to returns/per unit of money invested. Having comprehensive information on the capital costs of all of the feasible schemes, enables decision-makers to allocate budget to those schemes which deliver the greatest benefits.
Not all of the impacts of a transport investment can be quantified or measured in monetary terms and expressed as a component of the Benefit Cost Ratio (BCR). Advances have been made in recent years in valuing many of the environmental impacts of transport schemes – including carbon emissions and changes in the levels of traffic related noise and local air quality.
No adequate method has been devised for assigning monetary values to impacts such the effect of new transport infrastructure on the landscape or the natural environment. While ITS schemes can be expected to have a more limited impact on the natural environment than a new road – there may be cases where the gantries and masts that are often part of an ITS scheme, will be regarded as having an adverse environmental impact.
More importantly, perhaps, is the failure of conventional transport appraisal methods to identify who benefits from the scheme.
Most transport models contain data on – and forecasts for – the origins and destinations of trips on the network. The model does not, though, help the decision-maker to understand, for example:
While land use/transport models can provide some indication of the likely incidence of the benefits, the use of such complex models is only justified in the case of major infrastructure schemes.
Not all of the impacts of a transport scheme can be captured in the BCR. Where they cannot be valued in monetary terms but are likely to be significant, decision-makers will want to take them into account. They will not want to be restricted to only prioritising schemes and selecting options on the strength of a BCR alone. Analysts can help decision-makers by providing as much information as possible about these unquantifiable impacts. This might include information about the consequences of choosing a scheme with a lower BCR – and assigning funds to it that might otherwise been spent on a scheme with a higher BCR.
Decision-makers and stakeholders need to be informed about the expected impacts of a project in a way which is neither unnecessarily complex nor risks concealing important information.
Presenting the Results of Appraisals
The UK Department for Transport has published a note which explains a process by which decision-makers can make a judgement as to whether the unquantified costs or benefits are of sufficient magnitude to change the BCR – so that it exceeds a value of 2.0:1.0 (indicating benefits that are valued at twice the costs)
Since the majority of all transport investment is spent on schemes for which the BCR – including an allowance for unquantified impacts – exceeds 2.0, schemes that do not reach this threshold are unlikely to be approved.
A template for summarising the appraisal is presented in the UK Department for Transport’s “Appraisal Summary Table”. It can be downloaded as an EXCEL spreadsheet from: https://www.gov.uk/government/publications/webtag-appraisal-tables
The information about the unquantifiable impacts included in this table helps decision-makers to weigh up these impacts with the benefits that are quantified in monetary terms and which constitute the BCR.
Road traffic models provide a representation of the highway network in terms of the capacity it provides and the volume of traffic using it. Road networks have finite capacity and the function of the traffic (or transport) model is to show how the outputs of the network – in terms of traffic speeds and in some cases the variance of journey times around their mean – are changed when the volume of traffic changes:
By integrating the supply of road capacity with demand from road users, a model can show the relationship between costs (in terms of travel times and other elements of generalised cost) and traffic demand. It also shows the point at which equilibrium is reached (a balance between traffic demand and available capacity).
The model forecasts the responses made by road users in response to better traffic management or investment in road capacity improvements:
In order to reach a position in which road users are assigned to the routes that provide them with the lowest generalised cost option – the traffic model has to be run several times to reach equilibrium.
There are important differences between road traffic models in the way in which they represent capacity:
Many highway authorities have access to traffic models set-up to cover the networks from which they are responsible (See Appraisal of ITS road schemes in UK using the INCA traffic model).
There are a range of approaches to transport modelling that can be used for ITS applications.
Inter-urban transport models generally model the capacity of the network using a relationship between:
At low volumes, traffic will flow freely at speeds close to the maximum set by any national limit. As volumes increase, speeds fall until stop start conditions prevail.
Urban applications that use ITS to optimise traffic – such as urban traffic signal sequencing – make use of models which represent:
Traffic models such as SCOOT, CONTRAM and SATURN represent the capacity of urban networks and responses to changes in that capacity. Versions of these models – that also include different vehicle classes and occupancies – can be used to assess the impacts of public transport priority schemes. The case for schemes of this type, rests on the more efficient use of road space made by buses and trams – because they carry more people than other classes of road vehicle.
ITS schemes can improve traffic flow, resulting in journey time savings, reduce unreliability by giving road users less variance in day to day travel times, improve safety – and provide road users with information about conditions ahead. In most cases the specification of the performance of the ITS proposal will provide information on the improvements that it can be expected to deliver.
Many applications of ITS are either intended to influence drivers’ behaviour – or to have the effect of doing so by providing them with what they perceive as an improved journey. Some drivers will respond to these improvements by changing the route they take, the time of day of their trip, their destination or their mode. Transport models are designed to estimate these responses and take them into account in the estimate of the benefits.
On interurban and the less congested urban networks, it is often judged acceptable – in absence of a detailed transport model that covers all modes and choice of destination – to assume the same overall forecast of road trips in each option.
The traffic model:
In some cases the ITS scheme can affect the choice of route taken by some drivers. In addition to the benefits gained by drivers who would have used the route without any improvement scheme, drivers who re-assign to the improved route also gain a benefit. The transport model, because it is based on the assumption that drivers opt for the quickest or lowest cost route, shows the extent of this re-assignment and the benefits in terms of congestion relief on the routes from which the traffic has reassigned.
Schemes that have a substantial impact on road journey times, such as new inter-urban or urban motorways, will influence people’s choice of destination and mode, in addition to their choice of route. In such cases a multi-modal transport model provides the best tool for assessing the impacts of a scheme. It is unlikely that many ITS schemes will require this approach for modelling the responses and estimating the costs and benefits.
Transport models are representations of a typical time period – for example, the morning peak or an average day. They need to be augmented with estimates for other time periods in order to determine the overall benefits of ITS – and any other initiatives aimed at improving reliability. For example, one of the main causes of unpredictable unreliability on motorways are incidents.
The road operator for England (Highways England) has developed a model, based on past observations, on the probability of incidents and of the delays that they cause. This model is used to estimate the benefits of schemes which provide more capacity or give motorists advance warning of incidents – so reducing the probability of accidents, the risk of secondary collisions and the delays caused by incidents.
Many textbooks on transport modelling have been published. A general introduction to transport models and their use is provided as part of the UK Department for Transport’s WebTAG transport appraisal guidance at http://www.dft.gov.uk/webtag/documents/expert/unit3.1.php.
The UK Department for Transport has also developed a method of estimating changes in the standard deviation of travel times on the urban road network, where there are many more alternative routes, from changes in journey time and distance.
See: TAG Unit A1-3 section 6.3 available for download at: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/370878/WebTAG_A1.3_User_and_provider_impacts_May_2014.pdf