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Traffic Models

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:

  • traffic models are used to estimate the benefits, in terms of increases in speeds and reliability – of changes in highway capacity
  • traffic models can also be used to estimate the impact of policies to reduce demand through pricing or other restrictions

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:

  • it is assumed that drivers will choose their route in order to minimise the generalised costs of travel (or in some models their travel time)
  • on congested networks, an increase in capacity will generally result in a reduction in road user travel costs
  • lower costs will result in additional traffic being attracted to the improved route or network – so that some of the initial cost advantage is eroded

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:

  • models used for inter-urban schemes generally represent capacity in the form of a speed flow relationship – which shows, by the number of carriageways, the relationship between the number of vehicles in a typical hour (or day) and the average speed of that traffic. As traffic volumes increase beyond the free flow uncongested level – speeds fall, gradually at first and then more rapidly until stop-start conditions prevail
  • in urban areas “at grade” (same level) junctions provide the main constraint on traffic flows. Models have been developed which take into account the capacity of the major junctions and the likelihood of the queue at one junction clearing before more traffic builds up behind. Drivers in dense urban networks face a greater choice of routes than is usually the case on the inter-urban network and so – despite the need to cover a smalller geographical area – the network detail in an urban model is often greater

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).

Choice of traffic model

There are a range of approaches to transport modelling that can be used for ITS applications.

Inter-urban networks

Inter-urban transport models generally model the capacity of the network using a relationship between:

  • the capacity of each link of the road
  • the volume of traffic on each link
  • and the speed which traffic will achieve during a peak or a typical hour

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 networks

Urban applications that use ITS to optimise traffic – such as urban traffic signal sequencing – make use of models which represent:

  • junction and link capacities in urban areas
  • and the interaction between junctions in periods when flows exceed capacity

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.

Assessing the Impact of ITS

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.

Impact on Driver Behaviour

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:

  • is used – initially – to show the effects of the increase in future volumes of traffic on the performance of the unimproved network (the “do nothing” reference case);
  • is then used – once the changes in network performance delivered by the ITS scheme have been input –to show both the number of vehicles benefitting from the ITS scheme and the magnitude of those benefits.

Impact on Route Choice

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.

Impact on Destination and Choice of Travel Mode

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.

Impact on Journey Time Reliability

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.

 

Further Information

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

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