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