2.4.8 Use of Transport Models

Transport models can be a valuable aid in assessing existing and potential future problems.  Transport models can usually be easily interrogated to produce summaries of network-wide conditions as well as modelled flows and delays on particular links or at particular junctions.  If the models include public transport as a mode or modes, then data can be produced on route or corridor flows (and potentially overcrowding) as well as network-wide summary data.  Transport models can also be used to provide data on the gross volume of trips made in the modelled area, and in some cases mode split and journey purposes.  Plotting desire lines can be helpful.  Output from transport models can also be used to derive other measures that may help quantify problems. Examples include, but are not limited to, emissions of pollutants or accessibility indices.

Practitioners should, however, be cautious when using transport models:

• The availability of a transport model can result in a practitioner devoting much attention to extracting data which in reality may reveal little if problems lie elsewhere.  Scoping the exercise of extraction of data from a model will be important before starting the exercise.
• The analysis of problems that can be undertaken using a transport model is limited to the time periods and modes included in the model.  There may be significant problems in other time periods or with non-modelled modes.  The analysis of problems should not be limited to what can be extracted from a model; and
• Transport models are only as good as the input demand and supply data.  Analysts should carefully consider a model's calibration and validation before embarking on any significant work.  A poor model can lead to mis-identification of problems.

Transport models can be used to identify potential future, as well as existing, problems.  However, each and every forecast is:

• Underpinned by a set of explicit (or sometimes implicit) assumptions and exogenous forecasts of key variables (e.g. traffic growth); and
• Limited to the modes modelled and the interactions (assignment, mode split etc.) considered.

When a transport model is being used to identify future problems this must be accompanied by a clear statement of the assumptions that underpin the forecasts.

The Scottish Government's Transport Model for Scotland (TMfS) offers practitioners a potentially rich source of information on existing and potential future problems in their area.

In large-scale studies, model development often forms part of the process prior to testing and then appraisal.  The problem identification stage will usually precede any model development and application.

Naturally, practitioners should not feel that the absence of a transport model means that they cannot make assessments of what potential future problems may be.  A structured approach should be taken to scoping the future conditions relevant to the study in both the transport field and beyond.  Appropriate techniques include professional level consultation, opinion gathering techniques, and quantified projections, but in the last case it is important to make clear what assumptions have been made.