17.3 Modelling and Assessment Software

The development and application of models and software are important to many project appraisals for the following reasons:

• They provide an analytical framework to assess existing demands on the transport system, and to project these demands into the future to test the impact of transport and land-use options on a systematic basis; and
• They enable quantitative measures to be generated that act as key indicators in the appraisal process.

Some of these indicators can only be derived in a sufficiently robust, disaggregate manner by using a model.

At their most complex, models include:

• A road traffic assignment model;
• A public transport passenger assignment model;
• A mode choice model;
• Trip generation and trip distribution assumptions based on trip end data; and
• Modelling of transport and land-use interactions.

Demand models are most common and range from assignment only, to more sophisticated approaches involving the four main stages of generation, distribution, mode choice and assignment.  Demand models come in two main forms:

• The operational form where the networks and systems are represented in detail; and
• Policy sensitive forms where the trip ends are represented in greater detail.

For many appraisals, the pattern of travel demand is unlikely to differ significantly for the transport options under consideration.  In these circumstances, travel demands can be assumed to be fixed for each option and the assessment undertaken using only an assignment model.

Accessibility models represent the transport supply in relation to the spatial distribution of land-uses.  It is often assumed that road network supply is available to all locations so the modelling is regularly restricted to public transport supply.  However in some circumstances it is helpful to compare the standard of transport supply for road and public transport users so in these cases travel times and costs using the road network are also modelled.

Land-use/transport interaction (LUTI) models cover a wide range of types of model representing aspects of the interaction in different levels of detail.  At their simplest level they are sometimes taken to include policy sensitive demand models or accessibility models as described above, but in true LUTI models patterns of land-use are modelled rather than simply represented as a data input.