What is a model?
A model is a formal mathematical representation of a real world
system. A land use and transport model could represent how people’s
travel behaviour responds to changes in the transport system provided;
how the performance of the system changes as patterns of use change;
how these changes affect indicators such as congestion, pollution
and accidents; how land use changes affect patterns of use; or how
land use is itself influenced by changes in the cost of using the
Why do we need models?
The answers to these questions are complex, and it can thus be
difficult to estimate how the transport and land use system will
change in the medium and long term (Section
3) without some analytical tools to provide those estimates.
Moreover, the range of policy instruments (Section
9 ) and of ways in which they can be combined (Section
11 ) makes it particularly difficult to decide what is the best
strategy. Authorities need information on likely effects on their
land use and transport systems for a range of scenarios. Computer-based
mathematical models of the urban land use and transport system can
What types of model are available?
Any model is supposed to be a simplification of the system being
studied. It is not, and should not try to account for, everything.
It should instead be a well-made caricature, where the characteristics
of the modelled system are brought out with no more brush strokes
than necessary. This makes it easier for the modeller to understand
the system, and for others to use it. This in turn means that the
results are more likely to be trusted. However, simplicity cannot
be the main objective. The key to a good model is to drop unnecessary
detail and complexity. This will be a greater challenge when dealing
with integrated strategies in which more elements need to be modelled.
In the PROSPECTS Methodological Guidebook we provide advice on
three types of model, in order of increasing complexity and the
specialist skills required:
• Policy explorers, which provide a very simplified representation
of a hypothetical city, and help users to understand the types of
impact which a policy might have
• Sketch planning models which represent the main interactions
between demand, supply and land use at a strategic level for the
city in question, without giving detailed information on transport
networks or land use patterns; and
• Land use—transport interaction (LUTI) models, which
represent transport networks and land use patterns and their interactions
in greater detail, while still focusing on strategic issues.
In addition, there are conventional network and transport planning
models, which are less complex than full LUTI models, but which
typically ignore the land use effects.
PROPOLIS provides a valuable example of the use of a range of LUTI
models to test a common set of strategies in seven cities, and argues
that such models are essential for understanding the complex interactions
between transport and land use in larger cities. ISHTAR has developed
a suite of programs which go further in assessing the impacts of
transport on pollution and the built environment.
What are the limitations on models?
There are dangers both in over-use and under-use of models. The
traditional rational, analytical approach to planning (Section
4 ) can all too easily lead to over-reliance on models, and
a failure to realise that other issues are important, and that others
will mistrust the experts and their results. Model-based analysis
therefore needs to be used as a contribution to strategy formulation,
rather than being seen as the whole process. Model assumptions need
to be made clear, and results need to be able to be presented in
a user-friendly way to decision-makers and to stakeholders as part
of the participation process (Section
5 ). Ideally models should also be available for non-experts
to use, as a tool to support “deciding together”. However,
most current models are unfortunately not well designed for this.
Many cities do not use models themselves, often because they do
not have the resources or expertise to do so. Cost, and the need
for specialist skills, have often been a barrier, but the sketch
planning models which we describe are now much less expensive, and
much simpler for those without specialist expertise to use. Another
concern is that models may be unreliable. It is certainly the case
that, because models are simplifications of reality, they will omit
some of the interactions in the real system, and approximate others.
In our review of the requirements and capabilities of models we
identified the limitations in the box as of particular importance.
There are also approaches that do not depend on mathematical models;
for example, the EU project ASI has developed a qualitative ‘tool
box’ to assess the effects on ‘life quality’ of
urban transport and mobility policies. In practice it may be sensible
to combine mathematical modelling to predict indicators which can
be quantified with qualitative approaches for those indicators (such
as ‘life quality’) which cannot.
These are all areas for further research and development. Even
so, it will be easier to plan a land use and transport strategy
for a city with a model, in the knowledge of these imperfections,
than to estimate the effects without one. Indeed, there is a need
for further research to develop guidance for the prediction of impacts
when models are not available.