Improving Transport Project Appraisals
In the Islamic Countries
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forecasting. Usually they build on data on past and current traffic to study how they are linked
to other variables of the context and actors’ preferences and behaviours and combine them to
other data on population, employment, travel costs and travel times, etc. Once the model is in
place, if populated with projected data on population, employment, and other variables it results
in estimates of future demand. Different models and techniques may indeed be suggested or
required, for example deterministic/stochastic, static/dynamic, analytical/simulation
approaches. Building a model requires trading-off time and resource (especially budget and
capacity) constraints. Models that forecast the traffic demand include travel costs, travel time
and an alternative specific constant. The specification of the model is the way each variable is
linked to the other and, in particular, what depends on what and how. Depending on the level of
specification they can provide different degree of details on the demand. They have to be
adapted to freight and passengers traffic and should take into consideration network and multi-
modal effects on traffic and, where relevant, corridor level interactions among projects and
modes. For example, network effects require that models include: a sufficiently wide
geographical areas, various multimodal responses, (both in terms of competition and
complementarity), interaction in responses between passengers and freights, consider
congestion effects and changes in departure times.
The traffic demand forecasts form a key input to the project appraisal. In particular, in addition
to determine the infrastructure capacity and other more technical specifications, it affects the
calculation of socio-economic and environmental effects (which, in some cases, can be included
as direct output of the model itself). As such, any errors present within the demand forecasts
will undermine the reliability of the appraisal 19. The accuracy of demand forecasting is therefore
important in the delivery of the appraisal. In turns, the quality of the forecast depends on the
quality of the underlying models. The standard transport model structure adopted in the UK is
presented in the following figure in a flow chart format.
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