The Role of Transport Models in Evaluation

The use of transport models to estimate demand for travel in urban transport networks is well established.

Models simulate travel demand between each each origin and destination zone (the study area is divided into analysis zones) and assigns those trips to the road and transit transport networks.

Urban transport models used to predict changes in travel demand resulting from transport system and demographic changes are based on the concept of perceived generalised travel costs.

It is assumed that travellers make their decisions on the destination, the mode of transport, and the route used, based on how they perceive the attributes of each travel option.

The generalised cost represents the sum of the various components of the total perceived cost, such as: fuel cost, parking fee, transit fare, walking and waiting time and in-vehicle travel time. Weighing factors are used to represent the relative importance of each cost component to users.

The values of time used for evaluation purposes is usually the same for all users irrespective of income level or mode used. Two values of time are used for evaluation purposes, namely: working time (trips made on employer’s business) and non-working time (all other trips). Walking and waiting time is commonly valued at twice in-vehicle time.

The level of confidence which can be placed on any model output is directly related with the level of confidence which can be placed on the underlying model assumptions.

For example, it is important to know the assumptions methodology regarding:

  • The elasticities of demand for transit with respect to modal attributes
  • The estimation of generalised costs for each mode
  • The form of the models used for trip generation, trip distribution and mode choice
  • The level of detail of the calibration results. The latter need to provide sufficient detail to enable an assessment of the calibration process to be made.

guest post by Prof Luis Ferreira

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