Types of model

The tools typically available internationally for modelling accessibility fall into three categories as shown in figure 4.1.


Figure 4.1     Types of accessibility model

 

The models used by facility providers such as retailers and public transport operators generally start from the analysis of the local population and output information on the potential client mix within the local catchment. This allows providers to plan appropriately for the people able to access their services.

Some models have been derived from road or public transport journey planners and tend to be restricted to an analysis of the time it takes to reach different destinations.

Although transport and land use models tend to be much more complex than the journey planners or catchment tools they do not necessarily provide better information for accessibility planning unless linked with other models. These network and transport system models tend to have very few user groups. It is therefore necessary to use travel time information from these and link them with separate accessibility models to generate high-quality accessibility measures.

Due to the different functionalities of the different types of accessibility model available there is confusion about what an accessibility model comprises.

Instead, it is more useful to specify what indicators are needed and then identify what tools can be automated to calculate these. The standard functionality available in current databases and geographic information systems (GIS) is usually more than sufficient for most modelling applications, and by adding simple programming or macros to these, the repetitive indicator calculation can be automated.

There is generally very little transferability of models between areas. Most local authorities in countries with well-developed accessibility planning approaches have automated calculation procedures to meet their needs and in some places these have been ‘branded’. These brands have been important to give the models credibility within an industry that has been traditionally heavily dependent on sophisticated models.