The figure above shows the public transport accessibility level for the Helsinki metropolitan area in 2009 (click for a larger view). The accessibility level is a subjective measure indicating the usefulness of the public transportation for commuting between the residential districts and the service and supply oriented areas. In the image the red colors indicate areas where public transportation is the most effective for everyday commuting. At the blue areas the public transportation is the least effective, and the yellow indicates the areas of moderate effectivity.
The most efficient areas (red color) are located in the vicinity of the main transportation hubs and shopping malls, since the service level at these locations is high and a high number of transportation routes intersect at these locations. Moderate efficiency is distributed along the train and subway stations, which provide the most time efficient commuting.
The accessibility level shown here is based on the spatially varying accessibility index calculated as a weighted and normalized sum of the following variables:
- the traveltime from the point of interest (POI) to the major service and consumer good suppliers,
- the traveltime to the nearest public transportation hubs (the hubs are determined from the routing data),
- the traveltime to the Helsinki city centre, and
- the average walking distance to the nearest public transportation stops from POI.
In this formulation of the accessibility index, the traveltime to the main transportation hubs is considered as the most important variable. This assumption shapes the results considerably. However, the definition of the accessibility level is a subject of constant debate, and as of yet there is no agreed consensus on how the accessibility index should be formulated. For further discussion, refer to this wonderful editorial of Environment and Planning B journal.
The data for calculating the accessibility index was generated using the crawler and analysis tool which I wrote for fetching traveltime and routing data for public transportation organized by YTV (Helsinki Metropolitan Area Council YTV). Total of 126000 routes were used to construct the accessibility indices for the image. Visualization is based on the SciPy -library.