The accessibility calculation methodology described in this report can be broken down into seven steps as shown in figure 15.1, forming two distinct phases. The first is a network analysis (steps 1–4) and the second involves aggregating the results of the network analysis to produce the final comprehensive accessibility score (steps 5–7). Steps 1–4 are useful whether or not there is a clear accessibility policy to back them up. The travel time results generated can be used by policy makers and planners in many different ways as shown in the UK. In contrast steps 5–7 require the scores to have some meaning in policy. This is like the German or Netherlands approaches where accessibility goals are specified in policy and then measuring techniques are used to support the management and monitoring of progress towards the goal.
The first phase involves the preparation of the input datasets, interrogating the network dataset to find the quickest path(s) from the site being assessed to as many destinations of the activity type under consideration that are within the 95% percentile travel range. A series of raw accessibility indices are created for the site. The raw accessibility indices (R) are combined to create the aggregated accessibility score for the site. These steps are repeated for each of eight activity types and four transportation modes. This generates 32 datasets which are then aggregated in phase 2 (refer figure 15.1, steps 1–4).
The second phase takes these 32 datasets and combines them based on the relevance of the eight activity types and four transportation modes for the six different age groups. The final result from phase 2 is the comprehensive accessibility score, a single value which measures the overall accessibility to key activities from the site. The outputs of the intermediate steps in phase two yield results for accessibility to a single activity type by age group (eg accessibility to doctors for children under five) and the overall accessibility of the site to key facilities by age group (eg overall accessibility for people aged over 66) (refer figure 15.1, steps 5–7).
The following sections describe each of these seven steps in detail and explain the terms which have been introduced above.
Figure 15.1 Overview of the accessibility calculation methodology
Note: step numbers match the headings in the following sections