Public transport accessibility levels (PTAL)

The study aimed to find out how well different commercial centres in Christchurch were served by public transport. The public transport accessibility levels (PTAL) methodology developed by the London Borough of Hammersmith and Fulham in 1992 (TfL 2010) was used to inform this assessment.

A detailed city-wide walking network was developed using Environmental Systems Research Institute (ESRI) ArcGIS. The walking network included various time-based deterrence functions to a maximum walking time limit or equivalent walking distance. The time-based deterrence functions included delays for road crossings based on crossing type, traffic volumes and road hierarchy. This was a significant advancement on the ‘as the crow flies’ distance buffer that is often used. Factors on the network such as delays at road crossings and urban design factors significantly affect straight line maximum walking distances.

A computer model was developed using Python and ESRI ArcGIS to automate the analysis. The model was run on all district centres and business retail parks. A grid consisting of a point of interest every 100m over the Christchurch urban area was created and PTAL calculated for more than 19,000 grid points. The model was validated against a number of manual calculations.

Although PTAL indicators are controversial in the UK, being described as lacking in a credible theoretical foundation and certainly only suitable for analysis of dense urban areas such as in London, performing the detailed PTAL analysis resulted in some interesting and insightful outcomes regarding the pattern of public transport accessibility in Christchurch. The accessibility index (AI) value calculated for each point of interest was categorised into one of eight PTAL bands. It was found that applying London PTAL bands to Christchurch AI values resulted in what appeared to be an unfair representation of Christchurch public transport accessibility. Consequently new PTAL bands were generated specific to Christchurch that provided a fairer representation for this community based on existing performance.

PTAL only measures access to public transport and is not a true accessibility assessment as it does not include demand for the specific public transport routes and interchange options provided at the point of interest. Nevertheless, the PTAL analysis undertaken for Christchurch was useful to describe to practitioners where enclaves of poor public transport accessibility might exist. It also proved useful to demonstrate that existing measures, such as access to a bus stop within so many metres, could be improved upon.