Prior to the launch of accessibility planning guidance in the UK in 2004 ACCMAP was the most widely used software by local authorities. DfT therefore partnered with the developers of ACCMAP to add indicator calculation functions that were likely to be needed for more integrated accessibility analysis. The upgraded software was named Accession and marketed by Citilabs UK. Accession is limited to local models as in common with ACCMAP. The software is based on a Microsoft Access platform that restricts the file sizes possible. Accession has been marketed by Citilabs outside the UK including to local authorities in New Zealand.
The tool helps users to manage spatial and timetable-based data inputs but is designed for compatibility with UK data sets and in other countries there are different data standards, particularly for the public transport data.
Accession and other similar tools allow users to create and delete data for executing ‘what if’ queries and understanding the impacts of planning decisions, and include both spatial and tabular data. Features such as this, as well as the ability to import and export various GIS and transport data formats, are generic functionalities and Accession seeks to strike a balance between policy relevance for accessibility planning and the same flexibility available in generic GIS software such as ARCinfo.
Although its primary purpose is for use with public transport data, Accession also includes analysis of walking, cycling and vehicular transport although there are some simplifications with regard to the walking and cycling analyses in particular, which reduce the quality of the outputs.
In assessing the suitability of Accession for analysis in New Zealand it is of note that Accession does not use specific walking or cycling networks for accessibility calculations, rather walking and cycling trips are based on road centreline data, and walk and cycle speeds are set as a model parameter. This reduces the accuracy for neighbourhood accessibility analysis.
Accession also gives the option to use a straight line ‘as the crow flies’ walk distance for the calculation of public transport interchange (stop to stop) and connection (origin to network or origin to stop) trips. This straight line distance is converted to a generic detour distance (actual distance travelled) using a conversion factor of 1.4 (as shown in figure 10.1). This is a significant assumption considering the variability of walking environments and although it can be varied by the user, it would require validation to each study area or each part of the study area.
Figure 10.1 Straight line walk distance factor diagram
The limits on file sizes also mean that Accession is not suitable for more strategic analysis covering wider areas. One way of increasing the size of the model is to use a larger zone size and to alter the walking and waiting times to reduce the computation demands when estimating the journey times.
In the New Zealand analysis the model was assessed over various time periods limiting the maximum travel time. The time interval set is a trade off between accuracy and calculation time. For example, reducing the sample time to five minutes doubles the processing time but cuts the error in journey time calculation by 75%.
To reduce processing times, walking, cycling, public transport or vehicle networks can be further simplified by reducing the number of modelled nodes. This approach requires a testing methodology to find the optimum network quality which will reduce processing time, but will not compromise the quality of accessibility calculations.
A more detailed description of Accession including a trial applied to Christchurch New Zealand is included in appendix A.