In order to aggregate the outputs of the network analysis to obtain the final comprehensive accessibility score it is necessary to assign relative weightings to each of the datasets produced. Four matrices are required, which describe:
· how each mode of transportation varies with age group and activity type
· how each activity type varies by age group
· each age group as a percentage of the total population
· each working age group as a percentage of the total working population.
Not all transportation modes are appropriate for all age groups when travelling to certain land use activities. For example, the elderly are less likely to cycle due to their physical capabilities and a driver licence is only available to those over a certain age. For the purposes of developing the comprehensive accessibility score methodology, the weighting of each transportation mode when travelling to each activity type for each age group is assigned on a ‘best assumed’ basis to allow the methodology to be tested. In order to improve the integrity of the methodology, further research to adjust or confirm these values if the comprehensive accessibility score is deemed useful, is required.
The usefulness of a comprehensive accessibility score that aggregates more specific information is potentially limited given it smoothes out any accessibility limitations within a specific mode, activity or age group. If a purpose of accessibility modelling is to take a more disaggregate look at travel patterns, aggregating the results can disguise the findings. However an aggregate figure is also useful for showing how well transport systems provide coverage of a range of trip purpose and the components of the combined indicator can be weighted by the demand for or importance of the trip purpose for each resident. Nevertheless, it is included within this research as a potential ‘headline indicator’ should further research be commissioned.
The following logic was used to set up these initial weighting matrices:
· If all four modes were generally available to the age group, the relative mode weightings were set to 25% each (ie equal weighting).
· For convenience stores emphasis was given to walking and cycling modes, as the authors considered it more likely a person committed to using motorised transport would go to a supermarket that provided a higher level of service and typically a lower cost of goods.
· When visiting doctors and hospitals, physical modes of transportation were weighted lower than motorised forms due to the likelihood of illness or injury giving rise to the need for the journey.
· The under four-year-old age group relies on caregivers to accompany them when using the networks. For this reason cycling was given a reduced weighting as cycling with a small child is often considered a safety concern.
· For the 65 and older age group, a shift away from cycling was assumed in favour of walking, public transport and private vehicle, due to the physical capability of this age group. Public transport was given a greater emphasis than private vehicles as many of the older members of this age group no longer had a licence to drive and hence were no longer able to use this transportation method independently.
The fraction of the total population made up by each age group was calculated from 2006 Census data. The weighting assigned to each age group for work activities was based on the age group’s fraction of the total population aged 10 years and older in the 2006 census data.
All four weighting matrices used in the accessibility calculation methodology are presented in appendix E.