BUS606: Operations and Supply Chain Management

Accessibility

Accessibility refers to the ease of access to a particular neighbourhood feature with more accessible destinations having lower travel costs in terms of distance, time, and/or financial resources. Accessibility to built environment features is not only determined by their distribution across space but also by mobility factors such as private vehicle ownership or public transportation networks. Handy and Niemeier suggest three categories of accessibility measures: 1) cumulative opportunity measures which is simply a count of features within a given distance with an equal weight applied to all occurrences of a specific feature; 2) gravity based models where features are weighted by factors such as the size of the destination or travel cost; and 3) random utility-based measures where theory is used to inform the probability of an individual making a particular choice depending on the attributes assigned to that choice (e.g. attractive of destination or potential travel barriers) relative to all choices. An alternate accessibility measure that incorporates a temporal dimension has been proposed by Kwan. Space-time measures' incorporate the constraints imposed by the fixed locations an individual must visit during the day (e.g. location of work-place, child's school) when determining potential accessibility to discretionary locations that an individual may visit (e.g. supermarket) (see also activity space). Greater locational access to neighbourhood features may improve or worsen the health-related behaviours of local residents. For example, high levels of accessibility to a greengrocer or large supermarket may better enable the purchase of fresh fruits and vegetables while greater accessibility to outlets selling fast food may encourage the consumption of fast food at levels that are damaging to health. Traditionally, accessibility has been rather simply measured through the presence or absence of a resource in a particular locality because these data were readily available. These measures assume equal exposure for each person within the area unit irrespective of where they live in that unit, the amount of time that they spend in the area, and their ability to travel within and beyond the boundary of the administrative unit. GIS has improved measurements of accessibility by enabling the creation of more refined individual-level metrics such as density within buffers from a household location, proximity based on network distance, activity-spaces, and continuous surfaces of accessibility such as Kernel density estimations.

Scale

Selecting an appropriate spatial scale for measuring features of the built environment using GIS is an important prerequisite for research into neighbourhood influences on health. Different characteristics of the built environment are likely to influence the health and/or behaviours of local residents at various spatial scales. For instance, it could be argued that the influence of access to a local corner store may have a greater effect on the local resident population (i.e. neighbourhood), whereas the availability of a large supermarket is likely to extend to a wider geographical extent. Further, the most appropriate spatial scale for capturing the neighbourhood feature could be influenced by the socio-demographic characteristics of the study population (e.g. children compared with elderly) as an individual's capacity and motivation to travel longer distances is likely to be affected by personal mobility and activity space. Other considerations include whether the area of interest is urban or rural as environmental exposures in an urban setting may be confined to a more localised population, whereas rural features are likely to be utilised by those from a larger spatial region. Thus, for studies examining exposures related to the built environment no one scale can be recommended. Ideally the selection of the spatial scale will be informed by the theoretical understanding of the processes that link the neighbourhood characteristic(s) to health. However, it has been recognised that relevant theory does not always exist. In situations where this is so, for example amongst studies examining walkability, the result is that buffer sizes used for measures of walkability have varied from between 100 metres to 1 mile (approximately 1600 metres) or to even larger areas defined by the boundaries of administrative units. Decisions regarding scale are external to GIS, however the advantage of GIS is that many plausible scales might be investigated.