“…Likewise, the use of GIS to create circular buffers centred on residents' homes has been used elsewhere to study the associations between local contextual conditions and health outcome (Chaix et al, 2005a;Chaix et al, 2005b;Chaix et al, 2006). Buffers can be used to obtain summary measures of local factors which can then be examined in relation to the health outcome of interest, with the size of these circular surfaces (e.g., a given radius around each residential address) being based on the study's purpose (Berke et al, 2007;Crawford et al, 2008;Pate et al, 2008). For IDUs as well as the average resident, a 10-minute walking distance, corresponding to a 500-meter buffer, is generally recognised as a reasonable radius by which to represent access to local services.…”