Study area

We assessed the spatial distribution of WRD and walkability for 2007 in the Sydney metropolitan area. Sydney is Australia’s largest city, located on the eastern seaboard with an area of over 3700 km² and had an estimated resident population of 3.6 million in 2006 (average density 990 persons/km²). Both walkability and WRD were calculated at the 2006 Australian Census Collection District (CCD) level, the smallest geographical area used for reporting Australian census aggregated household data by the Australian Bureau of Statistics (ABS) [20]. We obtained digital boundaries for CCDs from the ABS [20]. The average CCD in the study region included 200 dwellings and 550 residents and covered an area ranging from 0.002 km² to 125.40 km² (median 0.20 km², 10^th percentile 0.02 km², 90^th percentile 2.40 km²) prior to CCD exclusions due to missing data [21]. Usual resident population, percentage of total population, and land area (km²) were calculated for each CCD using data from the ABS 2006 Census Basic Community Profile DataPack (ABS cat no 2069.0.30.001). Percentage of people (employed adults aged at least 16 years) within each CCD walking entirely to work on the 2006 Census day was also calculated using data from the 2006 ABS Census.