Aboriginal and Torres Strait Islander people and primary health care: patterns of service use, preferences, and access to services 2024

Overview

Drive times refer to the estimated time it takes to travel by car from one location to another, based on the road network and speed limits, via the fastest route. This estimate may differ from the actual travel time.

Drive times between population centroids (ABS Population Grid 2021) and nearest service locations (in terms of time) were calculated using Esri software. Drive times were adjusted for the distance to the road network and imputed in the absence of a valid route.

Details

• Origins: Land centroid of each populated grid cell from the ABS Population Grid 2021.
• Destinations: Service locations (geocoded and checked).
• Road network: ArcGIS StreetMap Premium Asia Pacific 2023 Release 1.
• Barriers: None (people may use any service, nationally).
• Traffic consideration: None (driving unimpeded).

Adjustments and imputation

In the vast majority of cases, drive times needed little adjustment: geodesic distances from population centroid to road network (start journey) and from road network (end journey) to service location (‘snapping distance’) were penalised at 1 minute per kilometre (equivalent to 60 km/h). It was especially important to consider the snapping distances in remote areas, where the Esri road network could be incomplete. A speed of 60 km/h was chosen to return moderately conservative results.

For locations where the road network analysis detected no service within a 2-hour drive, the geodesic distance to the closest service was used to impute a drive time, at 2 minutes per kilometre (equivalent to 30 km/h). It was especially important to impute results for islands where ferry routes were missing from the road network. A speed of 30 km/h was chosen to return extra conservative results. The use of a 2-hour drive time cut-off for this treatment resulted in some population centroids scoring potentially ‘false positive’ access to services when farther than a 2-hour drive but within 30 km of a service. 

For locations where the snapping distance accounted for more than 25% of the total drive time: the lesser of total drive time and the geodesic distance time was used. This was applied in case any populated location had a nearby service but incomplete road network data.

Limitations

The drive time analysis is very accurate in most cases. However, there are some general limitations:

• Minimum drive times are employed as an objective measure relating to the ease with which people can use services, where distance presents a barrier. In reality, there:
  • may be more important, non-spatial barriers affecting access to services
  • are subjective differences of opinion relating to the same travel time
  • are many unmeasured spatial factors, such as traffic, parking, road quality, fuel costs, people movements (for work or other activities).
• People are assumed to be able to travel by motor vehicle, when necessary. For people without this option or for whom this is more burdensome, distance presents additional challenges.
• As well, the digital road network may be missing potential routes, or may include routes that are not accurate.

More specifically to this analysis:

• Residents within each 1×1 km grid square are assumed to have the same drive times to services. This does not account for the differences in how people are distributed within those grid squares, which can have a considerable impact in certain areas (for example, where water or other barriers separate 2 communities).
• Grid squares with between 0 and 0.5 estimated residents (Aboriginal and Torres Strait Islander people) are not shown on the map, but their modelled populations still contribute to results when aggregated to larger geographic areas.