Table 6 Summary of activity space measures. SDE1 = standard deviational ellipse at 1 standard deviation; SDE2 = standard deviational ellipse at 2 standard deviations; RNB = road network buffer; STT = standard travel time polygon; RTT = relative travel time polygon.
From: A suite of methods for representing activity space in a healthcare accessibility study
| Â | Representation of accessibility | Type and shape of measure | Data sources | Advantages | Disadvantages | Specific applications |
|---|---|---|---|---|---|---|
SDE1 SDE2 | Statistical approximation; abstract space | Euclidean measure; ellipsoid shape | Household + activity locations (multiple points), frequency-weighted | Captures spread and orientation of points; can be weighted by frequency | Capture 67% and 95% of points, respectively; Euclidean measures do not fully capture surface effects; poor representation of actual activity space; captures opportunities not in activity space; requires a minimum of three unique points to generate ellipse | SDE1 has statistical/predictive power (in this study area) |
RNB | Descriptive, Actual access | Network-based measure; shape is buffered network ("worm") | Household + activity locations (multiple points), road network | Captures 100% of activity destinations – best representation of actual space determined by nodes and routes | May be too restricted for predictive purposes | Bypassing; accessibility in actual activity space |
STT | Normative, potential access, single norm (30-minute travel time threshold) | Network-based measure; shape is network-derived polygon | Household (single point), road network, travel time standard | Results are fundamentally different from other measures; captures the highest number of SDPs; area is indicator of relative location within road network | Not based on activity destinations; "arbitrary" travel time limit? Most strongly conditioned by location relative to road network | Evaluate accessibility according to standards/guidelines. Area is indicator of proximity to primary roads, services |
RTT | Normative, potential access, relative norm (individually determined) | Network-based measure; shape is network-derived polygon | Household (single point), road network individual travel time threshold (maximum travel time to routine activity destination) | Captures 100% of activity destinations; highly correlated with RNB (can be used in combination) | May overlap with STT; also conditioned by location within road network hierarchy | Relative accessibility; area has strongest correlation with health destination |