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.

	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