Novel Methods for the Estimation of Cost-Distance Decay in Potential Accessibility Models

In much accessibility research, arbitrary estimates of the distance-sensitivity parameters have been used to represent the distance decay-parameters in potential accessibility models. These estimates might be considered arbitrary, since the choice of value and the choice of the distance-decay function is often motivated by statistical indicators of the goodness-of-fit on spatial flows, given the fact that measures on ‘real’ accessibilities are missing. Starting from these considerations, in this chapter we introduce a new approach, the half-life model, originating from the natural sciences, to estimate distance-decay parameters. This method is compared with the conventional approaches – originating from spatial economic science – for the computation of distance-decay parameters, which concerns the calibration of both the unconstrained and the doubly-constrained spatial interaction model. The emerging distance-decay parameters will be then considered in the construction of the accessibility indicators based on the potential accessibility introduced by Hansen in 1959. In this context, both the mean and the median distance will be taken into account in order to identify MAUP-related issues. The exploration of these three approaches focuses on empirical analyses of accessibility in Sweden, at the municipal level, for the years 1993 and 2008. All the emerging accessibility indicators are compared in order to analyse similarities and differences in the hierarchical accessibility levels of the Swedish municipalities. The chapter concludes with some methodological/empirical remarks on the adoption of these three approaches, in the light of possible forecasts and related policy analyses.