Permeability and interface catchment: measuring and mapping walkable access

Abstract

The relationship between urban morphology and walkability is central to urban design theory and practice. In this paper, we develop new measures for pedestrian permeability and catchment areas, suggesting that their joint use can progress our understanding of the ways urban morphology mediates walkability. Existing measures of permeability do not account for heterogeneous morphologies. Likewise, measures of pedestrian catchment do not account for what it is that is caught. The proposed “area-weighted average perimeter” and “interface catchments” together integrate both street width and block size, measuring both walkable access and what one gets access to. What is at stake is not only correlations with health and transport, but also measures of walkable access that are geared to the social and economic productivity of the city.

Keywords:

• Permeability
• catchment
• interface
• morphology
• mapping
• walkability

Acknowledgment

We thank Simon Wollan for an insightful critique of an earlier version of this article.

Notes

1. The relationship between pedestrian movement and street networks has also been explored through spatial syntax research, which studies the links between topological measures of urban networks (such as integration) and urban intensity (Hillier and Hanson 1984; Hillier 1996; Hillier and Vaughan 2007). While network integration is an important form of urban analysis, it is not strictly a measure of permeability or catchment and is excluded from this analysis.

2. There are two other mathematical measures of block shape that we have considered but discarded. First is the method used by Louf and Barthelemy (2014) which measures each block in terms of the ratio between the area of the block and that of the circumscribed circle. Second is the ratio between the block area and that of a circle with an identical perimeter, known as the “isoperimetric quotient” (Colaninno, Cladera, and Pfeffer 2011). In both cases as block shape approaches a circle the ratio approaches 1 and is considered to have maximum permeability. However, neither of these measures is scale based – the measure for a small square block will be identical to that of a large impermeable square block.