Permeability (spatial and Transport Planning) - Permeability and Connectivity

Permeability and Connectivity

Stephen Marshall has sought to differentiate the concepts of "connectivity" and "permeability" (see drawing). As defined by Marshall, "connectivity" refers solely to the number of connections to and from a particular place, whereas "permeability" refers to the capacity of those connections to carry people or vehicles.

Traffic studies on the influence of street patterns on travel generally overlook this distinction and two metrics are instead used to characterize a street pattern for trip purposes: connectivity and intersection density, both of which refer to regular city streets. This omission is often the result of unavailability of data for connectors that do not appear on maps and cannot be geo-coded. Consequently, the potential effect of connectors that are accessible only to pedestrians and bicycles on mode choice and extent of travel is missed. The aforementioned study shows that differentiating between normal paved streets and non-vehicular paths yields clear and positive results about the influence of the latter.

The distinction between permeability and connectivity becomes clearer in practice. Widening roads within a network that lead to destinations would increase the network’s permeability, but leave its connectivity unchanged. Conversely, transforming existing streets that are part of a grid plan into permeable, linked culs-de-sac, as was done in Berkeley, CA and Vancouver, BC, retains their connectivity intact but limits their permeability to pedestrians and bicycles only, while it "filters" out motorized transport.