The Connectivity of Streets: Reach and Directional Distance

We introduce two measures of connectivity that are applicable to standard GIS-based representations of street networks. The reach of a point measures the total street length covered by all paths extending out from that point that are no longer than a given threshold value. The directional distance of a street network from a point is measured according to the minimum number of direction changes required to reach any part of the network from that point, consistent with typical measures used in space syntax. However, our measure of directional distance requires no prior commitment as to the relational elements that make up the network. Any part of the network which is accessible from a point without a change of direction greater than a given threshold angle is treated as a single directional element for the purposes of computation. Street segments are characterized by the reach and directional distance of their midpoints. Networks are characterized by the average directional distance of the corresponding street segments. The measures render explicit the interplay between metric and topological properties of networks. Preliminary studies show that the measures discriminate well between different morphologies of street networks. When used to compare urban morphologies they are well correlated with standard measures used in the literature, with the added advantage that they can discriminate between street segments within the same urban area. Using field observations we also show that the measures can be used to model the effect of spatial configuration upon movement in ways which compare favorably to standard space syntax.