Radius-radius refers to the radius at which space syntax axial analysis attempts to maximise the radius of the analysis without inducing edge effect, (that is, the tendency for the edges of spatial system see more segregated than they might be in reality, simply because they aren’t connected onwards). It is calculated by taking the mean depth from the most globally integrated line in the system and using this as the maximum radius. See: Edge Effect.
Hillier, B. (1996, 2007), Space is the Machine: A Configurational Theory of Architecture. Space Syntax: London, UK. pp.120-121