Urban nuclei and the geometry of streets: the 'emergent neighborhoods' model

A controversy remains among planners and urban designers about the proper location of the non-residential core (nucleus) of a neighborhood in relation to thoroughfares. One school of thought suggests that the nucleus should be located along the busiest thoroughfares; a second school holds that it must be some distance away from them - which, because of their disruptiveness, should form the edge of the neighborhood; and a third school proposes that it should be somewhere between the two as an 'eccentric nucleus'. The three schools may be overlooking the underlying variables that govern this problem under different conditions, and so we propose a model for establishing the best location and distribution of urban nuclei as these conditions vary. This requires firstly, a redefinition of the 'neighborhood' as distinguished from a 'pedestrian shed'. We argue that a 'neighborhood' can either emerge within a 'sanctuary area' between thoroughfares, or span across both 'sanctuary areas' and thoroughfares, if the latter are properly designed; a 'pedestrian shed', by contrast, can overlap with neighborhoods and with other pedestrian sheds. We propose a '400 meter rule', a surprisingly small maximum spacing of main thoroughfares that empirical observation shows that traditional, pedestrian-governed urban fabric has always tended to obey, for reasons that are likely to have to do with the self-organizing logic of pedestrian movement and social activity. In so doing, we advance a more fine-grained, permeable, potentially lower-carbon model and illustrate its advantages with several historic and modern examples.