Territorial permeability analysis
Introduction
permeability or connectivity describes the extent to which urban forms allow (or restrict) the movement of people or vehicles in different directions. The terms are often used interchangeably, although distinct definitions also exist (see below). Permeability is generally considered a positive attribute of an urban design, as it allows ease of movement and avoids dividing neighborhoods. Urban forms that lack permeability, for example those cut by arterial roads, or with many long cul-de-sacs, are considered to discourage movement on foot and encourage longer trips by car. There is some empirical research evidence to support this view.[1].
Permeability is a central principle of New Urbanism, which favors urban designs based on the traditional street grid (particularly in a North American context).
Reservations
There are two main reservations related to permeability. The first relates to property crimes. Although the topic is disputed, there is some research evidence to suggest that permeability may be positively correlated with crimes such as burglary. New research has expanded the discussion on this disputed topic. A recent study conducted an extensive spatial analysis and correlated several social, site plan and building factors with crime frequencies and identified nuances in contrasting positions. The study analyzed, among others: a) types of housing, b) unit density (site density), c) movement on the street, d) dead ends or grids and e) the permeability of a residential area. Among their conclusions are, respectively, that: a) apartments are always safer than houses and the wealth of the inhabitants matters; b) density is generally beneficial but more at ground level; a greater effect than being on a cul-de-sac or being on a through street. It also reestablished that simple, linear cul-de-sacs with a good number of dwellings linked by streets tend to be safe. Regarding permeability, it suggests that residential areas should be permeable enough to allow movement in all directions, but no more.
The second reservation concerns the effects of permeability for private motor vehicles. Melia (2012)[5] proposed the terms "unfiltered permeability" and "filtered permeability" to distinguish between the two approaches.
Unfiltered permeability is the view supported by New Urbanists that urban designs should follow "traditional" or mixed-use streets, where pedestrians, cyclists, and motor vehicles follow the same routes. The main advantage claimed for this approach is that it "leads to a more uniform distribution of motorized traffic throughout the area and therefore avoids the need for distribution roads".[6].