a narrow street down the middle. Many old world cities have grown by accretion over time rather than being planned from the outset. For this reason, a regular pattern of even, square or rectangular city blocks is not so common among European cities, for example. An exception is represented by those cities that were founded as Roman military settlements, and that often preserve the original grid layout around two main orthogonal axes. One notable example is Turin, Italy. Following the example of Philadelphia, New York City adopted the Commissioners' Plan of 1811 for a more extensive grid plan. By the middle of the 20th century, the adoption of the uniform, rectilinear block subsided almost completely, and different layouts prevailed, with random sized and either curvilinear or non-orthogonal blocks and corresponding street patterns. Structure variations The concept of city block can be generalized as a superblock or sub-block. Superblock A superblock, or super-block, is an area of urban land that is bounded by arterial roads and the size of multiple typically sized city blocks. Within the superblock, the local road network, if any, is designed to serve only local needs. Definitions and typologies Within the broad concept of a superblock, various typologies emerge based primarily on the internal road networks within the superblock, their historical context, and whether they are auto-centric or pedestrian-centric. The context in which superblocks are being studied or conceived gives rise to varying definitions. An internal road network characterised by culs-de-sac is typical of auto-centric suburban development primarily in Western countries throughout the 20th century. The Oxford Dictionary of Architecture and Landscape Architecture's definition is rooted within this typically suburban conception:“Area containing residential accommodation, shops, schools, offices, etc., with public open space (e.g. a green), surrounded by roads and penetrated by cul-de-sac service-roads. It is linked to other super-blocks and a town centre by means of paths over or under the roads (e.g. in Radburn planning).”Though the aim of such superblocks is generally to minimise traffic within the superblock by directing it to arterial roads, the effect in many cases has been to entrench automobile dependence by limiting pedestrian and cyclist permeability. Superblocks can also contain an orthogonal internal road network, including those based on a grid plan or quasi-grid plan. That typology is prevalent in Japan and China, for example. Chen defines the supergrid and superblock urban morphology in that context as follows:“The Supergrid is a