Street Access To Neighboring

FIGURE 6.3 A site analysis should be conducted before a subdivision design type is selected and any subdivision layout created. In general, the analysis should include a topographic analysis, identifying areas of steep slopes; the locations of hilltops, ridges, and scenic overviews; an analysis drainage patterns; a vegetation analysis; a delineation of soil types and characteristics; an identification of water bodies, wetlands, woodlands, and flood hazard areas; an identification of the boundaries and characteristics of environmental corridors; and identification of structures having historic or other cultural value. The site analysis should also identify the classification of existing streets and highways adjacent to the development parcel, and the location of desirable and undesirable points of entrance to and exit from the site.

FIGURE 6.3 A site analysis should be conducted before a subdivision design type is selected and any subdivision layout created. In general, the analysis should include a topographic analysis, identifying areas of steep slopes; the locations of hilltops, ridges, and scenic overviews; an analysis drainage patterns; a vegetation analysis; a delineation of soil types and characteristics; an identification of water bodies, wetlands, woodlands, and flood hazard areas; an identification of the boundaries and characteristics of environmental corridors; and identification of structures having historic or other cultural value. The site analysis should also identify the classification of existing streets and highways adjacent to the development parcel, and the location of desirable and undesirable points of entrance to and exit from the site.

It is important to note that the subdivision layouts shown were prepared assuming that a local comprehensive plan was available to specify the type and density of land use to be accommodated, and assuming further that the comprehensive plan concerned did not include a neighborhood unit development plan or platting layout. The comprehensive plan was assumed to set forth a specific overall residential development density for the 80-acre tract concerned; and identified the arterial street pattern and related right-of-way widths. It is particularly important that the comprehensive plan set forth a specific maximum density of development in order to ensure that the public infrastructure and services are adequate and appropriate to serve development within the community.

Table 6.1 sets forth a comparative analysis of the three designs shown. To facilitate the comparison, a common development density of approximately two dwelling units per gross acre—or a total of 160 dwelling units for the tract—was assumed. In an actual design setting, the density used might be varied between the three designs, as long as it did not exceed the maximum density specified in the comprehensive plan.

Curvilinear Layout

The curvilinear subdivision design type has been the most common type of subdivision developed in the United States in the post-World War II era. The curvilinear subdivision has generally been designed to provide relatively large lots that permit larger homes with more private open space on each lot. The design of a curvilinear subdivision facilitates adjustment to fit the topography and minimize required grading while providing good drainage and efficient sewerage.

The urban cluster subdivision design type maintains a significant portion of a site in common open space bye minimizing individual lot sizes while maintaining the required overall density of development. The cluster subdivision, whether located in an urban or rural area, can effectively protect environmentally sensitive areas by maintaining such areas in open space, while concentrating lot into small groups or "clusters." Each residential lot in this subdivision layout has direct access to the common open space, and the use of cul-de-sac streets enhances residential quiet, privacy, seclusion, and safety.

Small Lot Subdivision LayoutsRadial Design Subdivision

The new urbanism design type is based on urban development patterns of the past, with consideration given to the open space concerns of the present. New urbanism subdivisions attempt to provide a central public common that is surrounded by residential lots, or, in some larger subdivisions, may provide for a neighborhood business or civic center. Residential lots are often double-fronted, with one face to the street and one to a utility corridor or alley running behind the lots, where garages are located. The new urbanism design is better suited to relatively level sites where the desired density of development is easily achieved without substantial grading. The grid street pattern provides more connections to adjoining tracts, and often results in shorter walking and bicycling distances, but may also result in higher volumes of through traffic using subdivision streets, increasing noise levels and safety concerns.

FIGURE 6.4 (a) Conventional curvilinear subdivision layout, (b) urban cluster subdivision layout, and (c) new urbanism subdivision layout.

The curvilinear subdivision design is intended to maximize the use of developable land for lots, while limiting open space to environmentally sensitive areas that are to be protected based on recommendations contained within the local comprehensive plan, or restrictions contained in the local zoning ordinance. The design process focuses on the street layout, considering desirable block lengths and widths, and desirable lot configuration. The curvilinear street pattern is fitted to the topography of the site to minimize earth work, achieve good drainage, and facilitate the provision of gravity drainage sanitary sewerage. The design provides

TABLE 6.1 Comparative Analysis of Subdivision Designs

Subdivision Type

Conventional Curvilinear

Urban Cluster

New Urbanism

Lot information Number of lots Average lot size Average lot width Average lot depth Total area within lots Percent of site area within lots Street information Total street length Total area within street rights-of-way Percent of site area within street rights-of-way Open space information Total area within open space

Percent of site area in open space Grading information Volume of earth work

160 lots

17,002 square feet 95 feet 179 feet 62.5 acres 77.0 percent

160 lots

6,028 square feet 60 feet 101 feet 22.1 acres 27.3 percent

160 lots

7,325 square feet 62 feet 118 feet 27.3 acres 33.2 percent

10,363 lineal feet 10,730 lineal feet 17,154 lineal feet 14.7 acres 12.8 acres 16.6 acres

18.1 percent

4.0 acres 4.9 percent

15.8 percent

46.4 acres 57.2 percent

20.7 percent

37.4 acres 46.1 percent

51,000 cubic yards 93,000 cubic yards 130,000 cubic yards relatively large lots that can accommodate larger homes with more private open space on each lot. Grading for a curvilinear subdivision is typically minimum and preserves much of the existing topography and vegetation. Disadvantages of a curvilinear subdivision design may include higher costs of infrastructure per lot due to the relatively large size of the individual lots. Also, such subdivisions usually provide smaller areas of land for public or common open space. The urban cluster design type maximizes the provisions of common open space by minimizing individual lot size while maintaining the required overall density of development.

A cluster subdivision can be used to effectively protect environmentally sensitive areas by maintaining such areas in open space, while concentrating lots into smaller groups or clusters. The urban cluster design provides relatively small lots fronting on a public street, with all lots being adjacent to common open space. This design type usually results in the lowest percentage of combined area devoted to streets and lots, thus providing the greatest amount of common open space. The design is somewhat more conducive to use with more level sites, where terracing of the small lots making up the clusters is not necessary.

The advantages of a cluster subdivision design include protection of natural resource features and the opportunity to maintain a significant portion of the site in common open space. This can provide attractive recreational opportunities for residents of the subdivision; maintain the scenic beauty and biodiversity of the site; and decrease the amount of impervious surface area. If the subdivision is laid out in a way that concentrates lots on a limited portion of the site, the cost of providing infrastructure may be less than that required for a curvilinear type subdivision. Disadvantages of cluster subdivisions may include relatively small lots that require careful design of lot arrangements and building placement in order to provide privacy for the individual residences. The control of architectural design may be needed in urban cluster subdivisions due to the relatively small lots and the proximity of the clustered homes. A homeowners associations is required to own and maintain the common open space and related facilities. Provision must be made, however, for public assumption of the maintenance if the homeowners association fails to meet its responsibilities, with the associated costs assessed back to the lots in the subdivision.

Because the subdivision design shown in Figure 6.4 is assumed to be located in an area provided with a full range of urban services, including public sewer and water supply, the design presented is for an urban cluster. Cluster subdivisions may also be located in rural areas, in which case they are often known as conservation subdivisions. Although such rural locations may, in the absence of a comprehensive plan, raise concerns about urban sprawl, the cluster design can be effectively used to maintain agricultural uses in an area, preserve and protect wildlife habitat, and provide outdoor recreational opportunities. A rural cluster—or conservation—design subdivision is shown in Figure 6.5. Such rural conservation subdivisions may also facilitate the provision of sanitary waste disposal and the provision of water supply in rural areas not served by centralized sewerage and water supply systems. This may be illustrated by a simple example. If a rural area is planned and zoned to permit residential use at a gross density not to exceed one dwelling unit per 5 acres, a 100-acre tract might be subdivided into twenty 5-acre lots, using a curvilinear design each with its own on-site sewage treatment and disposal system—usually a septic tank system—and with its own on-site well for water supply. If the development were to use a cluster design with a net lot size of 1 acre, while maintaining the overall density of the tract at 20 dwelling units, approximately 24 acres of the site would be used for residential development, including supporting streets, and 76 acres would be maintained in common open space. This would permit the provision of a sanitary sewage collection and conveyance system tributary to an integrated anaerobic or aerobic treatment unit and an integrated soil drainage tile disposal facility located on a carefully selected part of the open space area. This approach could even be used with a centralized holding tank, the accumulated wastes in the tank being tracked to a recovery wastewater treatment plant. Similarly, water mains properly sized, valved, and equipped with fire hydrants and served by a common well could be provided. In this way occupants of the subdivision would perceive no difference between the sanitation services provided and such services provided in

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