Land Use Density and Clustering
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TDM
Encyclopedia
Victoria Transport Policy Institute
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Updated
25 April 2008
This chapter describes how increased density (number of people or employees located in an area) and clustering (locating related activities close together) tend to reduce travel distances and improve travel options. Density and clustering support and are supported by many TDM strategies.
Description
Density refers to the number of people or jobs in a given area. Clustering (also called Compact Development) refers to Land Use patterns in which related activities are located close together, usually within convenient walking distance. Clustering improves Accessibility by reducing travel distances and improving Transportation Options. It is an important part of land use management strategies including Access Management, Location Efficient Development, New Urbanism, Smart Growth and Transit Oriented Development.
Typical Densities
Definition
|
Population Density |
Typical Housing |
|
Rural |
Less than 0.5 residents per acre. |
Houses on large lots (>5 acres) |
|
Low-Density – Suburban |
0.5-5 residents per acre. |
Houses on lots 0.5 to 5 acres |
|
Mid-Density – Suburban Cluster or Urban |
5-12 residents per acre. |
Houses on lots 0.2 to 0.5 acres (2-5 houses per acre) |
|
Compact – Urban |
More than 12 residents per acre. |
Various combinations of detached houses on small lots,
duplexes, townhouses, and low-rise (under 4 story) apartments and condominiums. |
|
High-density |
More than 20 residents per acre |
Low- and high-rise (more than 4 story) apartments and
condominiums. |
Density and Clustering are somewhat different concepts. Density refers to the number of people or jobs in an area, while Clustering to the location and mix of activities in an area. For example, simply increasing population densities in a residential-only area may do less to improve accessibility clustering destinations such as schools and shops in the center of the development. Rural and suburban areas have low densities, but common destinations such as schools, shops and other public services can be clustered in villages and towns. This increases accessibility by making it easier to run several errands at the same time, increases opportunities to interact with neighbors, and creates transportation nodes (rideshare stops, bus stops, etc.).
Clustering is more effective at reducing automobile use if it includes other TDM strategies. For example, automobile commuting tends to decline if employment centers are clustered with shops, restaurants and daycare centers (destinations that employees want to visit during their breaks), and if such areas have Pedestrian Improvements, a Rideshare program, Transit Improvements and Commute Trip Reduction programs. Put another way, other Commute Trip Reduction strategies tend to be more effective if worksites are clustered.
Density and Clustering can occur at various scales and in many different ways. Office buildings, campuses, shopping malls, commercial districts, towns and cities are examples of clustering. Density and Clustering at a neighborhood level (areas of less than a mile in diameter) with good pedestrian conditions creates multi-modal centers (also called urban villages, transit villages or walkable centers), which are suitable for walking and transit.
Clustering is illustrated in Figure 1.
- This shows a
conventional suburban development with buildings surrounded by parking and
isolated from each other. There are often no paths connecting the
buildings or sidewalks along the streets. Only automobile transportation
can effectively serve such destinations.
- This shows the same
buildings sited so they are clustered together and oriented toward the
street, with main entranceways that connect directly to the sidewalk
rather than being located behind parking. This creates convenient
pedestrian access between them, for example, making it easier for an
employee in an office to visit an adjacent building with a childcare
center or shop, and for employees from two different buildings to
rideshare.
This type of clustering also facilitates Shared Parking, particularly if the buildings have
different types of land uses with different peak demands. For example, if two
of the buildings are offices with peak parking demand during weekdays, another
is a restaurant with peak demand during the evenings, and the fourth is a church
with peak demands weekend mornings, they can share parking and reduce total
parking requirements, which allows even greater clustering.
- This shows eight
buildings clustered around a park. As the cluster increases in size the
efficiency of pedestrian improvements, rideshare and public transit
service and other TDM strategies also increase, due to economies of scale.
- This shows the
eight-office building integrated into a park or campus, creating more
convenient and attractive pedestrian connections between the buildings,
further improving access and supporting transportation alternatives. It
also creates a more enjoyable environment for employees and visitors
compared with isolated buildings surrounded by parking.
Figure 1 Clustering
At the Building or Block Scale
Clustering can be implemented in urban, suburban or rural conditions, either incrementally or as part of a master-planned development. Clusters can range from just a few small buildings (for example, a restaurant, a medical office and a single retail store) to a large commercial center with hundreds of businesses.
Examples of Density
Here are some examples to provide a feel for various types of densities.
A typical apartment has 800 to 1,500 square feet of floor area. If a three-story apartment building has twelve units averaging 1,200 square feet each, its footprint (the area of land the building actually covers) is about 5,000 square feet (4 x 1,200, plus a little extra for hallways). Apartments typically have 1.0 to 3.0 occupants, depending on the number of bedrooms.
A typical modern house has 2,000 to 3,000 square feet of floor area. If a 2,500 square foot house is two stories, it will have a 1,250 square foot footprint. Houses typically have 2.0 to 4.0 occupants, depending on the number of bedrooms.
In addition to the building (apartment or house) itself, a development may also include sidewalks, driveways, parking lots or garages, porches, decks, outbuildings and greenspace (lawns and gardens). The portion of a site of land that is covered with pavement or buildings (together called impervious surface) is called the coverage, measured as a percent of the total land area.
A typical city lot is 50 feet wide by 100 feet deep, totalling 5,000 square feet, or about one-eighth of an acre. If such a lot contains 2,500 square foot single-story house, a 500 square foot two-car garage, a 40-foot driveway, a 5-foot sidewalk, the coverage will total 3,570 square feet, or 71% of the land area, leaving just 1,430 square feet for greenspace.
However, if the same size house is built in two stories, and the garage is incorporated into the house or accessed from a back alley, minimizing the driveway length, the lot coverage declines to 2,250 square feet, leaving about half of the lot as greenspace. With single-family housing, a setback of 5 to 8 feet is needed between each house and the lot line. Sharing walls (building a duplex or townhouse) eliminates the need for setbacks, allowing the narrower lots.
With 3.0 average occupants per house, density averages 24 residents per acre for 5,000 square foot lots, about 33 residents per acre in duplexes on 4,000 square foot lots, and 43 residents per acre in townhouses on 3,000 square foot lots. Density in single-family housing can be increased by adding secondary suites (also called granny flats), that is, a small rental unit incorporated into the house or in an outbuilding.
A four-story, low-rise apartment or condominium with 16 total units has a footprint of about 4,500 square feet. If located on a double lot (100’ x 100’), half the parcel may be used for a combination of surface parking and greenspace. These will typically have 1.2 occupants per unit, about 20 total occupants per building or 80 residents per acre.
These represent net densities. Gross densities over the entire area are lower to account for land devoted to non-residential uses such as commercial and industrial facilities, schools, parks and recreational facilities, and undeveloped land.
As a general rule of thumb, 4-7 dwelling units per acre are required to create demand for “basic” bus transit service (20-40 buses per day), 6-15 units per acre are required to create demand for “frequent” bus transit service, 9 units per acre are needed to create demand for light rail transit, and 12 units per acre are needed to create demand for rapid transit (Transit Evaluation). However, these density requirements vary depending on additional factors, including the size of the Downtown and other commercial areas served by transit, Parking Management practices (such as whether parking is priced), and whether there are Commute Trip Reduction programs at worksites.
New Urbanism and Transit Oriented Development involve clustering developments into walkable neighborhoods of 0.5 to 1.0 mile in diameter (a typical walking catchment area for commercial centers and transit stations), an area of 125 to 500 acres. Ideally, this includes a mixture of higher-density multi-family and small-lot single-family. For example, if a transit village has 200 total acres, of which 150 are devoted to residential, 25 acres are 4-story apartments, 25 acres are townhouses, and 100 acres are single-family houses on 5,000 square foot lots. The table below summarizes the total residents in such a community.
Table 1 Total Residents Within A Walkable Area
|
Type |
Units Per |
Occupants Per Unit |
Occupants Per |
Acres |
Total |
|
Multi-Family |
80 |
1.2 |
96 |
25 |
2,400 |
|
Townhouses |
43 |
2.0 |
86 |
25 |
2,150 |
|
Single-Family |
8 |
3.0 |
24 |
100 |
2,400 |
|
|
|
|
|
|
6,950 |
Criticism of Density and Clustering
Some people have a negative attitude about density and clustering. They believe that it is harmful to individuals and society, and that consumers always prefer lower-density development patterns (Moretti, 1999). However, many consumers value clustered development if it is well designed, affordable, increases accessibility, and incorporates other valued amenities such as personal security and good schools. One survey (NHBA, 1999) found that 83% of consumers prefer suburban housing, but the features respondents value most are neighborhood security, quality schools and neighborhood quality. This suggests that some households would choose higher density, multi-modal locations if they had such amenities.
Demand for New Urbanist communities, loft apartments and urban infill is strong, provided that they offer personal security, school quality and prestige comparable to suburbs. A study by Eppli and Tu (2000) found that homes in New Urbanist communities sold for an average of $20,189 more than otherwise comparable homes in more conventional communities, an 11% increase in value. One survey found that 43% of homebuyers who currently choose rural and suburban locations are good candidates for higher density, traditional neighborhood developments (Heart and Biringer, 2000). Similarly, a survey of the Puget Sound region housing market found that although the majority of respondents prefer a detached home, most care more about the quality of their neighborhood and owning their own home than about housing type, and more than 90% would willingly trade low-density housing for a medium- or high-density home if it had other desirable features (Decisions Data, 1994).
Many families already choose relatively higher-density
housing, but it is not clustered with other common destinations and so does not
increase accessibility. For example Moudon and Hess (2000) found that 40% of
residents in suburban areas of
How It Is Implemented
Clustering is usually implemented by local governments and developers. Clustering often requires changes to development policies and practices that allow and encourage higher densities and more flexible parking requirements.
Special effort is often required to increase density and clustering. Incremental increases can be achieved by expanding existing buildings, for example, by adding rooms and secondary suites. Urban redevelopment, such as conversion of commercial buildings to residential, or redevelopment of old industrial areas, can be an opportunity to increase density and land use mix.
Because existing residents often oppose density increases, special care may be required to provide address concerns and provide incentives. For example, developers may be required to help fund community amenities, and Residential Parking Permits may be applied to insure that existing residents have access to onstreet parking spaces. Many of the objections to increased density can be addressed through good design and mitigation (New Urbanism).
Travel Impacts
Density and clustering tend to reduce per capita automobile travel (Land Use Impacts on Transportation) by reducing travel distances to common destinations and by improving transportation Options, particularly walking, ridesharing and public transit by increasing the demand for such services (Kuzmyak and Pratt, 2003).
In an extensive review of studies
Density at both origins and destinations affect travel behavior. Work trips and shopping trips are affected by population and employment densities. One study found that increasing urban residential population density to 40 people per acre increased transit use from about 2% to 7%, while increasing densities in commercial centers to 100 employees per acre resulted in an additional 4% increase in transit use, to an 11% total mode share (Frank and Pivo, 1995). Barnes and Davis (2001) also found that densities at employment centers are particularly important for encouraging transit and ridesharing.
Aesthetically-pleasing urban character and amenities at worksites, such as shops and restaurants within walking distance, can reduce errand trips and increase transit and rideshare use, because without these, employees may feel the need to have a car to run errands during breaks (Cambridge Systematics, 1994). One study found that the presence of worksite amenities such as banking services, on-site childcare, a cafeteria, a gym, and postal services could reduce average weekday car travel by 14%, due to a combination of reduced errand trips and increased ridesharing (Davidson, 1994).
Table 2 Travel Impact Summary
|
Objective |
Rating |
Comments |
|
Reduces total traffic. |
3 |
Reduces travel distances and supports alternative modes. |
|
Reduces peak period traffic. |
3 |
" |
|
Shifts peak to off-peak periods. |
0 |
|
|
Shifts automobile travel to alternative modes. |
3 |
Supports alternative modes. |
|
Improves access, reduces the need for travel. |
3 |
|
|
Increased ridesharing. |
2 |
|
|
Increased public transit. |
3 |
|
|
Increased cycling. |
2 |
|
|
Increased walking. |
3 |
|
|
Increased Telework. |
0 |
|
|
Reduced freight traffic. |
2 |
|
Rating from 3 (very
beneficial) to –3 (very harmful). A 0 indicates no impact or mixed impacts.
Benefits and Costs
Density and clustering can provide a variety of economic, social and environmental benefits (Forman, et al, 2003, p. 332; Litman, 2004).
Density and clustering improve Accessibility (by reducing the average distance between common destinations) and Transportation Options (walking improvements and transit services are tend to be most feasible and cost effective with clustered land use), encourage use of alternative modes, and reduce per capita automobile costs and impervious surface. Clustering reduces the costs of providing public infrastructure and services such as roads, utility lines, policing and schools (Land Use Evaluation). This can help reduce regional traffic congestion, road and parking facility costs, consumer transportation costs, crashes, energy consumption, pollution emissions and urban sprawl. It protects greenspace (NEW, 2004). These benefits tend to be greatest if complementary land uses are mixed and supported by other TDM and land use management strategies, such as Smart Growth.
Density and clustering tend to provide agglomeration benefits, which consist of the accessibility and network effects that increase economic efficiency and productivity (Coffey and Shearmur, 1997). Published research indicates that doubling urban population density produces approximately 6% increase in productivity (Haughwout, 2000; Harris and Ioannides, 2000). This explains why cities and commercial centers develop and are so important for economic development: clustering of common destinations reduces the costs of activities that require frequent interactions. These benefits can be very large, as indicated by the much higher land values that occur in major commercial centers.
Clustering can increase Livability if it is implemented in conjunction with pedestrian and cycling improvements, traffic calming and other Streetscape enhancements. It can increase opportunities for neighborhood interaction and community cohesion. However, clustering can also increase exposure to noise and air pollution.
Density and clustering increase some costs, including some types of infrastructure costs (such as some utility costs), local traffic congestion, although regional traffic and pollution emissions tend to decline if clustering reduces total vehicle use. Although clustering may increase local traffic congestion, and therefore reduce average vehicle travel speeds, it tends to bring common destinations closer together, so total travel costs are reduced. Reduced automobile use and improved opportunities for Parking Management can reduce road and parking facility costs.
|
Do
Clustering and Density Cause Social Problems? Many
higher-density urban neighborhoods have higher rates of social problems
(crime and poverty) than lower-density suburban neighborhoods. Some people
assume that this indicates that clustering and density cause social
problems. But, although studies find an association between crowding
(density measured in residents per residential room, an indication of poverty)
and social problems, there is no such association with density
measured in residents per acre (1000 Friends of Oregon, 1999). For example,
there are also high crime rates in some rural areas with low densities but
high poverty, and therefore crowding. This
indicates that the association between density and social problems reflects
the tendency of distressed households to concentrate in higher-density, urban
neighborhoods, not that higher-density development causes social problems.
This suggests that clustering does not increase social problems, and urban
infill could reduce such problems if distressed households become less
segregated (Litman, 2001). |
Density and clustering tend to reduce the amount of greenspace in a particular area, although they can increase total regional greenspace by reducing per capita road, parking and building area requirements. Most of these negative impacts can be reduced with appropriate design features (such as noise insulation and carefully located parks), but these mitigation activities may also involve additional costs.
Table 3 Benefit Summary
|
Objective |
Rating |
Comments |
|
Congestion Reduction |
1 |
Can increase local congestion but reduces regional congestion. |
|
Road & Parking Savings |
2 |
Reduces road and parking requirements. |
|
Consumer Savings |
2 |
|
|
Transport Choice |
3 |
|
|
Road Safety |
2 |
|
|
Environmental Protection |
2 |
|
|
Efficient Land Use |
3 |
|
|
Community Livability |
1 |
|
Rating from 3 (very
beneficial) to –3 (very harmful). A 0 indicates no impact or mixed impacts.
Equity Impacts
Density and clustering can have a variety of equity impacts. Changes to development policies and practices may benefit some people and disadvantage others. In particular, it can add value to urban land values and keep urban fringe land from appreciating in value as quickly as would occur otherwise. Policies that support clustering often involve reducing cross-subsidies for low-density, urban-fringe development (Litman, 1999). Policies that reduce residential parking requirements and improve transportation choice can be progressive (Location Efficient Development). Clustering can be particularly beneficial to people who are transportation disadvantaged, and improve Basic Mobility.
Table 4 Equity Summary
|
Criteria |
Rating |
Comments |
|
Treats everybody equally. |
0 |
|
|
Individuals bear the costs they impose. |
1 |
|
|
Progressive with respect to income. |
1 |
|
|
Benefits transportation disadvantaged. |
3 |
|
|
Improves basic mobility. |
3 |
|
Rating from 3 (very
beneficial) to –3 (very harmful). A 0 indicates no impact or mixed impacts.
Applications
Density and clustering can be applied under most geographic conditions, although design, scale and magnitude may differ. For example, a rural cluster may be quite different than a suburban or urban cluster. Federal and state governments can encourage clustering in their own facilities and transportation investments. Regional and municipal governments can encourage clustering with supportive transportation and land use policies. Developers, businesses and campuses can implement clustering directly.
Table 5 Application Summary
|
Geographic |
Rating |
Organization |
Rating |
|
Large urban region. |
3 |
Federal government. |
1 |
|
High-density, urban. |
3 |
State/provincial government. |
2 |
|
Medium-density, urban/suburban. |
3 |
Regional government. |
3 |
|
Town. |
3 |
Municipal/local government. |
3 |
|
Low-density, rural. |
2 |
Business Associations/TMA. |
3 |
|
Commercial center. |
3 |
Individual business. |
2 |
|
Residential neighborhood. |
3 |
Developer. |
3 |
|
Resort/recreation area. |
3 |
Neighborhood association. |
2 |
|
College/university communities. |
3 |
Campus. |
3 |
Ratings range from 0 (not
appropriate) to 3 (very appropriate).
Category
Clustering is a Land Use Management strategy.
Relationships With Other TDM Strategies
Density and clustering support and are supported by Transportation Demand Management. They are an important component of Access Management, Location Efficient Development, New Urbanism and Smart Growth. Clustering tends to facilitate Pedestrian Improvements, and since most transit trips include walking links, it is important for efficient Transit. If located near transit stations or corridors it results in Transit Oriented Development. Clustering becomes more feasibility with Parking Management, particularly Shared Parking, to reduce the amount of land needed for parking facilities around buildings. The efficiency of Transportation Management Associations, Ridesharing and other Commute Trip Reduction strategies increases if worksites are clustered together.
Stakeholders
Major stakeholders for implementing clustered development include local officials, developers, existing nearby residents, future residents and employees and transit agencies.
Barriers To Implementation
Existing development policies and practices often favor lower-density, dispersed development. Transportation planning practices often favor road and parking facility investments which lead to lower-density, automobile-oriented land use patterns, over pedestrian and transit investments that lead to more clustered land use.
Best Practices
· Public agencies should encourage clustering in their land use and transportation policies, including the location and design of their own facilities.
· Existing policies that discourage land use clustering (such as single-use zoning, excessive building setbacks and parking requirements) should be eliminated or made more flexible.
· Clusters should include an appropriate mix of activities. For example, employment centers should also include shops and services that workers frequent during their breaks, and residential centers should include schools, shops and public services.
· Special care should be taken to create convenient and attractive walking conditions, and clusters should include bicycle, ridesharing and transit improvements as appropriate.
· Clustering should be implemented as part of overall land use management strategies such as
· Clustering should be implemented with other TDM strategies that encourage vehicle travel reductions and shifts to alternative modes.