Links to the 'context', 'choice' and 'custodianship' components of the seven Cs (Urban Design Protocol).
Density is the concentration of population and activity in an urban area. The most vibrant, diverse and exciting part of a city is often its centre. Density is at its highest at the centre, where there is the greatest range of people, buildings, public spaces, facilities, services and choices. Here, people can most easily exchange ideas and goods and services, both for business and for pleasure.
Urban design that promotes a higher density of buildings and public spaces (in conjunction with other conditions such as mixed use, good building design and adequate open space) can:
High urban density has potential costs in the form of congestion, noise and localised pollution. But low density development - urban sprawl - can also be costly, reflecting the higher economic and environmental costs of mobility. Much of the international research investigates this tension, examining the kinds of value (both private and public) created by dense versus less dense cities.
There is clear evidence about some of the savings offered by high urban density. Market demand leads to high land prices in dense city centres, and provides an impetus to economise on land resources. There are also infrastructure savings (eg, on roads, sewerage, schools), although these costs can rise again in cities with very high densities. High density also leads to energy savings, with significant reductions in petrol use and car dependence - especially in cities with multiple compact centres.
More general economic benefits of high urban density include enhanced ability to attract and concentrate businesses that are not space-intensive, such as knowledge-based industries, and to offer people better access to job opportunities.
Overall pollution from vehicle emissions can be less in dense cities (although there may be localised areas of higher pollution), providing development is carefully located and directed. Infill development is also shown to create less runoff and water pollution.
Urban density and green space are sometimes suggested to be incompatible. It is certainly clear that green space in the city contributes to public health, quality of life and biodiversity. This value is reflected in property prices around iconic green spaces. But it is less clear how much green space is needed to generate these benefits. Incorporating large tracts of green space into the city can create problems elsewhere. It may push development to the periphery where it changes the nature of adjoining rural areas, and generates more traffic and raises the costs of doing business in the wider urban area.
Cities in which compact centres are interspersed with green areas may offer the best solution to these problems.
There must always be some degree of trade-off between density and city greenery. Both the Urban Task Force in the United Kingdom and the United States Environmental Protection Agency, suggest a way through this challenge: the polycentric urban form (or cluster zoning) with high-density areas interspersed with green wedges or areas. Auckland's node-focused growth strategy has adopted this concept.
Opinions vary about the benefits of higher density: a place that attracts some people with its vitality and 'buzz' may deter others. High density city centres can provide a greater range of housing and lifestyle choices. There is also evidence that denser urban areas have a strong sense of community, connectedness and vitality - largely because people are in closer contact with each other. But there may be a point at which this ceases to happen. In very high density areas, people may in fact withdraw from others and seek privacy.
High urban density can be beneficial for public health because it encourages more walking and cycling. High density can also make public transport - which involves more walking than private vehicle use - more viable.
Although there is strong evidence about some of the benefits inherent in high urban density, it is clear that density alone does not deliver benefits unless other important design issues are addressed too. Successful intensification and higher density in cities requires good design that also meets other needs - for instance, adequate open space and pedestrian friendly streets.
The UK Urban Task Force (1999) says there is a sound case for greater urban density: "research has shown that real land economy gains are being achieved from increasing densities... [H]igher densities allow a greater number of public amenities and transport facilities to be located within walking distance, thus reducing the need for the car, and contributing to urban sustainability".
An Italian study showed that sprawl tends to raise transport costs. "Diffused, sprawling development" is associated with higher economic and environmental costs of mobility, and with low use of public transport. Density appears to have an impact significantly through influencing the average trip time of public transport.
Camagni et al, 2002
"... real land economy gains are significant" when housing concentrations are increased from low to medium densities (eg, 35-40 dwellings per hectare), according to a study by the Urban and Economic Development Group (URBED) in London, in 2000.
As density increased from 10 units per hectare for conventional development to 25 units per hectare, infrastructure costs per dwelling fell by 55%.
When they are carefully located and directed, concentrated forms of city development such as compact city, multi-nodal or edge city can lead to reduced pollutant emissions relative to business-as-usual, according to a 2000 study in the UK. ... "The compact city emerges as the most fuel efficient of all urban forms, with 43% less fuel consumption than 'business-as-usual' development."
The US Environmental Protection Agency has found that the most compact patterns of development result in less vehicle travel than dispersed patterns. This was borne out in a 1994 study of 28 Californian neighbourhoods by Holtzclaw, which found that "... a doubling of residential density levels produced 25-30 percent fewer miles driven per household".
Frank et al, 2003
The East Hills Development near Napier is an example of a rural cluster development that, while still car-dependent, nevertheless provides an alternative to large lot rural/residential subdivision. Relatively small house sites are placed strategically across the 76 hectare site to maintain privacy, benefit from views and blend in with the natural landscape. The balance of the land is designated as reserve, to be owned and managed by an owners' association. An extensive planting programme protects the local environment and enhances habitats.