THEME 4 - DENSITY
Density refers to the intensity of use of land. In UK planning practice, density is generally measured in dwellings per net hectare (dph), where the area includes developable residential land1.
There has been much previous work on the density and sustainability topic. Newman and Kenworthy (1989, 1999) famously developed the relationship between higher density and lower energy consumption for major world cities. The ways of achieving higher residential densities in design quality terms has been examined by many authors, including Rogers (1997), and has been influential on the urban renaissance movement.
A number of principles can be derived, with a focus on raising the density of development, particularly around public transport nodes:
- Transport energy consumption and CO2 emissions are generally lower at higher densities.
- Higher densities lead to greater scope for viable public transport services.
- Density can be an important factor in reducing car use in terms of both mode share and distance travelled.
There is continued discussion as to appropriate density levels. PPS3 (DCLG, 2006) advises an indicative minimum of 30 dph. Much higher densities can be achieved in many areas, up to 50-100 dph depending on context, and even 100-200 dph or more around important public transport interchanges. A set of density ranges can be developed for each local area reflecting contextual issues.
1. To what extent does density influence the demand for travel?
Various research studies have shown that density has an influence on travel behaviour, even when other urban structure variables, socio-economic and attitudes are accounted for. The main association appears to be with travel distance - at lower densities, travel distances tend to be longer. Car mode share also tends to be higher, but there is often less significance to the relationship. Trip rates remain similar across density ranges.
Data trends from the UK show that areas with higher densities tend to have shorter annual travel distances and lower car mode shares than average. Distance by public transport increases with density, particularly over 30 persons per hectare.
2. What range of densities best support viable public transport?
Minimum densities for viable public transport depend on the type of public transport, the local urban and cultural context, and the availability of alternative modes. It is usually accepted that people are willing to walk up to 400-800m to access public transport (with longer distances in larger urban areas). This distance is often used as a catchment area for development around public transport interchanges. The range of public transport options – including rail, light rapid transit, tram-train, ultra light rapid transit, guided bus, bus, demand responsive and para transport services – mean that a well-planned service can be financially viable in many areas. Everything else equal, higher densities support higher frequency and specification in services and help to justify dedicated rights-of-way.
Rudlin and Falk (2000), CABE (2005) and others give a range of density gradients (see evidence below) and indicative minimum densities for a bus service at 25 units/ha and for a tram service at 60 units/ha. Suitable densities for transport services are however contextually specific – depending on a range of issues, including local car ownership levels, mode split, trip distribution and proposed public transport service specification (including capacity, frequency and cost) (Halcrow, 2007).
“Local Planning Authorities may wish to set out a range of densities across the plan area rather than one broad density range. 30 dwellings per hectare (dph) net should be used as a national indicative minimum to guide policy development and decision-making, until local density policies are in place. Where Local Planning Authorities wish to plan for, or agree to, densities below this minimum, this will need to be justified” (PPS3, para 47)
“Local Planning Authorities should develop housing density policies having regard to:
- … The desirability of using land efficiently and reducing, and adapting to, the impacts of climate change.
- The current and future levels of accessibility, particularly public transport accessibility.”… (PPS3, para 46)
“Density is a measure of the number of dwellings which can be accommodated on a site or in an area. The density of existing development should not dictate that of new housing by stifling change or requiring replication of existing style or form. If done well, imaginative design and layout of new development can lead to a more efficient use of land without compromising the quality of the local environment.” (PPS3, para 50)
Planning Checklist: Density
In regional, sub-regional and local planning, practitioners are advised to:
4.1. Build to the highest density possible consistent to the local density range, and given quality of life and public transport availability considerations (existing and future). Increased densities need to be consistent with liveability objectives and type of accommodation needs, but the areas around public transport nodes (the 10 minute walk or approximate 800 metre radius catchment) can particularly be the focus for increased densities, again depending on context.
4.2. Consider the interrelationships between public transport accessibility, parking and density in the early stages of strategic planning processes (e.g. Regional Spatial Strategies, Local Development Plans), including across urban and suburban areas. Where new public transport capacity is installed, the aim should be to reconfigure development form to support patronage, particularly where suburban, low densities surround routes.
4.3. Ensure that local plans maximise the density of residential and commercial development while taking into consideration neighbourhood design and other constraints, as noted in PPS3 (para 46).
Evidence and Examples
The evidence given here is necessarily selective, but gives an introduction to the research on this topic. More details are found in the background technical report.
Some of the early pioneering studies (Newman and Kenworthy, 1989; Gordon and Richardson, 1989; Roberts, 1991; Breheny and Rookwood, 1993) were very influential in generating interest in the topic and provided an early understanding of likely relationships, though were often simplistic in methodological terms. They were mainly focused on the issues of density and travel. Some authors argue for the ‘compact city’ and that there is a strong relationship between density and transport energy consumption (e.g. Newman and Kenworthy, 1989 and 1999). Others are more cautious and suggest that land use factors are, at most, only a small part of the travel picture, and that other factors, such as income, are more important in influencing the variation in travel (e.g. Handy, 1995; Gordon et al, 1997).
There are issues raised around the acceptability of various policy stances, particularly the public acceptability of compaction (Breheny, 1992 and 2001). It is argued here that suburbanisation has been stimulated by lifestyle choice and that attempts towards urban compaction are running against the aspirations of the majority of the public. Breheny warns of the need to test compaction policies for ‘veracity, feasibility and acceptability’. The debate has also moved on somewhat from the compact versus dispersed argument, towards the spectrum ‘in between’, including polycentricity and ‘deconcentrated concentration’ as important principles – with growth concentrated at multiple locations (Owens, 1992; Breheny and Rookwood, 1993). Hall and Ward (1998) describe ‘sustainable social cities of tomorrow’, developing the early garden city ideals of Ebenezer Howard into notional development clusters along [re-opened] railway lines at various locations in the UK, as a complement to development within the urban boundary.
Ecotec (1993) and Banister et al (1997) provided early analyses of the UK National Travel Survey (NTS) dataset, finding that there are relationships with density (and other variables) and travel. Cervero (1996) considers that density exerts a stronger influence on mode share than land use mix. Stead (2001) and others have similarly used the NTS for analysis using multi-variate techniques, finding a range of associations, including density, and travel.
Rudlin and Falk (2000) and CABE (2005) summarises a density gradient by different contexts, which are useful for benchmarking purposes, as outlined below. Better Places to Live by Design: A Companion Guide to PPG3 (Llewelyn Davies and Alan Baxter Associates, for DTLR, CABE, 2001) also gives a number of useful illustrated case studies on this topic. Average densities of new development in the UK have been rising from 22 units/ha between 1981-91 (Bibby and Shepherd, 1997) to 44 units/ha in 2007 (DCLG).
|Low density detached - Hertfordshire||5||20||Urban Initiatives|
|Executive homes||5-10||20-40||Cabe, 2005|
|Average net density Los Angeles||15||60||Newman and Kenworthy|
|Suburban semis||15-30||60-120||Cabe, 2005|
|Milton Keynes average, 1990||17||68||Sherlock|
|Average post-war densities in the UK||20-25||80-100||Crookston, 2004|
|Minimum density for a bus service||25||100||Local Government Management Board, Sustainable Settlements Guide (housing occupied at full capacity)|
|PPS3 minimum threshold||30||120||DCLG, 2006|
|Victorian and Edwardian inner city areas (Forest Hill, London; Jesmond, Newcastle; Chorlton-cum-Hardy, Manchester)||30-45||120-180||Crookston, 2004|
|Garden cities||30-40||120-160||CABE, 2005|
|Georgian terraces, Canning Street, Liverpool||40-45||160-180||Crookston, 2004|
|Average net density, London||42||168||Newman and Kenworthy|
|Minimum density for a tram service||60||240||Local Government Management Board, Sustainable Settlements Guide|
|New Town high density, low rise - Hertfordshire||64||256||Urban Initiatives|
|Victorian/Edwardian terrace||60-80||240-320||Cabe, 2005|
|Urban villages||75-125||300-500||Cabe, 2005|
|Higher density flats, Kensington, London||80-120||320-480||Crookston, 2004|
|Vauban, Freiburg||90-100||360-400||Taylor and Sloman|
|Hammarby Sjöstad, Stockholm||145||580||Cabe, 2005|
|Kowloon, Hong Kong||1,250||5,000||Scoffham and Vale|
(Based on Rudlin and Falk, 2000; with more recent additions from Crookston, 2004; CABE, 2005; and elsewhere. Grey boxes show source figure; average dwelling size of 4 bedspaces assumed, which is higher than average household size in UK).
Llewelyn Davies (1997, 1998, 2000) has developed a number of ‘sustainable residential quality’ studies concerning the potential for and application of higher densities in urban areas, which influenced the development of urban capacity study methodologies. TCPA (2007) draws together on emerging good practice in the design of sustainable urban extensions and new settlements.
In the US, an empirical study of the New York Metropolitan region found that employment density at work exerts more influence on mode choice for work trips than residential density at home (Chen, Gong and Paaswell, 2008). Employment density and location as well as housing density are hence important issues to consider within strategic planning. In Seattle, higher housing density is associated with increased public transport and walk mode share of shopping trips, and higher employment density with public transport and walk share of both work trips and shopping trips (Frank and Pivo, 1994).
Recently there has been debate on the ‘network society’, hierarchies and polynuclear development (Castells, 1996; Graham and Marvin, 1996; Negroponte, 1996; Hall and Pain, 2006) which could have enormous implications for future travel patterns, including density (and wider urban structure) and travel relationships.
Further reading here
1Other metrics can be used. For example, gross density includes all land (i.e. including major roads, parks, service facilities, etc.) and is often measured in terms of dwellings per hectare or persons per square kilometre. Gross density is useful for comparing densities across larger areas and for estimating potential public transport demand. Habitable room densities allow different house types to be accommodated. Plot ratios and net site densities are also used. The more recent research is beginning to develop more effective measures of density and quality, e.g. number of useful facilities per area, such as bookstores or coffee shops.
Longbridge, Birmingham - The Longbridge Area Action Plan requires residential densities of 40-50 dph across the brownfield redevelopment site. Higher densities and a mix of uses will be clustered around a major public transport interchange while lower density housing will relate to existing development at the edge of the site.
Sherford, Plymouth - The Sherford (Plymouth) urban extension will be built at a density of 35-55 dph, with highest densities adjacent to the local centre. This is above the minimum PPS3 guideline.