Cities in numbers : The densest cities can be the most efficient, lively and sustainable – but only if they boast effective management and design to minimise overcrowding and pollution

Cities exist to provide access to people, goods, services and information: the better and more efficient this access, the greater the social and economic benefits of urban living.

A defining characteristic of cities is therefore the degree to which accessibility is based on the physical proximity between origins (eg where people live) and destinations (eg where people work) or on transport solutions which can overcome spatial separation, and the degree to which these solutions involve private or public motorised transport. More compact and dense cities are typical examples of providing connections through greater proximity.

Diagrams prepared by the Urban Age programme at LSE Cities explore the issues of residential and workplace densities, and the extent to which they overlap or not. The diagrams show the number of people living and working in each square kilometre of a city’s metropolitan region.

Facebook Twitter Pinterest New York’s employment peak of over 150,000 jobs per km2 is found in Midtown Manhattan above West 42nd Street, close to Times Square. Outside Manhattan, employment activities are relatively low. Photograph: LSE Cities

Facebook Twitter Pinterest Hong Kong has a close integration between residential and employment peak densities. This pattern is associated with a strong mix of uses and shorter travel distances. Photograph: LSE Cities

Density is a fundamental measure of urban structure that can be used to quantify the immense diversity in urban form across the globe. Higher urban densities can improve service delivery efficiency, promote urban vitality, and facilitate more sustainable public transport, walking and cycling. These advantages depend, however, on effective city management and urban design that minimises the negative costs of overcrowding and pollution.

Besides the density patterns that can be observed today, it is important to consider the changes over time that either keep cities compact or reduce densities. Total global urban land area has grown rapidly in the past century, doubling in OECD countries since the mid-1950s and increasing five-fold outside the OECD. In the US, the total area of the 100 largest urban areas increased by 82% between 1970 and 1990.

Overall, these land use changes have been part of a considerable reduction of residential densities in cities. However, alternative urban development trends are beginning to emerge and increases in urban density are recorded in many European and some North American cities. Examples of well-planned compact cities include Copenhagen, Stockholm and Hong Kong, whereas other cities such as London, Brussels, Boston, Tokyo, Hamburg and Nagoya have re-densified and moved back towards more concentrated forms.

Facebook Twitter Pinterest Since 2000, population growth in London has been concentrated within a 10 km radius of the city centre. Photograph: LSE Cities

Connecting places

To a certain extent, physical proximity in cities can be substituted by increasing the speed of travel through the use of rapid, motorised modes of public and private transport. It is important to note, however, that even then the overarching objective remains the provision of access to opportunities rather than mobility or movement itself.

Infrastructural features that define transport systems include the connectivity and surface coverage of roads, the quality of road and rail networks and other public and shared transport infrastructure. In addition, transport operations and service quality determine transport-based access and typically include the service level of public transport and the availability of privately owned vehicles.

Mostly as a result of their urban forms and transport infrastructures, cities today show a great variety of travel behaviours – most notably mode choice and trip lengths. This is the case even among cities at similar levels of wealth, indicating that socio-economic factors are only one among several determining factors.

Three main categories of urban travel are usually differentiated: public, non-motorised transport, and private motorised transport. Globally, public transport – commonly identified as the back-bone of urban transport – has surprisingly low shares in terms of actual mobility provision in cities. Estimates suggest that the mode share of all urban public transport trips worldwide was only 16% in 2005.

However, the role of public transport varies significantly between regions and individual cities, with public transport in Asian and some European cities accounting for up to 51% of trips, but fewer than 10% of trips in car-orientated cities in the United States or the Gulf Region. Non-motorised transport remains the predominant mode of transportation in most African and Asian cities, particularly those where incomes are low and the level of public transport is poor.

As the figures below demonstrate, walking and cycling also play a major role in cities in developed countries if pedestrian-friendly environments are provided. Similarly, the share of private motorised travel varies enormously, even when comparing cities at similar wealth levels. More than 90% of trips in some North American cities are by private vehicles, compared to less than 15% in Tokyo or Hong Kong. It is important to note that data on walking and cycling is often incomplete and, as a result, non-motorised travel tends to be under-represented in many cities.

Facebook Twitter Pinterest How people get around Hong Kong, Mumbai and Johannesburg. Photograph: LSE Cities

Modal changes

In terms of change over time, the most fundamental shift of travel behaviour in cities over the last five decades is linked to the rapid increase in global levels of motorisation, and a substantial shift away from non-motorised and public transport. Over that period, the share of public transport in urban agglomerations has either been constant or declining in almost all cities in the world, despite substantial efforts to support its growth. Similarly, the share of walking and cycling has mostly been declining, although together they still account for more than 70% of trips in some African cities, more than 50% in many Chinese cities, and more than 33% in most Indian cities.

Despite the global trend towards increasing motorisation, new and alternative patterns of urban mobility have emerged in recent years. In the developed world, several cities have increased their share of public and non-motorised transport and reduced car ownership while creating more attractive and economically prosperous inner cities. For example, between 2000 and 2010, levels of car ownership in New York, London and Berlin have been declining. Non-motorised transport, particularly cycling, is on the rise and public transport passenger numbers have bounced back.

With regards to current global trends, the most persistent urban transport development continues to be a strong growth in motorised trips and, above all, an increase in private vehicular traffic. However, there is a considerable risk of overestimating the growth of private vehicle stock, as most growth projections simply extrapolate historic trends without adequately incorporating evidence on changing patterns of mobility as a result of social and technological disruptions.

For example, analysis of recent traffic forecasting in both the US and UK has indicated that transport planners have consistently overestimated future car traffic growth in the previous two decades, with significant distortive effects on transport planning investments.

Access to the city

Within both developed and developing countries, there is a high level of correlation between the use of different transport modes and social class, with the poorer populations mainly (in the developed world) or almost entirely (in the developing world) reliant on public transport, non-motorised transport and walking. Poorer population groups in lower and middle-income cities rely on walking, cycling and (often informal) public transport.

Lower-income households are also disproportionately affected by key negative externalities generated by transport, including road accidents, air pollution and project displacement. Dense, well-managed urban development and the provision of accessible, affordable public transport can therefore have a positive direct effect on the poor and other disadvantaged groups by increasing their ability to access goods, services, and economic opportunities.

Urban Age is a worldwide investigation into the future of cities, organised by LSE Cities and Deutsche Bank’s Alfred Herrhausen Society. Its 10-year anniversary debates are held in conjunction with Guardian Cities.

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