by Carl Pierer
In a time of rapid urbanisation, cities distil contemporary issues. By 2050, more than two thirds of the world's population will live in an urban environment (DESA, 2012). Social questions, political problems, and environmental concerns are increasingly raised in an urban context. In particular, due to the concentration of people and consequently economic activity, cities are large contributors to the global greenhouse gas (GHG) emissions and responsible for 40% of GHG emissions from transport (I.E. Agency, 2008). This suggests that a greener organisation of cities can make a substantial contribution to mitigating climate change. The argument here is twofold. First, we present the argument for mitigating by changing the urban form, in particular to a dense and circular city. Secondly, we present the case of Curitiba, illustrating that urban form can significantly reduce GHG emissions from transport, even if the city does not conform to the ideal of high density and circularity.
The theoretical framework for modelling the city is relatively simply. The standard, classical economic model of the (monocentric) city supposes that there is a central business district (CBD) in which all economic activity occurs. City-dwellers commute to the CBD in the morning and back to their residences in the evening. Since this is an economic model, people try to maximise their utility given their income. The utility depends only on the size of living space and the length of the commute. Because a longer commute incurs higher costs to the commuter, the closer a place is to the CBD, the higher the demand from people wanting to live there. This in turn means higher rent prices, and so less living space for the same amount. The model is simplified by allowing a linear trajectory from each point of the city to the CBD. Consequently, the city according to this model is radially symmetric. If we look at the density of the city according to distance from the CBD, we see an exponential decay. That is, near the CBD the city is densely populated (meaning, in particular, high rising buildings), but further away the density is falling (bigger houses, fewer people, the classical suburb scenario).
Of course, this model is rather simplistic and many cities are not in line with the predictions. Some points along which this model has been developed include to allow for multiple centres of economic activity or for other desirable amenities such as distance to green spaces. However limited the model may be, it does illustrate nicely that the urban form is a major factor influencing certain sustainability issues (something that has been confirmed by empirical studies).
Emissions from transport depend only on the distance travelled. The longer the trajectory, the higher the emissions. The transport emissions for the whole city is simply the sum of all individual trajectories. From the city model it then becomes clear that to reduce the total emissions, the distances within the city should be reduced. In other words, a change in the urban form allows to reduce the emissions. By having people live closer to the city centre, i.e. a denser city, the length of daily commutes and thus the transport emissions are reduced. There are other positive side effects of a more densely populated city. For instance, it has been argued (Borck, 2014) that skyscrapers have lower emissions from heating, since the heat from lower flats heats those above. Others have suggested that denser cities lead to a stronger social network and less anonymity in the city, although this point is more contested.
But it is not only for reducing the overall transport emissions that density is desirable, it is also necessary for changing the kind of emissions. A classical paper (Newman & Kenworthy, 1996) has argued that the kind of transport available in a city is responsible for the shape of the city. The assumption underlying this idea is that people do not wish to commute for more than 30 minutes to reach amenities required on a daily basis. Since for many centuries the most common means of transport was walking, we observe that old cities (the "walkable city") are very dense, mixed use. They are at most 5km in diameter, and living areas mix with commercial and business areas. In the 19th century, with the advent of early massive public transport such as trams and trains, new kinds of cities develop. They are structured along the lines of these means of public transport, with centres developing around the stops of the tram or train. Because massive public transport allows to cover greater distances than walking within the 30-minute-window, the city can become bigger and more stretched out. Yet, in the centres, the land remains in mixed use. According to this classical paper, it is only during the 20th century, with the widespread use of individual motorised transport, that the modern car-dependent city is possible. Suddenly the cities spread further and further, a phenomenon called urban sprawl, since the individual transport allows people to cover even greater distances in just 30 minutes. Something else interesting happens: there is a specialisation of land use. Suddenly, certain areas of the city are mainly residential, whereas others are mostly commercial. It is in this car-dependent setting that the above-mentioned model is most accurate. While this classical paper has certain problematic aspects (for instance, it is not entirely clear why specialised land use comes about only in the car-dependent city), it does present an interesting perspective. For it argues that it is the mode of transport that influences the urban form. Conversely, it seems that if the urban form is controlled, a change in mode of transport can be brought about.
From the point of transport emissions, then, the optimal form is as circular and as dense as possible. This has repercussions for the kind of transport emissions. Because massive public transport requires a certain level of user density to be cost-efficient, a denser city offers more possibility to implement public transport. In a denser city, quite in line with the picture from Newman and Kenworthy, alternative means of transport such as cycling become more feasible, too, since the distances are shorter. One recent paper (Lohrey & Creutzig, 2016) finds a ‘sustainability window' for urban transport, i.e. an optimal population density. The minimum density for a sustainable city there is found to be 50 person/ha. There are, however, alternatives to this circular, dense cities. A famous case in point is Curitiba.
The Brazilian city of Curitiba is an exceptional success story. In the 1970s, a conscious decision was made by city planners to prioritise massive public transport over individual motorised transport, with massive repercussions. A master plan was adopted that aimed to structure the city's development according to these five principles:
- changing the radial urban growth trend to a linear one by integrated land use, road network and transport strategy;
- decongest the city centre and preserve its historical buildings and neighbourhoods with legislation and economic incentives;
- demographic control and management;
- economic support to urban development; and
- infrastructure improvement. (Friberg)
These principles have been guiding the development of the city for the past 30 years. Curitiba's form is dominated by structural corridors, which have the following form: two central lanes are exclusively dedicated to public buses. These were originally flanked by two local roads going in opposite directions, allowing local traffic to pass through the city. There are 60km of bus way along the structural corridors, which are fed by 300km of local bus transport allowing users to connect to the main transport axes. In the earlier years, zoning encouraged housing to be developed along these axes, with a high population density near the structural corridors that is rapidly decaying away from them.
In order to increase the efficiency of the bus system, the city has come up with a variety of inventions. First of all, they have several types of buses: direct buses (with a capacity of 3.200 passengers/day and a speed of 30km/h – the average speed of the MTA subway system in New York City is 32km/h), articulated buses and even bi-articulated buses (the latter with a capacity of 4000 passengers/day). Secondly, they have introduced boarding tubes, that is slightly elevated, sheltered bus stops to reduce boarding time by allowing passengers to pay before boarding and to board at an even level. Boarding times here approach those of subways. These tubes are fitted with elevators to allow handicapped passengers to board easily.
The benefits of this system are manifold. Use of public transport is very high (75% of commuters use the system on weekdays) and consequently the city's fuel consumption is "30 per cent lower than in eight comparable Brazilian cities" (Friberg). Despite having the highest per capita ratio of cars amongst major Brazilian cities, Curitiba does not have a traffic problem. About 28% of the direct bus users previous used their car. As a consequence, Curitiba has "one of the lowest rates of ambient air pollution in Brazil" (Rabinovitch, 1992). This well-developed public transport system also makes economic sense. Inhabitants devote only 10% of their income to transport, a low proportion in the Brazilian context. Moreover, the entire system works without direct financial subsidy (Rabinovitch, 1992).
In addition to these, the master plan allowed to direct commercial pressure away from the city centre. The city was one of the first in Brazil to establish a pedestrian network in the city centre in 1971. Further measures abound:
There are also several garbage recycling and collecting schemes, bicycle paths, and lots of green areas and parks giving the city not only recreational areas and beautiful landscapes but also important flooding control and protection for biotopes and freshwater sources. The city has also developed methods and strategies for the preservation of historical neighbourhoods, saving its culturally important parts from land speculation. There is also an incentive mechanism for developers to use part of their plots for green areas, giving them the opportunity to build higher than the land use legislation would otherwise allow. (Friberg)
Despite certain signs of old age, the planning scheme of Curitiba provides an inspiring example. It impressively illustrates the value of successful and intelligent city planning that focuses on sustainability and prioritises public transport of private car use. In conclusion, it also points to a (sustainable) alternative to the more traditional, more conventional idea of a dense, round city as the solution to reducing transport emissions.
Borck, R. (2014, April). Will Skyscrapers Save the Planet? Building Height Limits and Urban Greenhouse Gas Emissions. CESifo Working Papers.
DESA, U. (2012). World Urbanization Prospects: The 2011 Revision. New York: Tech. rep. United Nations Department of Economic and Social Affairs.
Friberg, L. (n.d.). Innovative Solutions for Public Transport; Curitiba, Brazil. Sustainable Development International , 153-156.
I.E. Agency. (2008). World Energy Outlook. Tech. rep. International Energy Agency.
Lohrey, S., & Creutzig, F. (2016). A 'sustainability window' of urban form. Transportation Research Part D, 96 – 111.
Newman, P. W., & Kenworthy, J. R. (1996). The land use-transport connection. Land Use Policy, 1-22.
Rabinovitch, J. (1992). Curitiba: towards sustainable urban development. Environment and Urbanization, 62-73.