RICKY BURDETT is professor of Urban Studies at the London School of Economics (LSE) and the director of LSE Cities and the Urban Age program. Professor Burdett was the architectural adviser to the mayor of London from 2001 to 2006 and is a member of the Hurricane Sandy Regional Planning and Design Competition. He is also co-editor of "The Endless City" (2007), "Living in the Endless City" (2011) and "Transforming Urban Economies" (2013). Are cities getting bigger?

At the moment 54% of the world’s population live in cities. By 2050 it could be 75%. Five generations ago, in 1900, that figure was 10%. Not only have cities become bigger, the speed at which cities are growing is something we’ve never experienced before. In Lagos, Mumbai and Dhaka, for example, there are roughly 35-45 people per hour being born or moving into those cities. That's over 300,000 a year. The impact of that is enormous—sewers, lighting, electricity provision, housing, hospitals. These urgent issues are not necessarily addressed by the cities that are growing the most.

Addressing that kind of growth sounds impossible.

There are very positive examples of how growth can be managed well in both the global North and South though the growth is unequal—it is concentrated now in Asia, Africa and parts of Latin America and has stabilised in Europe and North America. In some cases it’s actually reversed. Detroit and some Eastern European cities have had population decreases in recent years.

What slows growth?

It’s to do with migration reaching a saturation point. Eventually, people have found jobs and begun to do what they need to do. Let’s take Brazil—the economy has now stabilised so people are not as dramatically drawn to cities in order to get jobs. Work has been the reason why people move to cities for five thousand years and it will remain the reason.

But if people do keep coming then what happens?

There are many cities like Jakarta, Bangkok and Lagos, which are not managing growth well. They are expanding at an enormous rate with very little control on the infrastructure. Most of the growth is informal, unregulated and results in slums. This presents enormous problems for quality of life.

So the infrastructure just collapses?

The resilience of people and cities is extraordinary. Mexico City grew to 22m people in a very short time and the city hasn’t collapsed. However, it has run out of water. The city has sucked dry the water base—you can see cracks in the pavements and buildings. Sao Paulo is about to do the same. Most of the informal residents are on the outskirts where the water reservoirs are and all the settlements lack toilets and sewers.

Where do they get water from?

You need to reduce the amount of consumption and provide clean water to these very underprovided areas. What is happening in Latin America now is that a lot of mayors and city leaders are getting to grips with this. In Rio de Janeiro many of the favelas have been retro-fitted—water, electricity, gas and postal services now exist in places that still look pretty ropey. Not unlike, perhaps, an Italian medieval town 400 years ago. Narrow streets, not great quality of air and light but totally functioning environment and economy because this is where people live and work.

In walled cities like Siena and Lucca I imagine growth would have been physically contained and they must have felt reasonably stable?

This is the big debate in urban planning. Do you contain growth or do you let it grow forever? One of the ways you might manage growth is to contain it. In the late 1930s London began to develop the green belt. This is exactly the same as a wall, like that around Lucca. This means the city can only grow so far. Then you have a 20 mile green zone and you promote new towns or the organic development of older towns. Milton Keynes was designed at the same time as the green belt—people then commute from there into London. That model is a very sustainable model of an environmentally efficient city that tries to reduce the amount of commuting. Most of the big cities I’ve described, like Rio and Lagos have average commuting times of four hours a day. That’s average!

Is that inevitable?

No! If you take a city like Hong Kong, which has a population of seven or eight million, the average commuting time is 11 minutes. In Tokyo, the largest city in the world at 36m, the average commuting time is less than an hour. That’s because they have such extraordinary investment in public transport and it is all very integrated.

The key point here is that each city develops its special and political mechanism that is coherent with the point of development at which it sits. There is no perfect city but there are cities that are fit for purpose for their time and place.

But there are cities that work and cities that don’t.

Yes. I would say London with the green belt works, Hong Kong with its public transport infrastructure works, obviously the Scandinavian cities of Stockholm, Oslo and Copenhagen are extremely efficient, but places like Medellin or Bogota also show extraordinary low cost, simple innovation and intervention. In Bogota they’ve invested in more than 100km of cycle routes and it’s as successful as cycling in Copenhagen. They’ve also added the Bus Rapid Transit system—dedicated bus lanes. This is absolutely radical for a city like this and the effect is dramatic.

But a lot of people are still living in slums?

The UN projects that one in three new urban dwellers will soon be living in a slum. That creates a social time bomb in the cities currently growing at this incredible rate. London went through this problem 150 years ago. London was then hyper-congested, enormously polluted, with appalling living conditions, and average life expectancy for men was 24. In the 1860s, 1870s and 1880s, London invested massively in sewers under the Thames embankment, to avoid cholera epidemics, and in the social housing movement. There was an awareness that unless good quality housing is provided for the most deprived we aren’t going to solve the poverty problem. Institutions and interventions into the fabric of the city go hand in hand to improve the conditions of people who moving in.

What about pollution in these overcrowded places?

The defining statistic of the environmental issue is the fact that cities consume an enormous amount of energy and contribute 70% of the world’s CO2 emissions. If you can reduce their footprint by 10% you can do enormous benefit to the world. If Mexico City followed the Bogota model instead of the Los Angeles model it’s clear what the improvement would be. Even in America there are cities apart from New York that stand out. Portland, Oregon, Seattle and Washington have brought in the Urban Growth Boundary, like the green belt. It limits sprawl. If you do this you are automatically making a city more compact and you can then disincentivise the car. As a city mayor you can improve things.

Correction: The original version of this article said that 1900 was only three generations ago. This has now been corrected. Sorry.