Nobody really knows how big Lagos is. What's indisputable is that it's growing very quickly. Between now and 2050, the urban population of Africa could triple. Yet cities in sub-Saharan Africa are not getting richer the way cities in the rest of the world have. Most urban Africans live in slums ( 贫民窟 ); migrants are often not much better off than they were in the countryside. Why? The immediate problem is poverty. Most of Africa is urbanising at a lower level of income than other regions of the world did. That means there's little money around for investment that would make cities liveable and more productive. Without upgrades and new capacity, bridges, roads and power systems are unable to cope with expanding populations. With the exception of South Africa, the only light rail metro system in sub-Saharan Africa is in Addis Ababa, Ethiopia. Traffic jam leads to expense and unpredictability, things that keep investors away. In other parts of the world, increasing agricultural productivity and industrialisation went together. More productive farmers meant there was a surplus that could feed cities; in turn, that created a pool of labour for factories. But African cities are different. They are too often built around consuming natural resources. Government is concentrated in capitals, so is the money. Most urban Africans work for a small minority of the rich, who tend to be involved in either cronyish ( 有裙带关系的 ) businesses or politics. Since African agriculture is still broadly unproductive, food is imported, consuming a portion of revenue. So what can be done? Though African countries are poor, not all African cities are. In Lagos, foreign oil workers can pay as much as $ 65,000 per year in rent for a modest apartment in a safe part of town. If that income were better taxed, it might provide the revenue for better infrastructure. If city leaders were more accountable to their residents, they might favour projects designed to help them more. Yet even as new roads are built, new people arrive. When a city's population grows by 5% a year, it is difficult to keep up.