Sustainable Cities by taming urban population explosion
One reason why some cities have become unmanageable is because the size of population for which they were planned to support.
Take the example of the city of Nairobi, Kenya. Its said that this "place of cold waters" in the Masai dialect, was not initially meant to be the political and commercial capital of the East African nation. This was a railway stop-over for people transiting from the coastal town of Mombasa to the interior.
Supposedly, Kenya's capital was meant to be Machakos, a sleepy town some 40 miles South-East of Nairobi but due to the hilly topography surrounding the town, it was deemed not suitable for expansion, and so it was moved to Nairobi. Apparently, when Nairobi was undergoing construction, Kenya's population was less than 5 million, a size which the population of the city of Nairobi alone will outnumber in the next 5 years or less.
As the National population started to bulge after independence in 1963, resources became strained not only in towns but also in the countryside and hence people started migrating from rural to urban centres in search of health, education and ultimately better living standards.
About three years back, if one needed to acquire a basic document like a passport, one had to travel all the way to Nairobi, regardless of whether they had to spend a week on the road, due to the poor infrastructural development especially in the rural areas. As that was not enough, once you are in the city, you were not even guaranteed of acquiring what is deemed your birth right courtesy of corrupt government officials. This partly contributed to forcing people to start looking for cheap accommodation and hence the growth of slums. Others came purposefully to look for employment and also contributed to the mushrooming of the temporary structures in slums because, one could not afford better living elsewhere.
This has been occasioned principally by successive governments' indifference to the fact that amenities and services needed to be decentralized to create opportunities for other smaller towns to grow and provide opportunities for the local people so they don't have to move to the capital city for employment or government services.
I would suggest that governments ensure that almost every county or administrative region has been taken care of in terms of provision of Government services, which will in turn attract investors because there will be market for products and services. This will discourage the mass-migration we have witnessed in the last one or so decades to cities and hence enhance the sustainability of cities the world over.