towns and cities seem to be growing "naturally" wherever humans decide to settle and start to trade. Intriguingly, some key parameters of their growth appear to have remained constant from antiquity to this day, suggesting that neither technology nor political organisation has had much of a say over the human tendency to aggregate. Now, however, as the majority of people already live in towns and cities and billions more are to follow, we can't allow settlements to grow on their own. Good and sustainable planning is required to make sure that the dramatic urbanisation of our species doesn't lead to large scale disaster.
I've explored these issues in my latest feature which is out now in Current Biology:
The urbanisation of our species
Current Biology Volume 26, Issue 23, pR1205–R1208, 5 December 2016
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The mathematical equations relating the growth of population density of a city to its population size apply to medieval cities just as well as to modern ones. The photo shows a side road in the historic part of Mainz, Germany (own photo).