By John D. Macomber, senior lecturer Harvard Business School, HBR |

Today’s mega-cities have a footprint problem. They are developing horizontally, not vertically, with vast areas of low sprawl reaching out for miles from Sao Paolo, Lagos, New Delhi, Guangzhou, Jakarta, and many others. A central question our civilization must address is how we can avoid becoming a planet of informal slums.

Every year, hundreds of millions of people across the globe move from rural to urban environments in search of opportunity.  In a perfect world, governments would have the cash and the consensus to fund and coordinate the construction of the infrastructure required to sustainably accommodate a rapidly urbanizing world. But few governments appear to have the money or the political will to foot the up-front costs to prevent or fight fragmentation.