Growing and expanding cities need vast quantities of energy. Megacities (cities > 10 million population - there are only 2 in the US = New York and LA) need mega quantities of energy for industrial production (for employment prospects and the insatiable demand for fiat paper is why there is growth towards the Megacity) and to sustain the population. They also need vast quantities of fresh potable water and sewerage systems etc. And I haven't even mentioned transportation energy requirements and air pollution. The average city has approximately 3 days of food storage and requires a constant flow of this input. The flows of finance regulates this system. Megacities can only really continue to exist within a global domain as they sink the natural resource on a global scale. It could even be argued that the fall of a Megacity on one side of the planet could make the other 30 megacities fall one by one in a domino fashion as one relies on the other for its existence with a consequence for the majority of the worlds population.

Interruption of the flow of energy (electrical grid), Transportation fuels (gasolines), water and food and the typical Megacity of today would become a catastrophe within 7 days.

These interruptions could be the result of instability in the global financial system (highest probability in the short term), peak oil (highest probability in the medium term) or even a solar storm.

The wars to control the petroleum reminder of the peak oil problem started years ago and they continue a pace.

Of course the decline of the Megacity after its peak around 2030 might be quick (very very nasty), it might be managed ("If you want a picture of the future, imagine a boot stomping on a human face -- forever".)

There are no easy technological or scientific solutions to this issue as they have been tried before;

http://www.youtube.com/watch?v=NZ6t5JA7OBA

As for escaping a failing Megacity then I'm afraid the original ending of Bladerunner will remain a fiction as the elites security forces will instigate reverse sieges.