Ricky Burdett (LSE Cities) has just published a nice piece on the Guardian discussing how four cities have quite different ways to respond to economic, political and environmental shifts. It's a good article to disclose their research projects, which you may find here.
"Higher densities can facilitate more sustainable public transport, walking and cycling, making it more efficient to provide services and promote urban vitality. These advantages depend, however, on high-quality urban design and the effective city management to minimise overcrowding, stress and pollution."