Why are cites in Europe denser than in America?

Fascinating video that explains why European cities are much denser than US cities.

The answer is one word - transportation. People congregate to minimise commute times. Urban planning facilitates this by allowing vertical growth, especially around transit corridors, and having excellent public transit services inter-connecting population nodes with the city centre and with each other. 

Unfortunately, urban planning in most developing countries fail badly on both counts.