This is the dilemma China is facing now. They think they've found a solution though, with something called the "straddling bus," a cheaper, greener and faster alternative to buses and subways.
The bus is cool in that it is basically like a moving bridge; it spans the roadway like a bridge, allowing cars to go underneath, while passengers sit on the second level above the traffic. This keeps traffic from jamming up traffic every time the bus stops to pick up or drop off riders.
Powered by electricity and solar energy, the bus can speed up to 40 miles an hour carrying 1200-1400 passengers at a time without blocking other vehicles’ way. Also it costs about 10% of what it would cost to build a subway. Watch the video- pretty impressive.