Dedicated bus lanes (Bus Rapid Transit) has its own weaknesses, particularly in pre-built up urban areas. To make the best of your system, you need 4 lanes wide

Dedicated bus lanes (Bus Rapid Transit) has its own weaknesses, particularly in pre-built up urban areas. To make the best of your system, you need 4 lanes wide of dedicated bus space at the stops, or else you get really inefficient performance, that WILL NOT SCALE. At that point, once you've discovered that tunneling is the only option to get effective mass transit through your downtown core area, then you may as well just go light rail, since tunneling costs so much already anyways - may as well give the system a higher capacity ceiling, and higher speed.