It's not that complicated really - there's a three-level hierarchy in public transport systems: 1. If the road traffic is congested, the solution is bus lanes.

It's not that complicated really - there's a three-level hierarchy in public transport systems: 1. If the road traffic is congested, the solution is bus lanes. 2. If the bus lanes are congested, the solution is light rail rapid transit. 3. If the light rail is congested, the solution is a subway/train line.