This might confuse you. With half the number of vehicles Mumbai has overtaken Delhi as the most polluted city in India. In effect each vehicle in Mumbai is 4 times more polluting than a vehicle in Delhi.
How did this happen? The assumption is that Mumbai's roads are more congested and Delhi has more vehicles running on CNG. However I am not sure whether the building of flyovers has really helped Delhi. The traffic jams still have not gone away and standing vehicles still cause pollution.
The expanding of BRT is going to increase use of public transport, although this will take some time. On the first road modified for BRT connecting Chirag Dilli to Moolchand, one can see how more people prefer the buses since the dedicated lines make for an easier ride.
I think a large part of the reduction in emissions is also because of the Delhi Metro. Today when I took the Metro to Janak Puri, it was normal trip. I did not even have to think - should I take the car or the Metro? When I got off, I remembered the time when it was a long hot bus ride at the end of which you were fighting through the crowds to get down.
In Mumbai there is no respite for someone who wants to avoid using the car. Yes there is the famed suburban train system but it is run down. The trains have to be upgraded and also new routes have to be planned in alignment with directions that people travel to work. With more work shifting away from town, people need options. Maybe the Mumbai Metro has such routes planned for the future. Till then this is a crown it has to wear.