On my commute in I had the "opportunity" to ride behind 3 different city busses, which was more interesting than one would guess. They were generationally different, and by coincidence, in improving order which really drove the observation home.
The first was the most dated, a diesel bus who's emissions were not awful 3rd world carbon black, but had sufficient black color to make me wait and let the buss roll off instead of me being right in an exhaust vortex right behind it sucking in emissions.
The second was a more modern diesel. You could smell the exhaust, and it still wasn't desireable, but there was no black associated with it. It was fine riding a block behind as there was enough crosswind that the whole vortex behind didn't feel like a travelling gas chamber.
The third was remarkable. Given the first two, I looked up at the exhaust at the light to check what my strategy would be. I saw zilch for visible exhaust, just a little heat wave. Then I saw the sticker on the back - CNG bus. Quieter. Virtually unnoticeable exhaust - clear not sooty. No real exhaust smell. Really quite amazing. Being behind it didn't feel like it was instantly grossly polluting my body (I'm sensitive to exhaust, just doesnt' feel like something I want in me, and have always suffered in 3rd world cities, tunnels, our building parkade when everyone leaves and fans can't keep up, etc. where concentrations seem higher).
Calgary Transit operates like 1,100 diesel busses. They're piloting CNG - cheaper, quieter, will see on drivability and maintenance and range. But from a road user standpoint, 1,100x the difference I experienced in exhaust quality is amazing... and we're in exactly the right part of the world to have long, stable, cheap access to natural gas. I hope the operating metrics work out, as these were pleasantly quiet and clean additions to the downtown commute.
If I envision Calgary say a decade out - increased "core" density could be more pleasant than it is today. I see higher percentages of young people in particular using cruiser bikes as part of transportation. CNG busses are quieter and cleaner. The proliferation of hybrid cars damps both emissions and acceleration noise on streets. A certain number of cars has been displaced by the emergence of Car2Go as a viable part of the transportation mix... we're making strides it seems if these trends continue toward an urban renaissance not urban congestion and decay.