London style congestion tax. Every avenue in NY is a superhighway belching toxins into millions of New Yorkers. My health has dramatically improved after leaving that hell.
If you don't believe me, look at an 70 year old New Yorker living on the East side vs a 70 year old Californian (outside of LA).
On the other hand I had some of the worst allergies in my life when I lived upstate - I had to go on an inhaler it got so bad. Within a month of moving back to NYC, it completely went away. Perhaps my body, for better or worse has adjusted and formed an equilibrium with NYC's air.

Still, our rates of asthma and lung disease are incredibly high and the air needs to be cleaned up quite a bit.

I grew up in upstate NY also and had my allergies disappear in NYC.  It was great for a few years until the annual brochitis and other problems set in. Don't kid yourself. The city is one of the most toxic places to live other than downwind of a coal power plant. I've heard that Stratford, Ct is downwind of the city and gets their unfair share of the pollution. I'd bet good money cancer rates are higher there.

I have photos of myself in my last year in the city with dark circles and a pastey face. After five years in California, I can actually think more clearly and feel ten years younger. All my NY friends couldn't believe the change when I went back.  

I loved New York and I left because of a family emergency. I had no idea how hard it was on my health and I would encourage all New Yorkers to demand better mass transit and a 80% reduction in cars. Life in the city would be awesome. In the meantime, enjoy the rat race!

Just playing devil's advocate, are you sure it's the pollution that was responsible?  It could have been other things.  For example, you moved from NY to LA.  NY is at a high latitude and you probably were not getting enough vitamin D.  Vitamin D comes from exposure to the sun, and you'll always get more of it when the sun is shining more directly on you.  In the far North (or South) the angle of the sun in the winter is such that you can't produce any vitamin D at all.  That is not true of LA.  That's just one of many possibilities.  Another is that your brochitis was caused by cold weather and the weather is warmerin LA.  

Pollution could very well be a part of the problems you were experiencing, but at the same time LA is not renowned for its clear blue skies.  

Agreed - We are working very hard on this issue.
How about just... reducing the space available to cars. Widen sidewalks where necessary so that people don't have to walk in the street, make buffered bike lanes on major corridors, and convert bridge and tunnel lanes to subway, light rail, or busway use. Oh, and put tolls on the East River bridges, to pay for their continued maintenance costs as well as all the improvements that can be made to them. Then instead of slightly fewer cars moving a lot faster, you get far fewer cars, more pedestrian space, and better transit service.

Remove all on-street parking. If you have a car, you can afford a garage. It would discourage people from driving in the city, and do away with most of the traffic congestion that causes the pollution. The few things that do need to drive in the city (busses, delivery trucks) would be moving along quickly rather than being stuck idling in traffic.

Also, ban diesel anything, or at least require better emissions standards. I made the mistake of sitting near a bus route outside a cafe once. Every time a diesel bus rolled by, my water was full of black specks. Hybrid busses, nothing.