I'd like to see a study exploring the net health effect of exercising outside in a polluted city.

Also, if it's polluted outside, isn't it polluted inside too? Do these cities all have buildings with air purifiers?

Physically I would say yes (air purifiers). Central AC that is. Not sure the efficacy.

Also, particulate matter is much more volatile outdoors, because it's outdoors.

And, the downmods, two in a row, make zero sense. Else, explain it please.

We do have walk-only zones in most inner-cities in Germany. They are called Fußgängerzonen (pedestrian areas). They are usually the main shopping avenues.

I am surprised that something as everyday as these gets suggested as an outlandish experiment. ;o)

Were I the dictator, I'd just narrow the roads to one lane plus on-street parking and replace the freed area with trees or bushes (or tram tracks). Narrowing forces cars to slow down, making pedestrians safer and allowing bicyclists better blend with the traffic. Cutting down on the lanes adds congestion which effectively removes unnecessary car travel from the city area, encouraging people to walk or pedal. I would also remove huge parking lots as they alienate pedestrians and increase car travel. Certain downtown streets or blocks could be made pedestrian-only zones.

Wouldn't this affect business travel, such as deliveries to restaurants, offices, grocery stores, etc.? I think the primary thing to cut down on is luxury travel, where walking/cycling/public transit it a reasonable alternative - and to do it without hindering business.

I don't know that it would really (it doesn't in Japan), but it seemed like a potential catch.