You obviously live in the area Up the Junction, I'm not exactly sure where but I don't feel overly polluted where I live, which is 2 minutes away from the junction in question. However at the moment I do feel safe, something I can't see continuing if the road is blocked off. I know from previous comments, you don't believe it but traffic moving through the area is a form of deterrent to crime and anti social behaviour and in the past when the junction has been blocked for roadworks, there has been more drunks and groups of youths than normal hanging around at the junction and trust me as a woman walking through the area on her own, I certainly didn't feel safe, they may seem harmless but they're very intimidating, which doesn't make for a happy life. At the moment, they can only hang out by Save More but this plan will give them seats so they can sit whilst they heckle and abuse people as they go past and presumably also nice planters that they can then use as a toilet!
For me this isn't necessarily about the traffic situation, I have a car but it isn't my regular method for getting around, this is about my own personal safety, the police already don't like showing their faces around there but at least they have to drive through the area to get places so there is a bit of a presence, if the road is blocked off, that will stop and the police will only come when called.
There are other ways to make the route unpalatable to drivers, speed cameras, get rid of the free parking, make the road narrower, build in proper bike lanes, zebra crossings, pelican crossings, all those things would reduce the traffic and to some extent fulfil your desire for reducing pollution but without making the immediate area near the junction a ghetto.