I don't know about which city or urban environment, but my downtown area (largest city in our state), isn't crime ridden, has no smog, sure there's likely more pollution, but with the layout of our highways and the location of traffic, that's all further from houses and offices than led to believe.

The 'riots' you're thinking of took place in a rather racially segregated and historically significant part of my city that isn't downtown... Urban environments have existed for centuries, even millennia. Ancient cities were dense and had many a people within their walls.