Via Strange Maps, Thumb has this poster which overlays ring roads from around the world:
According to Thumb, "This poster is designed as a sort of calling card for Rice School of Architecture, located in Houston. We collected ring roads from 27 international cities and layered them all at the same scale. As it turned out, Houston has the largest system of those we surveyed. (Beijing was second)"
Strange Maps says that it's not entirely clear what is being referred to here as Houston's ring road. But it seems clear that the shape in the poster is that of the area contained by Highway 6 on the south and west sides of town and FM 1960 on the north side. The east side of this loop is a bit undefined, but it looks like it could be formed by Highway 146, which runs up the west side of Galveston Bay. Or maybe the east side of the "ring" is just open, represented by a notional north-south line that forms the right side of the poster.
Now, I can attest from personal experience: follow the route of this ring road and you will see nothing but the worst sorts of urban sprawl; it's truly a netherworld of placelessness, an interminable conurbation of strip malls and glass boxes which inspire the human spirit with nothing but alienation and loathing. I am sure that such is the case with most of the other US ring roads represented here.
As for the rest of the cities here, I can't say much; I've only been to a few of them, and never did I make a point of touring their ring roads. Is the spawning of endless sprawl by ring roads an international phenomenon? Maybe the very concept of the ring road is anti-urban: its function is to connect peripheral areas of the metropolis to each other, rather than to the central city. In the case of smaller rings, like Vienna's or Amsterdam's, the ring might be contained within an area that is basically urban; but for the larger rings, it seems inevitable that they'd promote auto-centric lower-density development - a.k.a. sprawl.
As noted above, Beijing has the second largest ring. I'd be especially interested in knowing what the character of development there is like. One thing that I'm curious about is the extent to which China is emulating the American style of urban development. China is, after all, adding cars to its roads at a furious rate. On the other hand, it seems to be adding to its mass transit infrastructure at an equally furious rate. And the other Chinese ring roads on this poster - Guangzhou and Tianjin - aren't obscenely huge. So, my legion of Chinese readers: what's the deal with urban growth there? Is it proceeding with a sensible consideration of the needs of a healthy urban environment? Or is it sprawling in the same sort of wasteful, inane, and crude patterns as the US has been for the last 60 years?