To be fair, I don't think he's denying white flight, and what I'm describing isn't really race-centric (though as with all things socioeconomic in the USA, race is always a factor).
It's true that many suburbs, especially the early ones, were explicitly racist (i.e., bylaw-segregated communities), that's not what's under contention. The question is if the physical design of suburbia is influenced by the desire to keep undesirables out, above and beyond any sort of legislative or extralegal measures already being undertaken at the time.
There aren't any easy answers here. I think it's quite self-evident, but obviously there is room for disagreement. One of the defining feature of modern suburbs is the "gate" - the limitation of access points to nearby arterial streets. It's fairly obvious (and even a perk for buyers) that this effectively separates the road network within from the outside and discourages through-traffic. The main claimed benefit here is safety - but from what?
One oft-claimed safety improvement is traffic. This isn't really true as pedestrian fatalities are still comparable to urban areas. But I am willing to accept that people believe they are getting improved traffic safety in these places.
The other oft-claimed safety improvement is crime. The idea is that if you make through-traffic a giant pain in the ass, then anyone who is there who doesn't live there is likely up to no good, and it makes troublemakers and potential criminals easier to spot. This is not illogical - and it agrees with Oscar Newman's Defensible Space theory.
The question is to what extent that is classist. This is where things get murky. We know a few things from the early days of suburbia:
- Cities were experiencing high crime and violence, and suffering from a particularly compressed housing market due to the sudden return of millions of servicemen. This is the primary force that purged people out of urban centers and into newly developed suburbs.
- Only the middle and upper-middle classes could afford cars. Suburbia (in that format) was fundamentally incompatible with public transit. Occasionally this was an enforced incompatibility - see: Robert Moses and the low overpasses on Long Island.
- Suburban developments were explicitly advertised as an escape from the gritty, dangerous cities of yore. Many were explicitly segregationist. I don't think it's at all a stretch to say that separation from the poor (and black, if you want to go there) was a substantial motivator for early suburbs. Like today, personal safety is one of the main selling points of suburbs, the contention is to what extent that was code for "because no poor people here".
- The design of suburban street layouts to minimize usefulness as through-roads is deliberate and an oft-touted selling point. The contention is to what extent this was an aesthetic preference vs. an exclusionary tactic. This is a hard one, since even during those times people were less-than-explicit about motivations against the poor.
The use of street design as a class/crime barrier has been very transparent and blatant in the past - even the recent past. For example: http://articles.baltimoresun.com/1996-05-03/news/1996124052_...
Most other cases are not quite as blatant and clearcut. There is grayness between simply wanting safe neighborhoods and wanting all poor people out.