I certainly think that noise, traffic and congestion are factors. They're factors that tend to get short shrift from New Urbanists, who tend to be . . . the kind of people who like living in cities. But along with the benefits of having more people around you comes a lot of stress. People who have chosen to live in a quiet area are probably people who have lower tolerance for that stress. If you're a renter, no problem: just move. But if you own, and the changes make your property harder to sell at the same time as they impair your enjoyment . . . well, it's not shocking that people resist.
That said, I think these are far from the only factors . . . and that in fact, "noise, traffic and congestion" are often covers for much less lovely motivations. In my fair city, Wal-Mart spends a lot of time defending its traffic management plans from challenges by groups whose backers (local merchants) could mostly care less about DC traffic patterns. What they care about is not having to compete with Wal-Mart. But of course, "Wal-Mart is going to undercut me by selling you cheaper goods!" doesn't sound nearly as alarming to local residents as "Wal-Mart is going to snarl traffic clear back to Maryland!"
In DC and New York, the two cities I am most familiar with, "Safety" is indeed a strong objection, at least to the "affordable housing" segment of the development market. The fact is that subsidized housing is associated with higher crime -- not just in the fevered imaginations of affluent homeowners, but in the crime statistics.
And while we can have all sorts of fun discussions about how Lehman bankers are much bigger criminals and our society's disparate treatment of white-collar offenses, the risk of falling victim to those crimes is not much enhanced by proximity to the perpetrators. Moreover, the fear of global financial chaos is somewhat less visceral than the fear of getting shot or having the snot beaten out of you. I know multiple people who have been hospitalized due to crime. I don't know anyone who has been hospitalized as a result of the Lehman bankruptcy. That's probably a pretty standard breakdown for most residents of urban centers.
In New York, at least, "views", aka "light", are also a big deal--in ways that I think it's hard for people in other cities to imagine, though you can come close by picturing what it would be like to sell your upstairs and move into the basement. Living in perpetual shadow, as many New Yorkers do, really is depressing. I didn't realize how depressing until I moved out of my first floor Manhattan cave and into a light-filled second floor apartment in DC.
When they tore down the burnt-out one-story "taxpayer" across 94th street from the building I grew up in, and started building 15 stories worth of condos, you could hear the howls of anguish clear to the George Washington Bridge. People who had been living in relatively light fourth-floor apartments for thirty years were suddenly going to be plunged into permagloom. Those people didn't gratefully acknowledge that they'd gotten the standard low-floor discount when they'd bought their apartments, and then gotten decades of "free" light. No, they spent a lot of time at community board meetings, trying to find some way to block the project, and no wonder.
They failed, because they had no right to that view, but it was still a nasty shock. The only people who got any light at all after the building was completed were those like my parents who had luckily purchased apartments on the top few floors.
For smaller towns, the cost of the new residents is a huge burden, particularly if the new homes are on the small side. It's very expensive to provide schools, sewers, roads, ambulance service and so forth to a bunch of new people, and in "starter home" developments, those people are rarely paying enough in taxes to cover their costs to the community. Larger communities can absorb this pretty easily, but if a new townhome complex is going to add 5-10% to the local population, you're essentially asking existing residents to raise their own property taxes so that other, poorer people can move in. Unsurprisingly, this is not popular, for the same reason that you do not earmark 5% of your income to support townhome development in far-flung exurbs.
Then there are the positive
externalities. People benefit
from having other people around them who are a lot like they are. That's not necessarily because they dislike those who are different. Rather, living near lots of people who want the same things out of life means that more of those things will be supplied conveniently close at hand, right there in your neighborhood. Professional DINKs want lots of bars and restaurants, and retail carrying the kind of high-end goods they can afford to splurge on. Middle income families with three children want good schools, affordable day-care, plenty of parking so that you can quickly move those children around, and places where the kids can spill soda on the plastic tablecloths. Poor families want affordable retail pitched to limited incomes and convenient social service providers. Immigrants want churches that speak their language, and grocery stores that carry all their beloved favorites from home. And so forth.
Who your neighbors are affects the services that are near you. So when an apartment building or development promises to radically change the population in your neighborhood, shifting the balance away from people who will consume the stuff you want to consume, it's natural to resent the change. This is not necessarily a case of rich people stomping out poor people, by the way; in DC, a lot of the complaints about gentrifiers center around things like bars and liquor licenses, and while some of this is undoubtedly a proxy for other issues like rising rents, I think some of it is just what the complainers say: those upscale bars are going into retail space that now won't be used for something else.
Too, if you live in a neighborhood with a strong community, a large influx of new residents threatens that. It's not that there's anything wrong with the newbies, necessarily; it's just that community cultures sustain themselves by transmission from older members to the new ones, and if there are too many new members all at once, they may swamp the old culture that community members treasure. And the more the newcomers differ from the current residents, the stronger this effect will probably be.
Then there are property values. Some of that is a reflection of the other factors I've run through, but there is also, of course, supply--if scads of new people can move into your neighborhood at an affordable price point, your house isn't going to be as valuable, is it?
But most of us do not want to say "I want to live around people who consume the same stuff I consume", or "I don't want to live near poor people", or "I would rather not have more families in town because they're just going to be a financial burden on me", or "I think it would be better if you had fewer alternatives to shopping at my store", so we talk about . . . noise, congestion, and traffic.