We had a barbecue on Saturday night, and unfortunately, on his way home from our house, Matt Yglesias was attacked on North Capitol. They weren't after his money; they just punched him and kicked him, and then ran away.
Typically, Matt looked for a lesson in the experience:
To offer a policy observation, higher density helps reduce street crime in an urban environment in two ways. One is that in a higher density city, any given street is less likely to be empty of passersby at any given time. The other is that if a given patch of land has more citizens, that means it can also support a larger base of police officers. And for policing efficacy both the ratio of cops to citzens and of cops to land matters. Therefore, all else being equal a denser city will be a better policed city.
That said, as a matter of personal ethics you really shouldn't run around punching random dudes in the back of the head irrespective of the prevailing level of population density or policing.
James Joyner asks if that's really true
Beyond that, while it's intuitively logical that densely populated areas would produce lower rates of crime for the reasons Matt suggests, is it really true? Certainly, one hears about muggings in Manhattan, Boston, and other very dense cities all the time. Surely, the incidence of violent crime per square mile rises rather than falls with population density? Certainly, I'd be less worried about random dudes assaulting me while walking around my subdivision than in downtown DC.
Obviously, it's not a continuous curve. There's a big increase in crime as density rises from rural to urban, because crime thrives on anonymity--you don't rob your neighbors, not necessarily because you like them, but because the likelihood of being identified is very high. In an urban environment, random assaults like Matt's are much easier to get away with.
But within the city, high traffic areas are generally much safer than low-traffic areas. I grew up just west of Broadway, and while there was a fairly high incidence of non-violent crime (prostitutes on the street corner, drug dealing in the SRO hotels down by Riverside), I wasn't very worried about getting mugged or otherwise assaulted. There was a subway stop at our corner and even in the wee small hours of the morning, a steady stream of foot traffic. The more people are walking through your neighborhood, the riskier assault becomes for the assaulter.
But of course, this is not foolproof--in the eighties a classmate of mine was pulled off Park Avenue at 10 am and beaten up by a gang of kids. And while quantity of cops matters, quality of policing also matters. Cops walking on the street are more effective than cops riding around in cars, and cops that are responsible for reducing crime in their area are more effective than cops who are responsible for taking reports of whatever crimes occur. And people have to identify more with the police than with the criminals--it doesn't do you any good to have high traffic if criminals know that bystanders won't bother to assist the police in identifying the culprits, either because they think the police are thugs, or because they think the police are so incompetent that nothing will happen even if they take the time to assist the police investigation. In 1991, when I was eighteen, my parents forbid me to take the train home from my part-time job, which ended at midnight--and they were right. In 2005, after Bill Bratton's policing revolution, neither of us thought anything of my hopping on the train at 3 in the morning.
More foot traffic in DC would definitely help, but even so, in my experience DC policing still looks a lot like the failed models that helped New York descend into chaos in the 1970s--all the police are in cars, and they're more reactive than proactive. That's something that density would definitely help (it's easier to have foot patrols if they don't have to cover several square miles), but so would a change in both the funding and the incentives of the MPD.