This post appeared in a previous blog and is here for posterity’s sake.

night time, originally uploaded by seth_holladay
New York Times has an article about where traffic comes from in the NYC region, and their hook is that it doesn’t come from the suburbs. In Traffic’s Jam, Who’s Driving May Be Surprising:

It’s a common enough thought among city drivers inching through traffic: Everyone around me came from the suburbs, making my life miserable. But it’s wrong, because more than half the drivers who crowd into Manhattan each workday come from the five boroughs.
…
Census data show that more city residents than suburbanites drive to work in Manhattan every day, according to Mr. Schaller. He estimated that 263,000 people in 19 counties in and around New York City drive regularly to jobs in Manhattan below 60th Street. Of those, 53 percent, or 141,000, live in the five boroughs, Mr. Schaller said. The greatest numbers are from Queens, with 51,300, and Brooklyn, with 33,400. About 23,900 auto commuters live in Manhattan, while 17,400 are from the Bronx and 15,200 from Staten Island. The suburban area with the most auto commuters to Manhattan is Nassau County, with 22,091 people driving to work in the borough, followed by Bergen County, with 19,975.

Outer Queens, Brooklyn, and the Bronx might as well be called the suburbs. Have you ever been out at Far Rockaway or past Shea? It might as well be Levittown, being that outer Queens and Brooklyn were the last to be developed in the post-WWII years.
It would be interesting to cross-reference driving habits by density, because I am sure they are directly related, and the lack (or lessening) of density of outer Brooklyn and Queens means owning a car becomes more necessary.
Below is a population maps of NYC with population density >10K resident per sq. mi (3861 per sq km):

population maps of NYC with population density >10K resident per sq. mi (3861 per sq km) by fake is the new real
And here is a color-coded population density map by Lamont-Doherty Earth Observatory at Columbia:

New York City population density 1990
You can see east of the Belt Parkway/Van Wyck corridors, the density fall rapidly. All still in Queens, mind you, but vastly different in scale than the rest of the city. The change in scale and lack of direct, and fast, public transportation makes personal vehicles a no-brainer, and often a necessity.
So, I think there is a good case to be made that a majority of traffic to the Manhattan core comes from what we would all call the “suburbs” – regardless if it happens to fall within the five boroughs. Either way, it doesn’t really matter: there are a ton of cars coming into the Manhattan core from outside the core, to the tune of 239,100 vehicles; 91% of vehicles below 60th Street are not from Manhattan. This is why public policy about parking and congestion pricing will be front and center for the foreseeable future.
Food for thought, I guess.