New Urbanists and their friends are constantly bringing up density, and since the ones I know aren’t generally all that dense, I have to assume they’re referring to population distribution. In which case, they might be interested in this:
University of California researchers David Brownstone and Thomas Golob have looked at the relationship between residential density and driving habits, and concluded that: “Comparing two California households that are similar in all respects except residential density, a lower density of 1,000 housing units per square mile … implies an increase of 1,200 miles driven per year … and 65 more gallons of fuel used per household.
Which further implies that they’re getting less than 18.5 mpg on those miles, but let that pass for the moment.
The numbers get worse as you get out into McMansionland:
[G]oing from a neighborhood designed on the post-war, upper middle class ideal — your own home on 2 private acres — to the reality in many of the Northwest’s more compact urban areas — a mixture of single family homes with small yards, together with some multifamily housing, with an average of around 10 housing units per acre — you increase density by just over 6,000 housing units per acre.
Now I know from acres: I live on a quarter of one. (0.26, actually.) No way are you going to get 1500 housing units in my yard. There being 640 acres to the square mile, I’m going to assume he meant “just over 6,000 housing units per square mile,” which is plausible.
Doesn’t change his point, though:
And, according to the numbers that these authors have crunched, living in a compact neighborhood rather than a sprawling exurb would lead to a decline in gasoline consumption of … wait for it … 395 gallons of gasoline per household per year!
That’s a lot of gas. By comparison, the average resident of the Northwest states consumes about 390 gallons per year; so living in a denser neighborhood does as much to reduce your driving as having one fewer person in your household.
If I had one fewer person in my household, it would be down to zero, which would indeed reduce driving.
My own neighborhood is postwar, if not exactly upper-middle-class; it’s maybe a tick or two above average for this particular ZIP code. Curious, I ran the numbers, and there are 23,209 housing units in 73112, an area which covers 7.7 square miles. We’re running, therefore, about 3000 units per square mile, about five per acre.
In the summer of 2001, the Sierra Club made some noise about a level of density called “Efficient Urban,” which called for 500 units per acre, or about 125 in my yard. People who could actually count noted that this exceeded the density of Kolkata — you may remember it as “Calcutta” — by a factor of seven. They have since, and by “since” I mean “within 48 hours of the original outburst,” recanted.
And 395 gallons of gas, now that I think about it, would propel Gwendolyn about 8700 miles, which is two World Tours or about 14 months’ worth around town. Better than the 18.5 mpg those Californians are presumably getting.