More density equals less driving: just an urban legend?

Vehicle miles traveled only decline substantially at density levels similar to Chicago’s.Photo: Doc Searls

Cross-posted from Per Square Mile.

Pushing high-density living may seem like a good way to get people out of their cars — saving them money, curbing emissions, and reducing oil dependence — but densification may not be a silver bullet, according to one recent study. The authors dug into the National Household Transportation Survey to examine per household vehicle ownership rates, vehicle miles traveled (VMT), and fuel consumption. While the results are by no means comprehensive or conclusive, they suggest that only the steepest increases in density could reduce car usage.

Despite a correlation between density and car usage, other factors seem to play more important roles. Density is responsible for a fraction of annual VMT; increasing density by 1,000 housing units per square mile — a titanic leap, given that the average household is 2.6 people — reduces VMT by just 1,171 miles, all else being equal. Since the average one-driver household in the study tacks on 10,100 miles per year, that represents just over an 11 percent drop in annual mileage.

If you look at the numbers another way, the case for density-reducing car usage looks even more tenuous. VMT only really declines substantially at the highest housing density — over 5,000 units per square mile, or about the same as Chicago. To halve the VMT of the highest mileage households, you would need to increase housing density in those areas 20 to 100 times.

The inflexibility of our automobile usage boils down to a few factors, with work being the most important. The more workers in a household, the more drivers, and the more drivers, the more miles. A one-driver household, as noted above, tallies 10,100 miles per year; a two-driver household racks up 18,800 miles; three drivers, 33,900; four drivers, 47,700.^[1] We are, by and large, beholden to our cars because we are beholden to our jobs. After that, driving increases as a result of income (richer people drive more), number of children (more and larger cars), education (higher education means more cars), and people’s life stage (households with older children have more cars).

While higher housing density doesn’t seem to reduce VMT, it does drive down fuel consumption. Households in the 50 to 250 houses per square mile range use 1,650 gallons of fuel annually, the most of any group. Every other group uses far less fuel. In the big cities, fuel usage drops to 690 gallons per household per year.^[2] The reason? People with the space to use pickup trucks, SUVs, and vans tend to buy them more than people who live and drive on tighter city streets — they typically drive smaller, more fuel-efficient vehicles. Yet this trend could be changing as we speak. Small car purchases have been increasing across the country, and anecdotally at least, I can confirm that large pickup trucks are harder than ever to sell these days.

One of the main arguments behind higher density living is that it will reduce our carbon footprint. While density may be a better long-term solution, right now the most expeditious approach is to increase fuel economy. Rebuilding neighborhoods will take decades. In that time, most people will buy at least a handful of new cars, primarily for commuting to work. It would be great if everyone had access to mass transit, but for many, mass transit isn’t just a poor option, it isn’t an option at all. Those who do travel by bus or train today may only be a job change away from having to drive. Modern life demands mobility, and few things are better at providing that than the automobile.

Endnotes:

[1] The increase from one to two drivers probably reflects some combining of trips by couples or roommates. The sharp increase from two to three drivers is probably the result of a family’s children driving to school or work.

[2] The lone outlier is areas below 50 houses per square mile, where households use 1,200 gallons per year. They probably have fewer nearby destinations, and so stay home more often.