Jose Gomez-Ibanez on greater urban density and less driving

Professor Jose Gomez-Ibanez’s study quantified the amount by which people living closer together – and driving less – can reduce the CO2 emissions now known to contribute to global climate change.

Jose Gomez-Ibanez leads research on land use and transportation at Harvard. He led a 2009 study by the National Research Council quantifying the way in which people living closer together – and driving less – can reduce the CO2 emissions now known to contribute to global climate change. Professor Gomez-Ibanez spoke more about the study with EarthSky.

Jose Gomez-Ibanez: The research shows that more compact, mixed-use development does reduce driving. A doubling in the density of development will reduce driving from some five to twelve percent per household.

In other words, double the population in an area – develop shopping and other services there – and driving does decrease, according to the study.

Jose Gomez-Ibanez: We’re not talking about moving people from single-family homes into multi-story apartment buildings, necessarily. In many metropolitan areas, the average density of new developments is one or a half-acre per dwelling unit. If you just had one-eighth of an acre per dwelling unit, you could still have single-family detached housing, but on much less land than we are using already.

So, according to Gomez-Ibanez, smaller house lots do reduce commutes, and greenhouse gas emissions, just by shortening the distance between where you’re coming from and where you’re going. He mentioned Portland, Oregon as a city that’s already been using denser urban development to reduce vehicle use. In Portland, city planners have implemented an urban growth boundary around the metropolitan area and tried to keep development contained within that boundary, rather than spreading across the countryside. The average lot size in Portland for single-family houses is about one eighth of an acre, in contrast to about one acre or more in other metropolitan areas. Gomez-Ibanez said that, as a result, transportation researchers believe driving hours in Portland have declined. He added, however, that it might not be a fast shift in other U.S. cities to denser residential areas.

Jose Gomez-Ibanez: The problem is that we are not going to knock down and rebuild existing housing stock to a higher level of density. It’s too valuable to do that. So most of the effect comes from the new stock. And it takes a while before the new stock is an appreciable portion of the total stock.

For the report by the National Research Council, Gomez-Ibanez reviewed close to a hundred studies on the driving patterns of people in the U.S.

Jose Gomez-Ibanez: These studies measure the amount of vehicle miles traveled by relying on smog check annual inspections, where in states like California and Massachusetts, the odometers are read every year when the vehicle is inspected. And so that would be the basic data for many of these studies, about how much a household was traveling.

Gomez-Ibanez compared the residential densities of different cities.

Jose Gomez-Ibanez: Obviously, if you compare the New York metropolitan area, or the Boston metropolitan area, with say the Atlanta metropolitan area or the Phoenix metropolitan area, the densities are very different, as are their level of transit service and the like. And that’s reflected in different quantities of travel by motor vehicle in the metropolitan areas.

Jorge Salazar