"The evidence indicates that Portland's policies to steer growth into more compact, mixed use development have paid off, not only in revitalizing the downtown and many of its neighborhoods, but also in changing travel behavior, the primary concern of this study," the report says.
Portland-area residents drive 17 percent less than the U.S. average, because denser development provides shorter travel distances and higher mass transit use reduces driving. From 1993 to 2003, the region's mass transit ridership grew 55 percent and housing density grew 18 percent, while the population grew 21 percent.
Now, I believe that dense living is easier on the environment and I have reported on some careful economic studies that have shown this to be true (at least in the form of apartment towers). And I am also pretty sure that dense living cuts vehicle trips for the very simple reason that you don't have to go as far when all the amenities are around you. But as I have said many times you have to be careful about making causal statements from correlations. In this case it is impossible to deal appropriately with self-selection.
Here is an example - a statement that says something like "based on the evidence from Portland if all people in the US lived in equally dense areas we would see an overall drop in vehicle miles travelled of similar proportion to Portland." But this would be incorrect. Many of the people that live in inner SE Portland have chosen to live there because that area matches their lifestyle preferences. They are already predisposed to walking, riding a bike or taking mass transit. If you were to take the average Happy Valley resident and move them into the inner SE, you would likely see a decrease in driving but of lesser proportion to the overall average difference between Portland and the US.
I say this not with the intent of debunking the report or its findings, for it seems rather careful about this precise problem. Rather, I say this with future users of this report in mind. For if we are going to get serious about promoting density, we need to be exceptionally clear and accurate about its social welfare benefits. Overstating them is not useful in the long run (to this academic, at least - I suspect my more politically minded friends would disagree).