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Kaid Benfield’s Blog

Considering the role of density in smart growth

Kaid Benfield

Posted April 28, 2009

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Later this week, I am going to be participating with my friend David Dixon and marketing whiz Laurie Volk in a seminar on development density at the annual meeting of the American Institute of Architects.  Our session is titled "Making Density Work:  Dollars, Design, and Sustainability - the Nuts and Bolts of Successful Density."  I think Laurie is "Dollars," David is "Design," and I am supposed to be "Sustainability."

Which is fine with me.  So, anyway, I have been pondering what it is that I want to say about density and sustainability.  The research certainly suggests that it makes a difference.  The graph below compares residential units per acre to annual driving rates, as measured in Chicago, Los Angeles, and San Francisco:

  driving goes down as density goes up (by: John Holtzclaw, et al.) 

Note that, as you move from low density sprawl on the left to high density on the right, driving goes down a lot.  This graph has become somewhat of a classic in the field, representing research by John Holtzclaw, my NRDC colleague David Goldstein, and their partners at the Center for Neighborhood Technology.

What I find most intriguing, though, is not the drop in driving rates per se but how much they drop as we change densities.  Look at the shape of the curve.  We get the greatest increments of performance improvement at the lower end of the scale, as we move from large-lot sprawl to smaller-lot single family homes and townhouses.  By the time we reach typical multi-family densities, the curve really flattens out.

So increased density is associated with reduced driving, but we don't need major changes to realize significant benefits.  This is especially the case when you realize that it is average density that matters:  we can still have some large lots, as long as they are mixed with smaller ones and apartments to produce a performance-changing result.

Here's a graph comparing neighborhoods in Baltimore, very similar in shape to the one above, but looking only at the lower end of the density scale, from large lots on the far left to townhouse-sized lots on the far right:

  the pattern in metro Baltimore (by: Baltimore Regional Council) 

Note that a change in density from ½-acre lots to 8 single-family houses per acre (see photo just below) is associated with a 33 percent reduction in driving in Baltimore. 

Every now and then I read or hear someone whine that smart growth advocates "want to cram everyone into high-rises at transit stations," when that is far from the truth.  There are certainly highly urban locations where high-rises are appropriate, but mostly I would just like to see us allocate land more efficiently for single-family houses.

two views of 8 houses per acre (from Massachusetts Smart Growth/Energy Toolkit)But there's something else going on with density research.  Neighborhood density works best when combined with synergistic elements such as location, transit service, connected streets, and mixed land uses. 

In fact, without some of these elements, it won't work very well at all.  Density appears to work well in the research, but almost all of the good density studies have been done at macro levels in places, or in areas that include places, that have been built for some time.  And, in America, the density tends to be in the older parts of our cities and suburbs, most of it from the pre-sprawl era.  Those are also the places that are centrally located, have smaller, well-connected blocks, shops and services nearby, and decent transit service.  So the density studies are actually measuring more than density.

The most sophisticated work that I have seen that attempts to separate some of these elements from each other comes from the highly regarded transportation consultancy Fehr and Peers, as represented on the graph below.  For each of the four factors represented, a doubling of intensity will reduce vehicle travel, on average, by the percentages shown:

  location matters most (research by Fehr & Peers, graph by me) 

Note that the most important factor in influencing travel behavior is location, or more precisely something called regional accessibility, the distance from a given place (say, a residence) to other important places (jobs, shopping, entertainment, other residents) in the same metro region.  This measure is highly favorable to central locations.  This is why the GIS-derived map of the distribution of carbon emissions from transportation in metro Portland that I have shown before indicates such large differences between central and outlying locations in their carbon footprint.

20 houses per acre in central Sacramento (By; Mogavero Notestine Architects)Or, as John Holtzclaw once put it to me in email, a high-rise in the middle of a cornfield isn't going to work for travel efficiency.  My take is that we should put the density in town if we want results, and add walkable blocks and a diverse mix of uses.

Incidentally, I have reason to believe that the Fehr and Peers formulas are in the process of being updated, and that the new version will also tease out the effect of transit service among the factors.  Transit will emerge as the second most influential factor after location.

Really, all this just confirms something I have always believed:  smart growth is not a principle, but a set of principles.  The best results come when they are applied together, and that will be my message on Friday.

Kaid Benfield writes (almost) daily about community, development, and the environment.  For more posts, see his blog's home page.

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Steve DavisApr 28 2009 11:00 AM

Well said, Kaid. It reminds me of the Green Dividend report about Portland — which showed that it wasn't just about everyone biking or taking the train everywhere — it was about the relatively high density, connectivity of the streets, and the fact that even car trips tended to be shorter than most major areas. And just like gas mileage improvements realizing the most efficiency going from 8 mpg to 15 mpg rather than 50 to 100, focusing efforts on the kinds of changes required to get away from the sort of widepread land use that results in 30 minute car trips for absolutely everything would be a great start.

Laying down a street grid, mixing uses, and raising the densities even incrementally can have a huge impact — and result in places where people want to live and have a high quality of life.

Kaid @ NRDCApr 28 2009 11:22 AM

Thanks, Steve. Smart growth's role in shortening driving trips is really underappreciated, in my opinion.

Tony ChaviraApr 28 2009 12:24 PM

Great to see that the three of you will be doing another presentation together! I'll be sure to fight my way through the ridiculous crowd to attend!

Kaid @ NRDCApr 28 2009 12:26 PM

Be sure and say hello, Tony. I remember we met very briefly before, but that was before we got to know each other's writing.

Dave ReidApr 28 2009 01:04 PM

It is not only that trips are shortened but that less trips are needed to be made via automobile. Further I'd add that density brings other advantages beyond just reducing the need to drive.

Kaid @ NRDCApr 28 2009 01:29 PM

Without a doubt, Dave. But I think the substitution of walking and transit for auto trips is better understood (and assumed) in our community than is the shortening of driving trips. At NRDC, we have demonstrated a significant reduction in VMT from a smart location even when there has been no reduction in auto trips at all.

As for your other point, you are absolutely right, as I have argued in here before.

Jim NoonanApr 28 2009 07:07 PM

Good article. Two points...

First is that it is good to see that the benefits are on the left side of the curve. One thing I have told many people is that it would be difficult for the average home purchaser to really see the difference between a house on a 2 acre lot and a house on a 1/4 acre lot. The only real difference is the drop in land consumed and the amount of yard one has to mow and fertilize. Once you are at a 1/4 acre, it is easier to imagine a town house or other (hopefully well designed) development nearby and perhaps even the kind of localized retail that used to be common in urban and small town neighborhoods.

Second point, I understand the point about density in a corn field.. But remember that density is usually associated with public water and sewer and therefore with growth around small towns. Few people build at any real density independent of a traditional community. Once again, you would see generally smaller scale development in terms of the total number of houses, at the same time that new homes are built using smaller and manageable infrastructure systems. That kind of growth also supports commercial and retail activity and growth in areas that need it, but might not have seen any for many years.

Kaid @ NRDCApr 28 2009 07:58 PM

Jim, I couldn't agree more. Thanks for stopping by.

SRM in DCApr 29 2009 10:50 AM

This comment came to me (Kaid) via email and, with her permission, I am sharing it here because I think it is an important perspective:

"You said once, a long time ago, that if you could figure out how to find a smart growth model that makes me (a lover of classic suburban sprawling neighborhoods) happy, you could 'solve sprawl.' The model described in your article -- regular single-family homes and townhouses, but on smaller lots and in neighborhoods that "are centrally located, have smaller, well-connected blocks, shops and services nearby, and decent transit service" -- makes me happy, especially when combined with the many very appealing pictures you have collected of such neighborhoods.

"I don't think it's much of an exaggeration to say that almost every person I have ever known is turned off by the image of living in 'high rises at transit stations.' I also think that most of those people would be happy to live in the communities described in your article."

Pete MurphyApr 30 2009 12:35 PM

Mr. Benfield, you are correct that increasing population density will have a dramatic effect on driving behavior. But I think that you're failing to consider a very unpleasant side effect. The reduction in driving will translate into a reduction in the per capita consumption of vehicles. You may wonder, "what's wrong with that?" The problem is that high population densities result in low per capita consumption of virtually everything. While this may be a wonderful thing from an environmentalist's perspective, you must remember that per capita consumption and per capita employment are inextricably linked. Anything that drives down per capita consumption - like increasing population density - will fuel rising unemployment and poverty.

I should introduce myself. I am the author of a book titled "Five Short Blasts: A New Economic Theory Exposes The Fatal Flaw in Globalization and Its Consequences for America." To make a long story short, my theory is that, as population density rises beyond some optimum level, per capita consumption of products begins to decline out of the need to conserve space. People who live in crowded conditions simply don’t have enough space to use and store many products. This declining per capita consumption, especially in the face of rising productivity, inevitably yields rising unemployment and poverty.

Consider a nation like Japan - a modern, prosperous nation like the U.S. but different in one important regard: it's ten times as densely populated. As a result, the per capita consumption of dwelling space - in other words, the size of their homes - is less than a third that of the average American's, not because they like living in tiny, cramped quarters but because there is no room for anything larger. As a result, their per capita consumption of all products involved in building, furnishing and maintaining such dwellings is proportionately affected. Thus, their per capita employment in these industries is very low.

Their per capita consumption of vehicles is similarly affected, just as your research into the relationship between population density and driving behavior predicts.

The problem is that their per capita consumption of everything is affected in the same way, to a greater or lesser extent, with the exception of a few things like food and clothing. This means that their per capita employment in producing goods and services for domestic consumption is very low, making them utterly dependent on manufacturing for export in order to sustain their bloated labor force.

Ultimately, advocating for an increase in population density is a prescription for economic disaster. The only way for all people to sustainably enjoy a high standard of living is through a reduction in our population.

I'd be very pleased to provide you with a complimentary copy of my book. Just contact me by E-mail with a shipping address and I'll put a copy in the mail to you today. You won't be disappointed. No discussion of the relationship between population density and per capita consumption is complete with considering the far-reaching ramifications for the economy.

If you'd like, you can learn more about this important new economic theory by visiting either of my web sites at or where you can read the preface and join in the blog discussion.

Pete Murphy
Author, "Five Short Blasts"

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