How dense is dense enough for smart growth?
Smart-growth advocates in Minnesota’s Twin Cities (average density: 1,800 people per square mile) say the Twin Cities area needs to be as dense as Portland (average density: 3,000 people per square mile).
Smart-growth advocates in Portland say Portland needs to be as dense as Los Angeles (average density: 5,600 per square mile). Smart-growth advocates in Los Angeles say Los Angeles needs to be as dense as Chicago (average density: 12,000 people per square mile).
Smart-growth advocates in Chicago say Chicago should be as dense as San Francisco (average density: 16,000 people per square mile). Smart-growth advocates in San Francisco want the city to be even denser still.
Where will it all end?
What’s your preferred density?
Fortunately for a puzzled nation, the Sierra Club has answered this question in a Web page that calculates the “environmental impacts of density”: http://www.sierraclub.org/sprawl/community/enviroimpacts.asp. You enter your preferred density in households per acre along with your idea of average automobile fuel efficiency (miles per gallon) and the price of gasoline. The Sierra Club then projects the environmental and social impacts of your density. For comparison, it includes the environmental and social impacts of the “efficient urban density” and a “sprawl density.”
On June 19, the Web site indicated that the efficient urban density is 500 households per acre. Since the U.S. has an average of 2.4 people per household, this represents 1,200 people per acre, or 768,000 people per square mile. Manhattan, at only 52,000 people per square mile, has a ways to go before it reaches smart-growth perfection.
Demographer Wendell Cox (http://www.demographia.com) points out that the Sierra Club’s “efficient urban density” is denser that the densest parts of Mumbai (Bombay) and Hong Kong. In fact, Cox adds, at this density everyone in the United States could fit into an area a little larger than Portland, Oregon’s urban-growth boundary.
Perhaps in response to Cox’s comments, on June 20 the Sierra Club modified the Web page to compare four different densities:
- “Dense urban,” which is 400 households per acre, slightly less than the “efficient urban” of the day before;
- “Efficient urban,” which is “only” 100 households per acre;
- “Efficient suburban,” which is 10 households per acre; and
- “Sprawl,” which the Sierra Club defines as one household per acre.
Twelve times Manhattan
Even the “efficient” urban density is incredibly dense compared to what most people are used to. One hundred households per acre is 153,600 people per square mile, or three times the density of Manhattan. The “dense urban” 400 households per acre is 614,400 people per square mile–nearly 12 times as dense as Manhattan.
Of course, 400 housing units could fit on an acre in a 20-story building, each story containing 20 apartments averaging a little over 2,000 square feet. Add four or five more stories for shops and offices and some underground parking and you have a nice dense city of 25-story buildings. But few cities have large areas of 25-story apartment/mixed-use buildings. Even in Manhattan, most residences are in four- to 10-story buildings.
But the so-called efficient density of 100 households per acre is scary enough. At that density, the population of the United States could fit in the Los Angeles urbanized area–call it “Sierra Club City.” The entire population of the world would fit into the state of Virginia.
The Sierra Club assumes that all or nearly all office and retail establishments would be mixed in with the residential areas. It calculates that the efficient density would provide 48 “shopping opportunities per acre,” whatever that means, as opposed to just 0.65 opportunities per acre at sprawl densities.
To be fair, some additional land would be needed for factories, warehouses, and other industrial areas. But that would still leave most of the rest of the world for farms, parks, and wilderness, which of course is the Sierra Club’s goal.
How much land would Sierra Club City save? At the present time, U.S. cities, towns, and other urbanized areas occupy about 109,000 square miles of land. Roughly one-third of that is industrial. If Sierra Club City replaced the other two-thirds, that would allow the restoration of about 72,000 square miles of land to farms, forests, or nature preserves. That sounds like a lot, but it amounts to just 2 percent of the land area of the United States.
Of course, if Sierra Club City stacks industry in 25-story buildings too, then up to 3 percent of the nation’s land could revert from urban uses to open space. Whether saving 2 or 3 percent is worthwhile depends on the costs of high-density living.
Dramatically increased congestion
Start with congestion. The Sierra Club says people living in sprawl densities of one household per acre would drive more than 32,000 miles per year. But at the efficient densities, says the club, they would drive only 7,600 miles per year, less than a fourth as much. Of course, with 100 times as many people per square mile, that means total driving would be nearly 24 times more per square mile in Sierra Club City than in sprawl.
Urban Americans drive an average of 40,000 miles per square mile of urbanized land each day. As the highest-density urban area, Los Angeles also has the highest density of driving: 124,000 miles per square mile of land.
But residents in the Sierra Club’s efficient city would drive 1.3 million miles per day for each square mile of residential area. That’s 33 times more than in the average urban area, and 10 times more than in Los Angeles.
Curiously, no matter what the population density, the Sierra Club model dedicates 93 acres of land per square mile to roads and sidewalks. It is not clear whether this is included in, or in addition to, household per-acre densities. Assuming it is in addition, then it represents about 13 percent of the land area, which is about right for suburban areas but far lower than the percentage of high-density urban areas that is devoted to streets.
Ninety-three acres divided into 12-foot lanes with three-foot sidewalks represents about 50 lane-miles of roadway. To handle 1.3 million miles of vehicle travel per day, each lane-mile of road would have to carry 1,100 vehicles per hour, 24 hours a day, seven days a week. Non-freeway arterial lanes can handle just that number, while freeway lanes can handle twice this amount, and lesser streets (collectors and locals) do less.
Thus, Sierra Club City will be about as congested as Manhattan during rush hour–only congestion in Sierra Club city will be 24/7. The Sierra Club’s “dense urban” density of 400 households per acre, of course, will be even worse: The club predicts people will drive more miles per lane mile than the best freeways can handle.
Increased air, water pollution
With this much traffic concentrated in a small area, Sierra Club City will be one of the most polluted cities in the history of humanity. The city will produce less pollution per capita than in a sprawling city, but it will be far more concentrated.
Typically, the Sierra Club model crudely assumes pollution is directly proportional to fuel consumption. The Web site asks you to enter the average fuel efficiency you imagine for your city and it calculates the pounds of hydrocarbons (volatile organic compounds), nitrogen oxides, particulates (PM10), and carbon dioxide that will be produced. The Sierra Club doesn’t estimate carbon monoxide emissions, but autos tend to produce about 10 times as much CO as hydrocarbons.
The Sierra Club presents pollution in terms of pounds emitted per household each year. For carbon dioxide, which is implicated in global warming, total emissions may be crucial. But for many of the other pollutants, the problem is not total emissions but the concentration of emissions. Heavy concentrations of carbon monoxide and particulates, for example, pose extremely serious health risks, while low levels can be tolerated and ignored by most people.
Sierra Club City will produce extremely dangerous levels of these toxic pollutants. Based on the Sierra Club’s numbers, automobiles in sprawl emit about 100 pounds of hydrocarbons, a half-ton of carbon monoxide, and 680 tons of particulates per square mile per day. But Sierra Club City will produce 2,340 pounds of hydrocarbons, nearly 12 tons of carbon monoxide, and more than 16,000 tons of particulates per square mile per day.
Since the Sierra Club model assumes emissions are proportional to fuel consumption, all pollutants will be 24 times as concentrated in Sierra Club City than in sprawl. But in fact, cars pollute more in stop-and-go traffic, and the extra congestion in Sierra Club City will make it even more polluted than the Sierra Club numbers indicate.
The Sierra Club model also claims high densities will produce less water pollution. “When more than 20% of the watershed is paved over and developed,” says the club, “water pollution skyrockets.” But a suburban neighborhood of one household per acre will have much less than 20 percent of its area paved over, while an urban jungle of 100 households per acre will be nearly all paved over. Thus, we can expect the most pollution from the urban area.
No less driving?
The Sierra Club’s model is optimistic about the effects of density on driving. The model assumes that, at any density, doubling density reduces per-capita driving by 20 percent. This is about four times greater than can be observed by looking at U.S. urban areas. However, it is about the amount generated by studies that looked at driving habits of residents of individual neighborhoods of various densities.
The problem with such studies is they usually fail to control for family size, income level, and other factors that influence driving. A disproportionate number of people in high-density areas are either poor or have no children. They either can’t afford to drive or have decided they would prefer to use transit. But this doesn’t mean forcing a middle-class family of four to live in high densities will lead them to drive significantly less.
Even without this error, Sierra Club City–a permanently congested and dangerously polluted area–will be far less attractive to most Americans than sprawl. But smart-growth advocates will nevertheless press for increasing densities in virtually every U.S. city.