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Wed, 11 Jul 2007
I have just been looking at the LEED for Neighborhood Development Pilot Rating System , part of a project that I heartily endorse. LEED, Leadership in Energy and Environmental Design, is a certification for green buildings. But the environmental impact of buildings is just as much about the context in which the building is located as about the building itself.
The LEED for Neighborhood Development Rating System integrates the principles of smart growth, urbanism, and green building into a standard for neighborhood design, high standards for environmentally responsible development. It is a collaboration between the U.S. Green Building Council, the Congress for the New Urbanism, and the Natural Resources Defense Council. The project is in a pilot stage and the list of registered pilot projects will be posted soon.
Although most of it is quite reasonable, with points for various aspects of environmental protection, reduced driving, access to services and open space, walkability as well as energy efficiency of buildings and infrastructure, there is a significant problem in some of the indicators, specifically between Neighborhood Pattern and Design (NPD) credits #1 and #3.
NPD credit 1 gives 0-7 points depending on the density (see table). It implies that the environmental benefits of density, and specifically residential density, are linear. That is to say it implies that the greater the density of a neighborhood the greater the environmental benefit. This is probably not the case in most cities. There is actually an optimal residential density above which environmental benefits decrease. I would gladly compare the evidence supporting each of these positions.
One of the factors that is responsible for this decrease in benefits at higher density is partially, but not sufficiently, compensated by the diversity index. Higher densities tend to be dominated by multifamily forms. On a regional scale too a large proportion of multifamily forms in one area, and in particular highrise apartment buildings, is as bad as too large a proportion of single-family forms. In fact they are two sides of the same coin - when many areas have an above-average proportion of one form, mathematically other areas will be below average, and economics and sociology will also exacerbate the pattern.
NPD credit 3, diversity in housing sizes and types is on a 3-point scale, using a Simpson Diversity Index, typically used to measure biodiversity. But it's pretty well the same as the Gibbs-Martin index normally used for this type of measure, but it indicates that the people who set up this LEED rating system majored in biology.
The density credit is only partially compensated by the diversity index. Having very high density made up of apartments only can earn 7 points at the risk of giving up the 3 diversity points.
The solution to this problem is to reduce the maximum score for residential density and increase the one for diversity. Higher diversity tends to imply higher density in any case, so the points for good compact mixed neighborhoods where everyone can live will be maximized and only the more sterile socio-demographic ghettoes will score a bit lower.
By the way, employment density does not have the same problem, although that is a matter of some debate and is probably related to transit infrastructure. Having some good employment destinations on transit lines is a necessary condition to letting the rest of the city increase its transit modal share.