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Martin Laplante

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Tue, 16 Dec 2008

Density as Persons Plus Jobs Per Unit Area

I've seen this formula appear more often in urban plans lately: density targets expressed as the linear combination of beds and desks (or whatever kind of work station). Is there any merit to these targets? Is the sum of the two an appropriate measure of density for the purpose of reducing land use, infrastructure costs, and GHG emissions?

As I and others have written about before, there are minimum and maximum residential densities beyond which various environmental indicators get worse. Minimizing total land use in a growing city is best done with relatively uniform residential density, while minimizing travel is best done with an inverted density curve: higher density further away from the centre. Economics makes these density profiles difficult to achieve.

Employment density, on the other hand, is a different kettle of fish. Location and density of employment can be used for two environmental purposes. One is to put destinations on high-intensity transit lines. Use of transit is highly influenced by the density of destinations such as employment and education, and transportation demand management works very well with employment, while the thrifty land requirements of high-efficiency transit are a good fit for very high density of employment. For interdependent businesses, with bicycle couriers and restaurants serving to connect different businesses, a lot potential vehicle trips are avoided by having a very high density of commercial and office uses. Separation of uses is handy there because having residential uses in such locations takes away potential for that wonderful synergy, without reducing vehicle kilometres travelled relative to having those same people live further away down the transit line.

For those types of areas to function, the weight of the employment parameter within the formula of the density target must be higher than the weight of the residential parameter, since they are more useful in achieving the objective. A higher weight lets separation of uses, which is good in very high density, be used to achieve targets.

In areas of lower density, the problem is different. There, it must be remembered that over 75% of all kilometres driven are for non-commuting trips, for instance for shopping or leisure. Lower density areas have the opposite problem from above, too much separation of uses. The destinations to which people should be able to walk or to drive only a short distance are too far away from the houses. Getting retail, personal service, and entertainment jobs to blend in with the low to medium density residential areas should be a priority.

It's relatively difficult for a developer and for zoning laws to create this blend, particularly when they have density targets to meet. The solution is to put a premium on these uses, to make them have a higher weight than residential uses. So in this case again, employment should have a higher weight in the formula, this time in order to overcome separation of uses.

For residential areas around transit stations, there is a tendency to pack in the apartment buildings. This does not increase ridership, quite the contrary. Households that live in apartment buildings are not influenced by the proximity to transit and density of uses as much as are households with children who live in houses. This is your most important target market. Putting community-serving uses aimed at households with children near transit is a winning formula. Walking your kids to school on the way to the transit station or picking up a few items at the store on the walk home makes transit more attractive and by combining trips also reduces the need to drive around as much. How do you get those uses near transit? You guessed it, put a premium on those forms of employment in the area around transit stations.

So you end up in most cases with a higher weight for employment than for housing. That makes sense. There are more people than there are jobs unless you run all children and retirees out of town.

Residential density is like a balloon: push too hard in one area and it will swell up somewhere else. Location-specific density targets may not have the desired effect, you have to constrain everywhere at once. Employment, on the other hand, does not react negatively to high local density targets. Quite the contrary. And unlike residential uses, there is no density that I know of beyond which the carbon footprint of the concrete construction overwhelms the presumed land and travel savings.

If you need to set local density targets, don't use the same weight for residential and employment density. Have a minimum residential density everywhere that will support transit at a cost you can afford. Don't allow residential density to go much beyond the density of neighbourhoods whose household size is the same as the entire metropolitan area's average household size. If environment is a factor, do not go beyond what can be built with wood frame construction. For employment density, go wild near major transit hubs and use TDM. Near transit in primarily residential areas, require mixing of retail and residential, and pay attention to the uses, not just the density.


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