The Density/Transportation Connection

Density is an issue that has stirred great controversy and heated emotions in many a community. While some advocate added density as the fix to our transportation woes and as an essential element of vibrant, full-service communities, others see it as a threat to the quality of community life with few compensating rewards.

A slew of past studies have found that spread out development patterns, where jobs, housing, shopping, and recreation opportunities are widely separated, result in a lot of people spending a lot of time in cars.  Intuitively, compact development provides more options–people can often walk or bike the shorter distances to work, shopping, or recreation opportunities. Higher densities supply more potential riders for transit, which, in turn, can support more frequent transit service and a greater variety of routes. So, compact development means more transportation options, fewer miles driven, and less time stuck in traffic, which translates into reduced energy use and greenhouse gas emissions.

Almost all the studies show a correlation between density and/or compact development and the number of vehicle miles traveled (VMT), the number of trips taken, and the means of travel. But study results have varied widely concerning how important density is in addressing transportation woes when separated from other aspects of compact development.  Most studies do not distinguish the specific effect of density versus other factors commonly associated with compact development, such as accessibility to employment centers, the mix of uses, distance to transit, and street network characteristics (such as block size and continuity of street network).

A number of recent studies, including several “meta-analyses” that pull together results from a large number of studies, shine a brighter light on the relative role of density compared to other variables in reducing vehicle travel. This composite picture can help communities develop more effective, targeted land use patterns and density to achieve efficient transportation while maintaining important community qualities.

While other attributes of compact development may be more important than density in reducing vehicle travel, threshold levels of density in the vicinity of transit stations and/or in major transit corridors are needed if you want frequent transit service.

I've highlighted a number of interesting study findings below. Traffic conditions are influenced by a complex interaction of market forces and population characteristics in addition to density and land use factors, so the findings can’t just be plugged into any and every community. It’s important to look at unique local conditions. Even so, they provide a useful framework.

• Compact land use patterns result in fewer vehicle miles traveled, in terms of both the length and the number of vehicle trips, than do sprawling land use patterns.
• The most effective way to minimize driving is to locate development in major centers with easy access to many jobs, shopping, and other attractions.
• The next most important land use factors are mixed land use and street network design (such as block size and frequency of intersections), which are more important than density.
• Walking is most strongly related to land use mix, intersection frequency, and the number of destinations (including jobs) within walking distance. Bus and train use are equally related to proximity to transit and street network design variables, with land use mix of secondary importance.
• Several of the most comprehensive studies suggest that doubling residential density across a metropolitan area may reduce VMT by 5 to 12 percent.  If coupled with higher employment densities, a mix of uses, and good street network design, the combined effect can be quite large.
• Three recent studies that focused on greenhouse gas reduction estimate reductions in VMT and energy consumption of between 8 and 18 percent when compact development makes up 60 percent or more of all future development between now and 2050.
• Market studies show that the demand for compact development is growing, and a significant amount of existing development will be replaced by 2050 (by as much as two-thirds, by one expert's models).
• Increased density may more likely have a significant effect in places where there is a demand for higher density housing and when combined with other more important land use factors, such as locations near jobs.
• A dense suburban development, when far from transit, employment centers, and stores,  probably won’t stimulate significant walking and transit use compared to those in city centers. Transit stops, highway corridors, and inner suburbs offer      better opportunities.
• Analysis suggests that light-rail systems need around 30 people per gross acre within a half mile of stations, and heavy-rail systems need 45 people per gross acre within the half-mile radius, to place them in the top one-quarter of cost-effective rail investments in the U.S. Densities within a corridor that are supportive of local bus service typically range from 4 units per net acre for 20 bus/day service, 7 units per net acre for 40 bus/day service, and 15 units per net acre for 120 bus/day service.