AZ DOT: Dense urbanism cuts traffic congestion
Compact, mixed-use development reduces automobile use and disperses traffic, according to a study of the Phoenix area.
Land Use and Traffic Congestion (pdf), recently released by the Arizona Department of Transportation, could be a landmark transportation study. Researchers found that automobile use is significantly lower in urban neighborhoods compared to suburban neighborhoods, confirming previous studies. The bigger news is that smart growth, with its shorter travel distances and connected pattern of streets, can also cut traffic congestion. Connected street networks avoid the conventional suburban problem of funneling traffic onto a few crowded arterial roads.
The study Prepared by J. Richard Kuzmyak for the Arizona Department of Transportation Research Center determined that urban corridors had considerably less congestion despite densities that were many times higher than a comparison suburban corridor:
The reasons were traced to better mix of uses, particularly retail share, which led to shorter trips, more transit and nonmotorized travel, and fewer vehicle miles of travel (VMT). Also recognized was the importance of a secondary street grid in the three urban areas, which allows for better channeling of traffic and enables walking.
The study further explained:
Higher-density and more mixed-use areas such as South Scottsdale, Tempe, and East Phoenix were found to behave significantly differently from lower-density/less mixed-use areas like Glendale, Gilbert, and North Scottsdale. ... The implications of these differences may be seen in various travel measures, including:
- Vehicle Ownership: 1.55 vs. 1.92.
- Average Trip Lengths: 7.4 vs. 10.7 miles for home-based work trips; 2.7 vs. 4.3 for home-based shopping trips; 4.4 vs. 5.2 for home-based other trips; and 4.6 vs. 5.3 for nonhome-based trips.
- Per Capita VMT: 10.5 miles per day vs. 15.4 miles per day.
The researchers looked further at four areas — three of them compact urban and one conventionally suburban — to examine traffic congestion.
Examining traffic congestion levels in the four study areas revealed some important and perhaps counterintuitive results in terms of the density/traffic question. ... This analysis found that despite having some of the highest densities in the region, the case study areas of Scottsdale, Tempe, and the North Central Avenue corridor had surprisingly good traffic flow, whereas the one suburban corridor — West Bell Road — showed significantly higher levels of congestion despite considerably lower development densities.
Credit for this seeming contradiction was attributed to both the nature of development in the higher-density areas and the existence of a street grid. The street grid in the three urban settings was found to handle and dissipate traffic demands rather efficiently. In addition to Phoenix’s ubiquitous one mile major arterial grid, these urban areas also contained a substructure of minor arterials and streets, on a spacing of one-quarter to one-eighth of a mile. ...
Efforts to increase density in new plans and projects in a way that also emphasizes mix of uses, pedestrian friendly design, and transit serviceability could reduce VMT generation rates in new developments by up to 45 percent. ... These findings buttress those of preceding tasks that concluded that properly designed developments, even if they entail significantly higher densities than currently exist, may be implemented without inordinate fears of crippling traffic congestion.
