IN THIS ISSUE:
Neighborhood Energy Challenge Spurs Investments in Efficiency
Mixed-Use Transit Village Leads Redevelopment Efforts
Arizona Study Suggests Dense, Mixed-Use Development Patterns Reduce VMT and Congestion
Grantee Spotlight: Planning for a Sustainable Michigan Street Corridor
Arizona Study Suggests Dense, Mixed-Use Development Patterns Reduce VMT and Congestion
The Scottsdale Road corridor includes a dense mix of
land uses that the study suggests helps to reduce traffic congestion.
A recent study released by the Arizona Department of Transportation demonstrates that higher-density, mixed land-use patterns reduce dependency on automobiles, traffic congestion, and residents’ vehicle miles traveled (VMT). The study, Land Use and Traffic Congestion, examined whether applying Smart Growth principles could reduce traffic congestion in the Phoenix metropolitan area. The study found that higher density and mixed-use developments designed to be walkable and accessible to regional transit could reduce residents’ VMT by an average of 25 percent.
Background
Previous studies have shown that as residential density increases, the level of automobile ownership decreases, leading to fewer automotive trips, higher rates of walking and transit use, and reductions in VMT. However, residential density is not the only factor affecting residents’ transportation choices. Other factors such as a mix of land uses, design choices that improve neighborhood walkability, and access to regional jobs (particularly by public transit) also reduce both the number of vehicle trips and overall VMT. These concepts — Density, Diversity, Design, and Destinations — are collectively referred to as the “4 Ds” of compact mixed-use development. Compact mixed land-use development has also been shown to reduce nonwork vehicle travel — the trips that account for most households’ daily living needs. Compact design and integrated land use increase the number of travel options and reduce the number and length of automotive trips. The 4 Ds were the main theme used to determine the effect of land use on traffic congestion in this study.
The Effect of the 4 Ds on Traffic Congestion in Arizona
Researchers examined the impact of the 4 Ds of compact mixed-use development on traffic congestion using existing data, modeling tools, and geographic information system (GIS) datasets from the Maricopa Association of Governments (MAG), which include modeling data and analysis, demographic information by census tract and block, and street-level land use and infrastructure data. To accurately identify the relationships among land use, design, and traffic congestion, the researchers used 2001 regional household travel surveys combined with GIS land-use datasets to develop a series of regression models that quantified the 4 Ds as well as the relationship between travel behavior and traveler demographic characteristics. These models showed that as the 4 Ds of land use increase, vehicle ownership and VMT rates proportionately decline.
For greater clarity, the study team divided the MAG region into 17 jurisdictional areas and used household-level data to examine differences in travel in relation to the 4 Ds. The team found that areas with higher density development and greater land-use mix or a high entropy index value had lower rates of vehicle ownership per household, shorter average trips, and lower per capita VMT (see table 1). Entropy is a calculation of the relative mixture of different types of land uses based on their proportions in an identified geographic area, indexed on a scale of 0 to 1. A value of 1 represents perfect mix and balance between each of the land use categories (single-family residential, multifamily residential, commercial/retail, office, institutional, industrial, transportation, and open space).
Table 1. Comparison of Land Use Characteristics and Impacts on Travel, MAG Region
Land Use Characteristics |
Higher Density/Mixed-Use |
Lower Density/Mixed-Use |
Residential density |
6.14–6.94 households/acre |
2.86–3.61 households/acre |
Land-use mix (entropy) |
0.53 |
0.30 |
Retail/service opportunities within walking distance1 |
42.4 |
15.4 |
Jobs access via transit |
59,000 |
27,000 |
Travel Characteristics |
|
|
Vehicle ownership per household |
1.55 |
1.92 |
Average trip lengths in miles |
7.4 (home-based work) |
10.7 (home-based work) |
Per capita VMT |
10.5 miles/day |
15.4 miles/day |
The study also modeled traffic congestion using an envisioned scenario in which the 17 jurisdictions had optimum land-use patterns: 10 residential households per acre, an entropy index value of 1.0, the number of walk opportunities increased to 100, and jobs access increased to between the minimum and maximum for each jurisdiction. Researchers calculated that the jurisdictions could reduce VMT by 20 to 45 percent, with an average reduction of 25 percent.
Researchers then sought to determine whether an increase in land use intensity would result in higher localized traffic congestion. Using surveys of local officials, researchers selected four corridors in the Phoenix metropolitan area perceived to be highly congested. Three corridors were urban, with varying degrees of land use mixture and design. A fourth, control corridor was a low-density corridor with multiple residential developments separated by individual entrances and commercial development along the main arterial corridor. Roadway congestion along the corridors in the urban settings was found to be lower than that found in the control corridor. Urban densities and land use mixes, combined with transit service, a well-organized street grid in the area, and regional accessibility through transit, provided the best opportunities to ultimately reduce traffic congestion. The 4 Ds contributed to reduced automobile ownership, handled pass-through traffic more effectively, and reduced VMT because many of the nonwork trips could be made more efficiently or through alternate means.
Recommendations for Arizona and Other Regions
The findings suggest that municipalities and metropolitan planning organizations wanting to decrease VMT and traffic congestion in their jurisdictions should consider improving land use efficiency, transit options, and the connectivity and walkability of local streets and sidewalks as well as rehabilitating or upgrading facilities in older, developed areas. The authors recommend that the benefits of compact mixed-use development found in this study serve as the basis for educational efforts, including planning and visioning exercises with the general public, the business community, and elected officials. The study also indicated that better modeling systems would prove beneficial to assessing the impact of these developments on local transportation networks.
Richard Kuzmyak, the study team lead, says that the study’s results can be applied in other states and localities. “If these relationships can be observed in Arizona, almost anywhere else should do even better, since the Arizona densities are still pretty low, even in the most urban places. With higher densities, better mix and better regional transit accessibility, [other regions] should be even more efficient in reducing their own auto dependency.”
