AZ DOT report finds compact development reduces VMT without increasing localized congestion

By Eric Sundquist

A first-ever analysis of land-use and transportation demand in Arizona contradicts fears that compact, “smart growth” development, while beneficial in moderating demand, will increase localized congestion.

The report, produced for Arizona DOT in March, also suggests that traditional travel demand modeling is outmoded, unable to reflect land use effects on demand, and it disputes notions that compact development is inequitable and costly.

Land Use and Traffic Congestion cites many previous studies and new findings from Arizona that suggest compact, mixed-use development can reduce the need for motor-vehicle travel, by shortening trips and making non-auto modes more feasible. The effect is particularly notable for non-work trips, which make up the bulk of household travel.

The report addresses concerns that, even with system-wide reductions in demand, compact development could cause substantial congestion in particular areas. Not so, it concludes:

“Fears about compact, mixed-use development leading to intolerable traffic congestion do not appear to be substantiated by what is seen in practice. While increasing development activity of any type will generate additional traffic, the nature and design and adequacy of the supporting infrastructure are critical variables in determining the severity of resulting traffic.”

One important variable, the report says, is a street grid with short blocks that allow for short local travel distances. A local street grid can also relieve the traffic-carrying burden of arterials. This sort of infrastructure is needed in Arizona, where Phoenix and Tuscon have one-mile arterial grids.

“While this may serve the purpose of regional vehicle movement, it is the opposite of what is needed to accommodate pedestrian and vehicle traffic in mixed-use activity areas. When development is more concentrated, the network must be similarly articulated to maximize circulation and access to specific destinations, with parallel streets being no more than one-quarter mile apart.”

The report recommends improving travel-demand modeling to take into account land-use-based solutions. Traditional modeling does not discount trip numbers and length for compact, mixed use development, and its zone-based approach does not lend itself to modeling short trips suitable for non-auto modes. The report compared modeling predictions in the Phoenix area to actual traffic volume, finding that the modeling significantly overestimated traffic volume, especially in dense areas.

SSTI has assisted Delaware DOT with just such modeling improvements. DelDOT has moved to tax parcel-based modeling, allowing short trips to be better modeled and demonstrating internal capture where land uses are mixed and connections high. Now it is deploying LUTSAM, a scenario modeling and simulation tool that enables the department to analyze various land-use arrangements in particular areas – for example large-lot homes on cul-de-sacs, versus a more compact neighborhood with better connections and neighborhood-serving commercial uses. Results can be displayed in a 3-D simulation, making travel demand implications clear to land use authorities. SSTI invites interested DOTs and agencies to contact us for more information.

The wide-ranging ADOT report addresses other issues relevant to the land-use/transportation connection, including:

  • The notion that compact development requires heavy land-use regulation. In fact, the report says, it appears that existing zoning tilts the balance away from market demand for compact development, toward single-use, low-density development.
  • The complaint that compact development is inequitable because it raises housing prices (the higher prices being an indicator of the market mismatch just noted). In fact, if the cost of providing infrastructure to low-density housing and the greater personal cost of transportation are considered, compact development is the cost-efficient option.
  • Elected officials are more confident they know the costs and benefits of compact land use than are planners and other professionals. Despite this confidence, the report suggests education on the issue is important. “It is fairly clear that the average citizen fears density as the enemy, linking it to a variety of ills including traffic congestion, influx of new and possibly different demographics, noise, crime, and loss of safety and security. These concerns are clearly passed on to elected officials, who are keenly tuned to the public’s sensitivities about change, and these trepidations are passed on to planners and administrators resulting in conservative plans, codes, and procedures. Reversing this stigma will not be easy and will only be achieved by demonstrating the inherent benefits of the alternative approach and giving assurances through example that many of the concerns are unwarranted.”

Eric Sundquist is Managing Director at SSTI. He can be reached at Erics@ssti.us.