What causes suburban sprawl, zoning or highways?  Opponents of new and expanded highways claim that highways do.  Yet state highway agencies have no authority to control land use; the local governments do.  What state highway agencies do have the responsibility of disclosing are the indirect and cumulative impacts of proposed highway projects reported in federal environmental impact statements (EISs).

So how do state highway agencies identify such impacts in EISs?  The following discussion provides three examples of how PB has approached indirect cumulative impact (ICI) analyses in three very different situations.

I-93 Improvements Project, Salem to Manchester, New Hampshire DOT

The project includes:

• Widening the four-lane highway to eight lanes for 29 km (18 miles) in New Hampshire just north of the Massachusetts border      
• Expanding existing interchanges and reserving the right of way for a potential rail corridor.

When I-93 was first constructed in the 1970s, suburban sprawl in southern New Hampshire was the immediate result of new accessibility.  Some 30 years later there was concern that:      

• Increasing roadway capacity would stimulate further suburban sprawl within and north of the corridor.      
• Many of the rural communities in that region did not have the planning capacity to effectively control additional growth and protect the environment. 

Because of the controversy surrounding the project, EPA asked that PB be added to NHDOT's consulting team to assemble an "expert panel" and guide them through a process that would allocate future growth, and to prepare the secondary land use chapter of the EIS.

We selected a 16-person expert panel with members from the fields of real estate, land use law, public policy, local and regional planning, and environmental public interest groups.  They were asked to allocate population and employment growth in 2020 without and with the project in a 29-municipality study area in both states, New Hampshire and Massachusetts (Figure 1).  PB prepared a briefing book containing data and information about the region, and provided additional material that panel members requested.  Three public sessions were held so panel members could ask questions and discuss their allocations, which were made independent of one another. 

We calculated a "blended average" of the panel's range of allocations to calculate the amount of land needed to accommodate such growth.  For example, the panel allocated nearly 41,000 more people with the build alternative than with the no build alternative by 2020.  Based on our conversion methodology, this amount of residential growth would require more than 19,000 acres.  With this information and calculations for employment growth, we could determine if there was enough upland area available that was suitable for development so that significant natural resources were not adversely impacted by this future growth.  The locations of new growth were identified using local future land use maps or zoning.  The results showed that impacts to significant environmental resources would be minimal.

While the panel's allocations were not challenged, NHDOT agreed to acquire extensive wetlands and provide funds to support land use planning at the state and affected local levels as "mitigation."  The Record of Decision was signed in 2005 and design of the project began.

Mountain View Corridor Project, Salt Lake and Utah Counties, Utah DOT

The Mountain View Corridor project proposes:      

The project addresses existing congestion in the transportation network by providing an alternate to I-15, the only other north/south route along the Wasatch Front, and addresses the significant current and projected future growth in the region.  Our scope of work included preparation of the induced growth chapter of the EIS. 

Our analysis built on:      

• The land use analysis performed previously by Envision Utah, a public/private partnership formed to guide the development of a growth strategy aimed at protecting Utah's environment, economy and quality of life, and the Growth Choices process in the Mountain View Corridor
• Local master plans that adopted the Growth Choices regional vision.  

An expert panel was not needed in this case.  The recent high quality master planning stimulated in part by the Envision Utah Growth Choices process had resulted in a regional consensus for transportation and land use.

Instead, we interviewed municipal planning departments and large developers with major land holdings to:

• Understand development trends and local land use plans      
• Learn how development of sensitive lands, such as wetlands, might change with or without the project.

Developers were asked to tell about their plans and if their plans would change with or without the new facility.  Communities were also asked if their long-term plans area were based on the Mountain View Corridor project.  Questions were asked involving whether or not land uses and densities would change around the transit stations.  A summary report of the information was gathered and made part of the administrative record. 

To analyze land use impacts and development over time, land use layers, natural resources layers, and satellite imagery were developed in ArcView GIS (geographic information system).  The Mountain View Corridor design files were also put into ArcView GIS database and combined with the environmental layers.  In GIS, land use impact boundaries were created around the interchanges and transit stations to show the areas of influence on the land uses.  Based on current research:      

• Interchanges have the potential to impact land uses within an 8-km (5-mile) radius.       
• Transit stops have an area of influence of a 0.8-km (0.5-mile) radius (Figure 2).  

Sensitive lands were located and documented within the identified areas of influence.  Development trends over time were evaluated by using National Aeronautics and Space Administration (NASA) satellite imagery that covered the study area.  The imagery showed development increasing in the study area especially since the late 1990s when the economy improved along the Wasatch Front.  Using the imagery, the area of developed land was calculated for the given year.  These images helped to show that patterns of growth from long before the Mountain View Corridor project was ever considered.  Maps and graphics were developed to show the findings.  

During the interviews, members of the communities and land developers indicated that growth and the projected land uses along the corridor were not expected to change with or without the project.  Development is increasing and will continue with the strong housing market.  Many of the large areas of undeveloped property are currently is some phase of the site planning and development process.  In addition, environmental impacts from indirect impacts were determined to be minimal, except potentially in floodplain areas in Utah County and other wetland areas in Utah County and Salt Lake City. 

During the DEIS process tolling was considered as a way to finance the project.  Moreover, additional alternative alignments are being considered in the southern end of the study area (October, 2006).  The DEIS is scheduled to be issued in early 2007.

Prairie Parkway Project, Kane, Kendall and Grundy Counties, Illinois DOT

The project area, which is located approximately 96 km (60 miles) west of Chicago, is experiencing growing regional development demand and increasing traffic congestion.  The project, which would improve regional mobility and access to regional jobs, address local road system deficiencies and improve safety, included:      

• A new 4-lane limited access freeway in Grundy, Kendall and Kane counties between I-88 and I-80      
• The widening of IL-47 for about 20 km (12 miles) north of I-80. 

The scope of the DEIS addressed both indirect and cumulative impacts.  The indirect impacts were associated with induced land development resulting from the improved accessibility and mobility provided by the project.  The cumulative impacts focused on the total impact of the project, induced development, and other reasonably foreseeable development that would occur with or without the project.

Using the expanded Chicago Area Transportation Study (CATS) travel demand model, we were able to determine that the build alternative would attract as many as 36,000 additional people and 46,000 additional jobs to the project study area from other parts of the CATS area.  This is an approximate 30 percent increase over the no build alternative.  Further, this would convert some 49, 000 acres of largely farmland to suburban development-a 12 percent increase over the no build alternative. 

We used several tools and techniques in the ICI analysis, including composite land use plans, spatial development trend maps, interviews with land use decision makers, and induced impact area analysis.

Land use plans for existing and future land uses were developed using parcel data for all three counties as the base file in GIS.  Existing land use maps were developed using aerial photography and zoning maps to classify the land use.  Future land use maps used any available comprehensive plans to classify the land use.  All of the plans used different land use categories, so we used Northeastern Illinois Planning Commission (NIPC) land use codes as a way to combine all of the land use categories into one consistent land use for the region.

Satellite imagery was geographically referenced into a GIS base and shapes were drawn around the areas of development for the years 1976, 2001 and 2004.  An exhibit was created using the shapes for each year to show the spatial development trends in the project area.

An induced impact analysis (IIA) area was created for the project using the assumption that the induced impact area for residential uses extends approximately 8 km (5 miles) from the project interchanges (Figure 3).  An 8-km (5-mile) circle buffer was created at each project interchange and then the appropriate traffic analysis zones (TAZs) were selected to form the IIA.  The IIA was then subdivided into four subareas, and:      

• An estimate of the land area that would be needed to accommodate the additional projected growth for each subarea with and without the project was calculated.      
• Additional population growth was converted to number of housing units needed. 
• The number of housing units was applied to an average residential density allowed by planning and zoning in the subareas. 

A similar procedure was used to calculate the land area needed to support the growth in jobs.  These jobs were assumed to be in four employment categories with different space requirements:      

• Governmental/service/other office space using jobs
• Manufacturing     
• Transportation, communications, utilities and warehousing
• Retail. 

The average space needed per employee was factored in to determine the building footprint.  In addition, the amount of land needed for parking, internal circulation, required open space and landscaped space, and vacancy rates were calculated using averages based on national industry standards and local zoning.  The DEIS was issued in November 2006.

What Causes Sprawl?

So what causes sprawl, zoning or highways?  They both do, in different ways.  Local jurisdictions have land use planning and zoning controls that can shape future development patterns. New and widened highways will channel growth to within 5 miles (8 km) of an interchange; but they normally, by themselves, do not attract additional growth to what is already projected for an area.