Estimating Vehicle Emissions in Response to Urban Sprawl

October 29, 2008 By: Tim Dolney

The modern landscape has been substantially altered by the rapid expansion of housing developments into rural and suburban areas along city edges. These developments are characterized by several land-use patterns that usually occur in unison: low density, random movement, single-use zoning, "leapfrogging," and automobile dependency. Growth spawns growth, and commercial and industrial developments are quick to follow residential expansion. Over the years, this movement has swelled into the phenomenon termed urban sprawl.

Urban sprawl is an undesirable form of development: open green space, critical nature areas, and in prime farmlands are overrun by housing and pavement for additional roads. Additionally, new housing developments are located in areas that are the least accessible from the established urbanized areas. Thus, there is an increasing geographic separation between housing developments and job markets. The population is traveling ever-greater distances to overcome this spatial division. Longer travel distances translate to increasing vehicle emissions, which negatively impact the environment. The spatial dimension of "dirty air," once primarily an urban problem, now extends to suburban and rural areas.

With the proliferation of GIS technology, urban planners can now simulate urban growth with various assumed trends to estimate how different growth levels could impact the environment. If a certain simulated development is deemed undesirable, alternate scenarios can be generated by simulating growth with different trends and assumptions. Ultimately, this process can help decision makers to find the best-case scenario that may encourage change in current polices and land use.

A number of computer simulation models have been developed to assist decision makers in quantifying land use effects brought about by future developments, but each falls short of extending the relationship to vehicle emissions. Thus, planners would benefit from a tool that estimates increases in vehicle emissions resulting from new housing developments to better understand the spatial dimension of dirty air. Planners recognize that increased growth in housing along the city edge will increase the amount of vehicle emissions generated, but to what extent? Additionally, how will these increases vary spatially?

Modeling Emissions

The VERTUS (Vehicle Emissions Related To Urban Sprawl) model was developed to help urban planners quantify the amount of vehicle emissions generated, given a particular level of urban sprawl at the township level. The model only calculates emissions generated during "home-work journey" travel — travel from one's regular place of residence to one's regular place of work. The population participates in this type of trip on a daily basis.

The initial development of VERTUS was based on data from Geauga County, Ohio.

The model determines the number of home-work journeys, according to demographic and social characteristics of the township where the development occurred. Emissions are calculated at two geographic scales: vehicle emissions generated by local traffic, and those due to travels on highways. The division between local emissions and highway emissions reflects the assumption that commuters start their journey from their home, travel through local streets, and eventually gain access to a highway for faster travel to their place of work. Thus, local emissions are those generated during the commute from home to highway access points (HAPs), while highway emissions are generated between the start of the highway and the place of work. This differentiation allows users to see emissions generated locally at the township level and the regional level across multiple counties. The model provides output for the following vehicle emissions: hydrocarbons (HC), nitrogen oxides (NOx), particulate matter 10 (PM10), carbon monoxide (CO), and carbon dioxide (CO2).

VERTUS was developed using data from Geauga County, Ohio, to test the feasibility of the approach. Geauga is a sprawling county that has absorbed a large portion of suburbanites moving away from the City of Cleveland.