Estimating Vehicle Emissions in Response to Urban Sprawl

October 29, 2008 By: Tim Dolney

Highway Emissions

The highway emissions interface where commuting characteristics from the HAP (green) to the 11 major employment centers in northest Ohio (red) are defined. The final output provides the amount of emissions generated in tons per year at the local and highway levels.

Inputs defined by the user and outputs generated by VERTUS are summarized; users can save results as a text file for further analysis.

Inputs defined locally are transferred to the highway emissions interface and cannot be changed. The only inputs that can be altered on the highway emissions interface are the commuting characteristics. The model calculates the percentage of home-work journeys traveling to each employment center, according to the chosen township. The user can interactively change the percentage traveling to each employment center. With these inputs defined, highway emissions are estimated by calculating the distance from the HAP to each employment center. The total distance is applied to each emission type to estimate the amount generated per year via highway travel as a result of new housing developments.

Assembling Data

The Year 2000 U.S. Census Transportation Planning Package (CTPP) was used to define all commuting characteristics: the number of workers, workers per household, non-commuters, PCs, LDVs, and commuting characteristics to employment centers. CTPP is a set of special tabulations from the decennial census designed for transportation planners. The data provide links among social and demographic characteristics of persons, households, and workers to journey-to-work characteristics, such as travel time and travel mode to work. It is the only Census product that summarizes data by place of work and provides information on the travel flow between home and work. The U.S. Census Bureau provides the data free of charge and includes all the tools needed to extract information from the CD in a standard format (Microsoft Excel or PDF). The inclusion of CTPP allows VERTUS to be applied to other geographic areas, as the commuting characteristics can easily be tabulated.

Locations of employment were established according to traffic analysis zones (TAZs). They are the only data source concerning intra-metropolitan employment. Similar to census tracts, which are population-based, TAZs are employment-based. The primary difference between the two is that census tracts are designed to have approximately equal population within their boundaries, whereas TAZs are designed to have equal work-based trips. For the region of study, employment density and total employment threshold values were established. Those TAZs that met the threshold values were considered employment centers. CTPP data were also used to establish these parameters.

To estimate emissions during the home-work journey, an emissions factor was established for each type of emission. This is the amount of each pollutant emitted into the air per distance traveled, usually expressed as gram per mile (g/mi). Emission factors for VERTUS were obtained from a U.S. Environmental Protection Agency (EPA) fact sheet entitled "Average Annual Emission and Fuel consumption for Gasoline-Fueled Passenger Cars and Light Trucks." This document provides the emission rates as grams per mile driven for PCs and LDVs. The estimates are derived from the EPA's MOBILE 6.2 computer model and are current as of July 2004, making them suitable for use in obtaining first-order approximations of vehicle emissions.