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Air Quality Planning for Transportation Officials

Mobile Source Emissions Inventory

One of the first steps in the development of a SIP is the preparation of an emissions inventory. An emissions inventory is based on the actual or modeled emissions from mobile, stationary, area, and biogenic sources for the relevant pollutant(s) within the nonattainment area. The inventory of mobile emissions is further categorized by on-road and non-road emissions. The emissions inventory helps define the extent of the pollution problem, relative to the air quality standards in current and future years.

The emissions inventory is the basis for all control measures and tracking of future progress. It is critical that the inventory be as complete and correct as possible. Periodic emissions inventories must be developed every three years. The on-road mobile source portion of the inventory should be developed in very close cooperation with State and local transportation agencies.

Travel and transportation factors are a key part of on-road mobile source emissions inventory development. Transportation agencies should work with their State and local air quality agencies to insure that the most up-to-date and accurate transportation data is not only used, but is interpreted correctly, and that travel data or projections are truly representative of the local area.

Emission estimates for on-road mobile sources are usually based on the combination of two fundamental measures: vehicle miles of travel (VMT) and emissions rates (the rate of pollutants emitted in the course of travel based on vehicle speed and other factors). Both measures reflect complex patterns of behavior and incorporate a variety of local assumptions. The EPA and the U.S. DOT have developed a series of tools and models to estimate the emissions produced by on-road mobile sources. Emissions estimates are influenced by the mix of vehicle types and ages on the highway, vehicle speeds, fuel characteristics, engine technology, operating conditions, meteorological conditions, and many other factors.

However, these factors are not static and technology is continually evolving which leads to changes in vehicle performance. Changes in fuel prices and economic conditions can affect vehicle sales and travel patterns. A substantial and on-going effort is required to accurately quantify these factors and their influence on travel patterns and emission levels.

EPA has historically relied on the Highway Performance Monitoring System (HPMS) as a source of VMT estimates. HPMS is a state-specific coordinated data base that is updated annually by State DOTs and MPOs. HPMS includes sample section data, which provides VMT through a systematic, stratified, random sampling process. However, HPMS is only statistically valid on a statewide basis. To accurately assess regional travel, local transportation data and input is vital to supplement HPMS data.

Transportation agencies play a vital role in developing the estimates of travel in on-road mobile source emissions inventories. Statewide and national data may be supplemented to improve estimates of existing travel. As emission reductions decisions are based on the existing contributions of all sources, an accurate estimate of transportation emissions must be developed to ensure that transportation factors are considered appropriately.

Updated: 8/4/2015
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