The Methods Behind the Estimates

The Texas Transportation Institute (TTI) is at the forefront in developing methods and procedures to apply the Environmental Protection Agency’s (EPAs) MOBILE emissions rate program to estimate on-road mobile source emissions in Texas.

The Clean Air Act (CAA) requires the EPA to set limits on the amount of certain pollutants allowed in the air. These “criteria pollutants” are ozone, particulate matter, carbon monoxide, lead, nitrogen dioxide and sulfur dioxide. For transportation in Texas, the primary pollutants of concern are oxides of nitrogen (NOx), carbon monoxide (CO) and volatile organic compounds (VOC). In addition, particulate matter, or PM, is a concern for some areas.

When the level of any of these pollutants exceeds the standard in an area, EPA designates that area in nonattainment (of the standard) for that particular pollutant. Once an area has exceeded the standard, the state is required to develop a state implementation plan (SIP) for those areas to implement, maintain and enforce these standards.

There are four areas in Texas, which are currently designated in nonattainment for one or more criteria pollutants. Another five are “near” nonattainment, meaning that they are close to having a problem, even though they have not been officially designated as violating a standard.

These nonattainment areas are subject to certain requirements. They must demonstrate that the emissions resulting from future actions (as documented in their official transportation plan) will not exceed the area’s emissions budget, a process known as “demonstrating conformity.” If conformity cannot be demonstrated by a specified deadline, or if the plan expires before a new one is adopted, the area enters into a “conformity lapse.” For areas in a conformity lapse, federal transportation funds cannot be spent on capacity enhancing projects (though certain safety, transit and air quality projects may go forward). There is, therefore, a critical need to accurately estimate on-road mobile emissions.

TTI researchers in the Transportation Modeling Program have developed procedures that estimate emissions for vehicles that travel on public roads based on EPA’s emissions rate program called MOBILE. MOBILE produces emission rates for 28 vehicle types, operating on four roadway types, under a range of conditions (temperature, humidity, emissions control programs, etc.). TTI’s approach allows the application of EPA’s MOBILE routines at an extremely detailed level. (see Key Activity Measures below for additional information)

“We apply an activity measure like vehicle mile traveled and speed for every hour of the day, for every link of the roadway network, by road type, by each of the 28 vehicle types,” says Dennis Perkinson, program manager of the Transportation Modeling Program at TTI. “This very disaggregate process is extremely sensitive to changes in the transportation system, whether these changes are due to area growth, changes in travel patterns, improvements in vehicle technology, or emissions reduction measures.”

Primary Air Pollutants

• NOx — is the generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts. These compounds are involved in the creation of ozone. NOx forms when fuels are burned at high temperatures.
• CO — is a colorless, odorless, poisonous gas. A product of incomplete burning of hydrocarbon-based fuels, carbon monoxide consists of a carbon atom and an oxygen atom linked together.
• VOCs — are precursors of ground-level ozone. Since all organic compounds contain carbon, volatile organic compounds are often called hydrocarbons (HC).
• PM — includes dust, dirt, soot and smoke. It is identified by size with a different standard for each. PM10 (particles less than 10 microns) and PM2.5 (particles less than 2.5 microns) are the sizes of interest for transportation emission purposes.

Key Activity Measures

Vehicle miles traveled (VMT) is a key activity measure for emissions estimation. All else being equal (which it rarely is), the more VMT you have, the more emissions you have. For a given area, VMT is estimated by year, seasonal day type (e.g., summer weekday), hour of the day and direction (peak or off-peak), for each roadway type (e.g., freeway, arterial, collector, etc.).

Emissions can vary dramatically by speed. In general, speed is a function of the ratio of roadway volume to capacity. Speed is estimated for each link (roadway segment) in the transportation system of an area.

Emissions also vary by vehicle type. The MOBILE program produces emissions rates for 28 different types of vehicles categorized by fuel type and gross vehicle weight rating. The mix of these 28 vehicle types is estimated by roadway type and time of day for each area.