November 13, 2012

Tacoma-Pierce County Nonattainment Area

Motor Vehicle Emissions Budget (MVEB)

Based on EPA’s final Transportation Conformity Rule Restructuring Amendments (77 FR 14979, March 

14, 2012), the on-road emissions for the most recent year of clean data, 2011, was selected as the motor 

vehicle emissions budget to satisfy regional conformity test requirements for the Tacoma-Pierce County 

nonattainment area. The MVEB is based on a 2011 winter weekday inventory of onroad mobile 

emissions of particulate matter less than 2.5 micrometers in diameter (PM2.5) and nitrogen oxides (NOx). 

This will be the first motor vehicle emissions budget in the State of Washington developed using EPA’s 

Motor Vehicle Emission simulator, or MOVES. 

To create the MVEB, EPA's guidance for using MOVES for transportation conformity1 was followed within 

the constraints of local data availability. MOVES is significantly more complex than its predecessor, 

MOBILE. The MVEB calculation method is designed to be as simple as possible, yet without sacrificing 

the accuracy of the emissions estimates. The basic calculation equation is shown below, followed by a 

more detailed description of the method and data sources.

Basic Calculation

E = (M x Fgmi) + (V x Fgpveh)

where E = emissions in grams

M = vehicle miles traveled (VMT)

V = vehicle population

Fgmi = emission factor in grams/mile

Fgpveh = emission factor in grams/vehicle

Vehicle Miles Traveled

PSRC estimated 2011 average daily weekday VMT for the nonattainment area. Per agreement with the 

consultation partners, the 2011 estimates were interpolated from 2008 and 2015 travel demand model 

outputs.

VMT is estimated from PSRC’s travel demand model for individual roadway links for five time periods of

the day: 

• Morning = 6am-9am;

• Midday = 9am-3pm;

• Afternoon = 3pm-6pm;

• Evening = 6pm-10pm;

• Nighttime = 10pm-6am

Data for each link and time period includes:

• Length

• Facility type

• Functional classification (for most links)

• Congested speed

• VMT

 1 Using MOVES to Prepare Emission Inventories in State Implementation Plans and Transportation Conformity: Technical Guidance 

for MOVES2010, 2010a and 2010b. Transportation and Climate Division, Office of Transportation and Air Quality, U.S. 

Environmental Protection Agency. EPA-420-B-12-028. April 2012
November 13, 2012

Emission Factors

EPA's MOVES model was used to generate emission factors for PM2.5 and NOx in grams per mile and 

grams per vehicle. The model was run at the county scale with local data for many of the inputs. Winter 

weekday conditions were estimated using the base year 2008 inventory median (50th percentile) 

temperature profile for peak PM2.5 conditions. A detailed documentation of the MOVES inputs and 

methodology has been prepared by the Department of Ecology. This document includes explanations, 

instructions and information on the source of data used for the MOVES inputs, such as roadway 

classifications and travel distribution, meteorology, vehicle fleet data, fuel parameters, etc. This 

document will be used by PSRC as guidance for future conformity analyses.

The grams per mile emission factors were output by hour, road type, and speed bin. The grams per 

vehicle emission factors were output by hour. The grams per mile emission factors were averaged over 

each of the five time periods in PSRC’s travel demand model. This resulted in one emission factor per 

road type, time period, and speed bin. The grams per vehicle emission factors were summed over each 

of the five time periods, resulting in one emission factor per time period.

Emissions Estimation

Once the MOVES emission factors were prepared, the factors were applied to PSRC’s VMT data to 

estimate total emissions. The grams per mile emission factors represent emissions that occur while a 

vehicle is in regular operation. The grams per vehicle emission factors represent emissions that occur 

when a vehicle is stationary (for example, start exhaust, extended idling, etc.).

Each roadway link from PSRC’s travel demand model was classified according to the MOVES road types: 

Rural Restricted, Rural Unrestricted, Urban Restricted, or Urban Unrestricted. Based on the congested 

speed, each roadway link was assigned to the closest corresponding MOVES speed bin. The MOVES 

gram per mile emission factors by speed bin and road type were then applied to PSRC’s VMT by speed 

and road type, for each of the five time periods. The emissions for each time period were then summed 

for an estimate of total daily emissions.

The grams per vehicle emission factors were applied to the estimated vehicle population within the 

nonattainment area. This estimate was derived from the vehicle population within Pierce County, 

adjusted by the percentage of the human population in the nonattainment area. The estimated emissions 

for each time period were then summed for an estimate of total daily emissions.

The two component emissions estimates were summed, and the resulting MVEB is identified below:

Pollutant Emissions (pounds per winter 

day)

PM2.5 3,002

NOx 71,598