Motor Vehicle Emission Budget Methodology 1

Tacoma-Pierce County PM2.5 Redesignation Request and Maintenance Plan

Appendix C. Motor Vehicle Emissions Budget

Methodology

A motor vehicle emissions budget (MVEB) is that portion of the total allowable emissions in the 

SIP that is allocated to onroad mobile sources, such as cars, trucks, and buses. It is the level of 

onroad emissions that the area can have and still meet the SIP's goals. Budgets are established in 

the applicable SIP as part of the air quality planning process.

In order to demonstrate transportation conformity, projected emissions from highway and public 

transportation use must be less than or equal to the budgets. In other words, the budget acts as a 

ceiling on emissions from the onroad transportation sector. This must be demonstrated regularly 

as a part of approval of a region’s Transportation Improvement Program (TIP) and Regional 

Transportation Plan (RTP).

The 2017 and 2026, onroad emissions presented in the proposed maintenance plan inventory are 

based on data from travel demand modeling from PSRC’s current RTP: Transportation 2040 

Update, adopted May 29, 2014.1The travel demand model assumptions include proposed 

transportation projects, user fee and other financial assumptions, and transit investments that 

meet the goals of the long range plan. 

The MVEB cannot be exceeded, so care must be taken to develop the MVEB. The emissions 

inventory is a realistic starting point, but it does not take into account the uncertainty of future 

projections. Uncertainty could arise from versions and inputs associated with both the EPA 

emissions model and PSRC’s travel demand model, as well as the status of available funding 

over time to implement the aspirational vision of Transportation 2040. 

The emissions inventory was developed using MOVES2010b. EPA is planning to release 

MOVES2014 in the summer of 2014, and EPA will likely release subsequent MOVES versions 

before the final year of the maintenance plan, 2026. In addition to EPA’s assumptions built into 

the model algorithms, Ecology regularly updates the local input files used to run the MOVES 

model. Local data includes fleet and fuel characteristics, travel estimates, and control programs.

The vehicle miles traveled (VMT) data used for the emissions inventory was developed using 

PSRC’s current travel demand model. This version was used for all modeling included the 

Transportation 2040 Update. Periodically, as new data becomes available, as new methods 

advance from theory to practice, and as computer resources become more productive, the PSRC 

travel model is subject to upgrades and improvements to better represent travel conditions. Not 

only do the tools evolve, but the roadway network that is modeled evolves with every update. 

Modifications may be made to reflect specific projects or region-wide assumptions related to 

policy decisions and economic effects. 
Motor Vehicle Emission Budget Methodology 2

Tacoma-Pierce County PM2.5 Redesignation Request and Maintenance Plan

To address the uncertainty of projecting future emissions, the MVEB was developed by adding a 

portion of the safety margin to the onroad inventory projections. A safety margin is the amount 

by which the total projected emissions from all sources of a given pollutant are less than the total 

emissions for 2011, the attainment year. 

The uncertainty was calculated with the following methodology. An additional ten percent of 

the projected onroad emissions from 2017 was added to the 2017 MVEB to account for potential 

changes to the modeled transportation network, and the evolution of regional policy and 

economic conditions over time. This was based on the alternatives analysis that was included as 

part of the Transportation 2040 Final Environmental Impact Statement (FEIS) conducted in 

2010. The range of emissions and the range of VMT was roughly 10 percent among all 

alternatives. 

An additional 5 percent of the projected onroad emissions from 2017 was added to the 2017 

MVEB to account for evolving model versions and inputs by PSRC, Ecology, and EPA. A total 

of 15 percent of the projected onroad emissions was therefore added to account for uncertainty.

The same methodology was applied to the 2026 MVEB. These assumptions were reviewed with 

the region’s Air Quality Consultation Partners.

Table 1 summarizes the proposed motor vehicle emissions budgets for 2017 and 2026 for PM2.5

and NOX. These values represent the maximum pounds per day of emissions that will be 

allowed from onroad mobile sources. The table also demonstrates the 15 percent uncertainty 

described above is within the available safety margin.

Onroad Motor Vehicle Emissions Budgets (Pounds per winter day)

Year PM2.5 NOx

2017 Onroad Inventory 1,642 36,339

2017 Available Safety Margin 662 26,452

2017 Safety Margin Allotted to MVEB 246 5,451

2017 Safety Margin Allotted to MVEB (%) 37% 21%

2017 MVEB 1,888 41,790

2026 Onroad Inventory 1,149 19,896

2026 Available Safety Margin 382 44,592

2026 Safety Margin Allotted to MVEB 172 2,984

2026 Safety Margin Allotted to MVEB (%) 45% 7%

2026 MVEB 1,321 22,880

 

1

Puget Sound Regional Council, 2014, “Transportation 2040 Update Report,” 

http://www.psrc.org/assets/10550/T2040Update2014.pdf