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SECTION 9 CONFORMITY 

9.1 Purpose of Section

This section establishes Transportation Conformity budgets for the on-road mobile source sector 

for EMA and WMA in the 2008 RFP milestone year and in the 2009 attainment demonstration 

year. The Transportation Conformity budgets set the levels of mobile source emissions that may

not be exceeded in the each area in the budget years. The budgets were developed using methods 

agreed upon by staff of the Massachusetts Executive Office of Transportation and Public Works, 

Office of Transportation and Planning (EOT), MassDEP, and EPA Region 1 through the 

conformity consultation process. Technical data files related to the Mobile 6 model runs used to 

estimate mobile source emissions in the budget years are in the Appendix 9. 

9.2 Background 

Transportation conformity is required by section 176(c) of the CAA to ensure that transportation 

plans, programs and projects receiving federal funds are consistent with the SIP. Conformity to a 

SIP means that transportation activities will not produce new air quality violations, worsen 

existing violations, or delay timely attainment of a NAAQS. 

EPA’s Phase 2 Implementation Rule provides that nonattainment areas establish motor vehicle 

emissions budgets for the RFP milestone year (2008) and for the attainment demonstration year 

(2009). The budgets are set at the level of emissions from the on-road mobile sector projected for 

these years. Emissions of VOC and NOx from this sector may not exceed the budget levels. 

Emissions in years for which no budget is set may not exceed the budget established for the most 

recent prior year.

MassDEP established conformity budgets for EMA and WMA under the 1-hour ozone standard. 

For WMA, MassDEP submitted to EPA a 1-hour ozone standard attainment demonstration in 

1998, which established budgets for the 2003 attainment year. EPA found the WMA budgets 

adequate for conformity purposes on February 9, 1999. For EMA, MassDEP submitted to EPA a 

1-hour ozone standard attainment demonstration in September 2002, which established budgets 

for the 2007 attainment year. EPA found the EMA budget adequate for conformity purposes on 

December 6, 2002. (The 2003 WMA and 2007 EMA budgets are shown in Table 9.1)

These 2008 and 2009 transportation conformity budgets, once deemed adequate by EPA, will 

replace the conformity budgets previously submitted and approved. 

9.3 Methodology 

Conformity budgets are set through a conformity consultation process. The 2008 and 2009 

budgets for EMA and WMA were developed using methods agreed upon by staff from the 

Office of Transportation Planning (EOT), MassDEP, and EPA Region 1 pursuant to the 

consultation process. EOT has summarized these methods as follows: 
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• The latest version of the Massachusetts Statewide Travel Demand model1 was the primary

source used to estimate travel speeds and vehicle miles of travel (VMT), based on the latest 

planning assumptions, socio-economic data, and professional judgment about travel 

characteristics and growth. The statewide model was used for all areas in Massachusetts 

except for the Boston and Berkshire Metropolitan Planning Organization (MPO) areas 

(where regional models were used to be fully consistent with the latest planning 

assumptions). 

• The travel demand model reflects the highway network and has a “base year” of 2000, which 

has been initially “calibrated” to replicate traffic patterns from that year, based on various 

information, including recorded traffic counts and 2000 U.S. Census data. 

• A 2008 RFP year and 2009 attainment year model were developed from the base year model, 

which contains the latest planning assumptions regarding growth projections, including 

socio-economic changes and updating the roadway network in the model to incorporate 

regionally significant highway projects reasonably expected to be completed by 2008 and by 

2009. 

• Emission rates (usually in grams of pollutant per mile at given travel speeds) are applied to 

VMT and speed results from the model to estimate total vehicle emissions. The emission 

rates were provided by DEP and are based on EPA’s MOBILE 6.2 model. These rates –

developed specifically for the years 2008 and 2009 – are based on a variety of assumptions 

about vehicle fleet mix and technology, fuel composition, and pollution control programs. 

Two sets of rates were used: One set for expressway travel and another for non-expressway

travel. (Emission rates are in Appendix 9A.)

• Estimates of emissions from the model are factored to VMT data from the Highway

Performance Monitoring System (HPMS), as federal regulations require HPMS VMT to be 

the primary measure of VMT within a nonattainment area. These "HPMS factors" are 

calculated from the results of the base year model; the factors are equal to the ratio of 2000 

HPMS VMT to 2000 modeled VMT, and vary by MPO region. (See Appendix 9 B) 

• A seasonal adjustment factor is also applied to the statewide travel demand model and regional 

model results to replicate summertime conditions (when ozone concentrations are typically 

highest). Most travel models represent spring or fall conditions, where certain input data 

(employment, schools) are more representative of year-round figures. The seasonal 

adjustment factor represents the ratio of summer traffic levels to annual average traffic. 

• Model VMT and speed output were separated by region, and further split into expressway vs. 

non-expressway results. Then the appropriate emission rates were applied to produce the 

rough emission totals. These totals were then factored to HPMS data, and finally seasonally

adjusted to produce the modeled emission totals. 

 

1 While the Statewide Travel Demand Model is often updated, there are no specific dates or version 

numbers attached to the updates. The version used for the 2008 and 2009 conformity budgets was the latest update 

as of 10/17/07. 
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• In EMA, net emission estimates from commuter rail, bus, and boat services are included in the 

totals, with most accounted for in the results for the Boston MPO region. The remaining "offmodel" estimates – covering several regions and categorized as “Other Eastern 

Massachusetts” – were calculated by the Central Transportation Planning Staff (CTPS) of the 

Boston MPO, and include: 1) Additional net commuter rail emissions generated outside the 

Boston MPO region, and 2) Emission estimates of daily VMT savings realized from 

carpooling at various park and ride lots located in several regions. The off-model estimates 

were added to the modeled emission totals to produce the final 2008 mobile source emission 

estimate totals used for the RFP budget development. (The “off-road” estimates are described 

in Appendix 9 C.)

• Calibration of a travel demand model is often a continuing process in order to achieve results 

that are more accurate and to keep pace with changes in the latest planning assumptions and 

growth projections. As in the past with other model sets, future model output and HPMS 

adjustment factors can change as the statewide model and regional models are made more 

accurate and updated assumptions are incorporated. 

9.4 Mobile Source Budgets

Application of the methods described above results in projected emissions for the 2008 and 2009 

years for the EMA and WMA attainment years as shown in Table 9.1. These emissions 

represent the 8-hour ozone standard budgets for EMA and WMA.2

Table TC 1 2008 and 2009 EMA and WMA Mobile Source Budgets

Tons per summer day

2008 2008 2009 2009

VOC NOx VOC NOx

Eastern MA 68.30 191.30 63.50 174.96

Western MA 11.80 31.30 10.73 27.73

Prior 1-hour budgets 2007 2007

EMA 86.70 226.36

WMA 2003 2003

23.77 49.11

 

2 Emissions estimated by Metropolitan Planning Organization Region are in Appendix 9 D.