Appendix C, Exhibit 3 

Emissions Modeling Assumptions 

Used to Develop Motor Vehicle Emission Budgets for the 

Atlanta Eight-Hour Ozone 15% Reasonable Further Progress Plan 

and Attainment Demonstration

On April 30, 2004, the United States Environmental Protection Agency designated 20 

counties within the metropolitan Atlanta area as a "marginal" nonattainment under the 

eight-hour ozone standard. The eight-hour ozone nonattainment area encompasses the 

previous 13-county one-hour ozone nonattainment area plus seven additional "ring" 

counties: Barrow, Bartow, Carroll, Hall, Newton, Spalding, and Walton. With an 

attainment deadline of June 15, 2007, marginal 8-hour ozone areas were required to attain 

the National Ambient Air Quality Standard (NAAQS) by the 2006 ozone season. 

Because monitoring data from the 2006 ozone season indicated that Atlanta had not 

attained the NAAQS, the area was reclassified to "moderate"). Regional emissions 

inventories for the 20-county eight-hour ozone nonattainment area are needed to develop 

Motor Vehicle Emission Budgets (MVEB) for NOx and VOC as part of the State 

Implementation Plan (SIP) revisions that will demonstrate reasonable further progress 

towards attainment and modeled attainment of the eight-hour ozone standard in Atlanta. 

In ozone nonattainment areas with air quality classified as "moderate" or worse, the 

reasonable further progress (RFP) requirement established in Section 182(b) of the CAA 

prescribes emission reductions from the baseline totaling 15% within six years of the 

base year (i.e., by the end of 2008 for the 8-hour ozone NAAQS). MVEB are being 

established for the year 2008, the RFP milestone year. Separate MVEB are being 

established for 2009, the last full ozone season before the June 15, 2010, "maximum 

attainment date" for a moderate ozone area. Below is a detailed listing of the procedures 

and planning assumptions that were used for the regional emissions inventories to support 

MVEB development. This document was submitted to the Interagency Consultation 

Group, in accordance with Section 93.105(b) of the Transportation Conformity Rule and 

Sections 106(g) and 106(h) of Georgia's transportation conformity SIP, all of which 

require interagency consultation for SIP development, on October 28, 2008. 

Section 1: General Methods and Assumptions 

• Regional emissions inventories completed to establish MVEB for Atlanta Eight-Hour 

Ozone 15% RFP Plan and Attainment Demonstration 

• Emissions Modeling Methodology 

o Eight-Hour Ozone Nonattainment Area: 

 Existing ARC link-level travel demand and emissions modeling process for 

20-county Atlanta ozone nonattainment area 
Section 2: Travel Demand Modeling Assumptions 

• Calibration Year: 2000 

• Project Listing: 

o As approved in Envision6 2030 RTP and 2008-2013 TIP; adopted by the ARC 

Board September 26, 2007; and received a positive conformity determination 

from USDOT October 10, 2007 

o As approved in 2030 LRTP and 2008-2013 TIP; adopted by GHMPO Board 

August 14, 2007; and received a positive conformity determination from USDOT 

October 10, 2007 

o Project coding for network years 2005 and 2010 reviewed by state and local 

project sponsors in March and April 2007 as part of Envision6 2030 RTP/FY 

2008-2013 TIP development 

 Year 2008 and 2009 networks needed for budget inventories derived from 

2010 travel model network 

• Travel demand model refinements implemented as part of model expansion to 20 

counties, from the previous 13-county platform: 

o Mode choice model updated to improve performance of suburban intra-county 

trips 

o Attraction share model incorporated into four-step modeling process 

o New commercial vehicle and truck model developed to improve traffic 

assignment for both passenger vehicles and truck traffic 

• Demographic Data: See "Demographic Data Used to Develop Motor Vehicle 

Emissions Budgets for the Atlanta Attainment Demonstrations" 

• Speed Data: Free-flow Speed by Area Type and Facility Type 

Note: Within the ARC travel demand and emission modeling process, free flow 

speeds are adjusted to reflect the increase in delay and travel time on a roadway 

segment as traffic volumes build and congestion levels increase. Link-level 

congested flow speeds are used to estimate mobile source emissions, consistent with 

the requirements of Sections 93.122(b)(1)(iv) and 93.122(b)(2) of the Transportation 

Conformity Rule. 

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Free Flow Speed by Area Type and Facility Type 

Area Type

 Facility Type 

Urban Very 

High Density

Urban High 

Density 

Urban 

Medium 

Density

Urban 

Low 

Density Suburban Exurban Rural 

Metered 

Ramps 

0 Zone Centroid Connectors 7 11 11 11 11 14 14 

1 

Interstate / Freeway Free 

Flow 55 58 58 61 61 63 65 

2 Parkway 50 50 55 55 57 60 60 

3 HOV Buffer Separated 55 58 58 61 61 63 65 

4 HOV Barrier Separated 55 58 58 61 61 63 65 

5 

High Speed Ramp / CD 

Road 50 50 55 55 57 60 60 15 

6 Medium Speed Ramp 50 50 50 50 50 50 50 10 

7 Low Speed Ramp 40 40 40 40 40 40 40 10 

8 Loop Ramp 30 30 30 30 30 30 30 10 

9 Off Ramp w/ Intersection 25 25 25 25 25 25 25 

10 On Ramp w/ Intersection 40 40 40 40 40 40 40 5 

11 Expressway 40 42 45 48 52 55 60 

12 Principal Arterial - Class I 26 30 33 36 42 46 55 

13 Principal Arterial - Class II 24 27 30 34 40 44 48 

14 Minor Arterial - Class I 22 25 28 31 38 42 45 

15 Minor Arterial - Class II 20 23 26 29 34 38 42 

16 HOV - Arterial (all classes) 20 27 30 33 36 39 42 

17 Major Collector 18 22 25 28 31 34 38 

18 Minor Collector 15 18 21 24 27 30 35 

19 

Planned Ramps w/ 

Intersections 30 30 30 30 30 30 30 5 

20 Planned Directional Ramps 45 45 45 45 45 45 45 10 

• Transit Modeling 

o Model recalibrated to 2000 transit ridership estimates, provided by transit 

operators 

o Reflects results from Transit On Board Survey 

o Routes updated to reflect current operating plans 

o Transit mode split is estimated using the mode choice model 

 Estimates individual modal trips from the person trip movements developed in 

the trip distribution model. 

 Composed of three nested logit models: 

• Home based work trips, which includes home based university trips; 

• Home based other trips, which include home based other, home based 

shopping and home based grade school; and 

• Non-home based trips. 

 The mode choice models is organized in terms of seven characteristics: 

• Mathematical structure; 

• Trip purposes and choice sets; 

• Limitations on choice sets; 

• Analysis of transit access; 

• Treatment of HOV lanes; 

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• Stratification by income groups; and 

• Analysis of alternative transit paths. 

o Transit Fare Modeling 

 Fare structure and operating plans supplied by the local transit operators 

• Fares remain constant over time, across all network years 

• Fares reflect current transit operating plans 

 Transit fare structure uses a fare matrix on a zone to zone level with a 

universal fare structure (flat fare) for all bus and rail lines 

• Changes to the existing fare structure and service frequency are coded 

directly into the model 

• Current fare values in the model are weighted according to the percentage 

of riders using a discounted fare pass; changes to these assumptions can be 

incorporated directly into the model 

• Peak and off-peak fares are equivalent 

Section 3: Emissions Modeling Assumptions 

• Emission Factor Model: MOBILE6.2.03 

• MOBILE6.2.03 Inputs (13-county portion) 

o Average hourly temperature, humidity, and average barometric pressure for the 10 

highest ozone days during 2000-2002 

o Stage II refueling 

 Started in 1992 

 Three phase in years 

 81% efficiency 

o Anti-tampering program 

 Started in 1982 

 Covers model years 1975 – 1995 

 All LDG vehicle types are covered 

 Annual program 

 97% compliance 

 Catalyst removal only 

o I/M Program 

 Exhaust and Evaporative (OBD and gas cap pressure test) for 1996 and newer 

vehicles 

• Began in 1982 

• Annual inspection required 

• Computerized test and repair OBD – Exhaust 

• Computerized test and repair OBD & GC - Evaporative 

• Applies to all LDG vehicle types 

• Three year grace period 

• 3% waiver rate for all vehicles – Exhaust test 

• 0% waiver rate for all vehicles – Evaporative test 

• 97% compliance 

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 Exhaust and Evaporative test for 1975 – 1995 vehicles 

• Began in 1982 

• Annual inspection required 

• Computerized test and repair ASM 2525/5015 Phase-in – Exhaust 

• Computerized test and repair GC – Evaporative 

• Applies to all LDG vehicle types 

• 3% waiver rate for all vehicles – Exhaust 

• 0% waiver rate for all vehicles – Evaporative 

• 97% compliance 

• 25 year and older model years are exempt 

o Fuel1

 Low Sulfur, Low RVP Georgia Gasoline Phase 2 

• 100% market share of 10% ethanol-blend gasoline (E10) assumed 

• volatility waiver for E10 allows 8.0 psi RVP 

o 2002 regional fleet age distribution 

 Derived from R.L. Polk & Co. registration data for 13-county area 

 Applied to 15 of the 16 MOBILE6.2.03 composite vehicle classifications - 

LDV, LDT1, LDT2, LDT3, LDT4, HDV2B, HDV3, HDV4, HDV5, HDV6, 

HDV7, HDV8A, HDBS, HDBT, MC 

• Default for HDV8B 

o Default VMT fractions 

• MOBILE6.2.03 Inputs (7-county portion) 

o Same temperature, humidity, and barometric pressure inputs as 13-county area 

o No Stage II refueling 

o No anti-tampering program 

o No I/M program 

o Fuel 

 Same as 13-county area 

o 2002 regional fleet age distribution 

 Derived from R.L. Polk & Co. registration data for 7-county area 

 Applied to 15 of the 16 MOBILE6.2.03 composite vehicle classifications - 

LDV, LDT1, LDT2, LDT3, LDT4, HDV2B, HDV3, HDV4, HDV5, HDV6, 

HDV7, HDV8A, HDBS, HDBT, MC 

• Default for HDV8B 

o Default VMT fractions 

• VMT adjustment factors 

o Adjusts link-level VMT produced by travel model for ozone precursor emissions 

modeling: 

 

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1

 In 1999, Georgia's two-phase gasoline sulfur control program limited average sulfur in gasoline sold in 

the 13-county Atlanta area and in 12 surrounding counties to 150 parts per million (ppm). In addition, 

there is a seasonal (June 1 to September 15) 7.0 pounds per square inch (psi) Reid vapor pressure (RVP) 

cap on gasoline sold in this Phase 1 area. In 2003, Phase 2 of Georgia's gasoline rule reduced average 

sulfur to 30 ppm and added 20 additional counties outside the 20-county ozone nonattainment area to the 

sulfur and RVP control program. 
 HPMS adjustment in base year of calibration, year 2000, in accordance with 

Section 93.122(b)(3) of the Transportation Conformity Rule which 

recommends that HPMS adjustment factors be developed to reconcile travel 

model estimates of VMT in base year of calibration to HPMS estimates for 

the same period2

 

 Summer (seasonal) adjustment to convert from average annual VMT to 

summer-season VMT3

 Factors applied to VMT estimates generated by ARC travel demand model for 

13-county and 7-county portions of 20-county nonattainment area, separately 

VMT Adjustment Factors – 13 County Portion

Functional Class Name Factor 

Rural Interstate 0.67 

Rural Prin. Arterial 1.02 

Rural Min. Arterial 1.16 

Rural Major Collector 0.88 

Rural Minor Collector 1.07 

Rural Local 0.64 

Urb. Interstate 1.05 

Urb. Other Fwy 2.44 

Urb. Prin. Arterial 0.67 

Urb. Min. Arterial 1.21 

Urbanized Collector 1.21 

Urbanized Local 0.95 

VMT Adjustment Factors – 7 County Portion

Functional Class Name Factor 

Rural Interstate 0.89 

Rural Prin. Arterial 0.99 

Rural Min. Arterial 0.98 

Rural Major Collector 1.81 

Rural Minor Collector 1.81 

Rural Local 1.10 

Urb. Interstate 0.86 

Urb. Other Fwy 0.85 

Urb. Prin. Arterial 0.97 

Urb. Min. Arterial 0.96 

Urbanized Collector 1.80 

Urbanized Local 1.06 

• Off-Model Calculations 

o The Senior I/M Exemption emissions debit calculated for 2002 was considered 

conservatively high and was also added to the 2008 and 2009 budget year 

inventories 

 

2

 40 CFR Part 93.122(b)(3) 

3 Procedures for Emission Inventory Preparation, Volume IV: Mobile Sources, Section 3.4.2.6, EPA420-R92-009, USEPA Office of Air and Radiation, Office of Mobile Sources, 1992. 

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