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INTRODUCTION

The purpose of the vehicle miles travelled (VMT) emissions offset demonstration for the 

1-hour and 8-hour ozone standards is to respond to U.S. EPA’s final action entitled 

“Disapproval of Implementation Plan Revisions; State of California; South Coast VMT 

Emissions Offset Demonstrations” published on March 28, 2013 (78 Fed.Reg. 18849). 

In that final action, U.S. EPA withdrew its approval of, and then disapproved, the VMT 

emissions offset demonstrations in the 2003 1-hour ozone State Implementation Plan 

(SIP or plan) and the 2007 8-hour ozone plan. U.S. EPA’s action is in response to a 

decision of the Ninth Circuit Court of Appeals in Association of Irritated Residents v U.S. 

EPA, (9th Cir., reprinted as amended on January 27, 2012, 686 F. 3d 668).

BACKGROUND

In 1979, U.S. EPA established a primary health-based national ambient air quality 

standard (NAAQS) for ozone at 0.12 parts per million (ppm) averaged over a 1-hour 

period. See 44 Fed.Reg. 8220 (February 9, 1979). The Clean Air Act (CAA), as 

amended in 1990, classified areas that had not yet attained that standard, based on the 

severity of their ozone problem, ranging from Marginal to Extreme. Extreme areas were 

provided the most time to attain, until November 15, 2010, but were also subject to the 

most stringent requirements. In particular, Severe and Extreme areas were subject to 

CAA Section 182(d)(1)(A), which requires state implementation plans to adopt “specific 

enforceable transportation control strategies and transportation control measures to 

offset any growth in vehicle miles traveled or numbers of vehicle trips in such area....” 

U.S. EPA designated the South Coast Air Basin as “Extreme” on November 6, 1991 (56 

Fed.Reg. 56694). Thus the South Coast Air Basin was subject to this requirement. 

The U.S. EPA has historically interpreted this provision of the CAA (now called “VMT 

emissions offset requirement”) to allow areas to meet the requirement by demonstrating 

that emissions from motor vehicles decline each year through the attainment year. See, 

e.g., 57Fed.Reg. 13498, at 13521-13523 (April 16, 1992).

NEW OZONE STANDARD

In 1997, U.S. EPA replaced the 1-hour ozone standard with an 8-hour standard of 0.08 

ppm [62 Fed.Reg. 38856 (July 18, 1997)]. The U.S. EPA promulgated rules 

implementing the new standard. The “Phase 1” rule was issued on April 30, 2004 (69 

Fed.Reg. 23951). That rule includes anti-backsliding requirements that meant that 

many requirements remained applicable even after the revocation of the 1-hour 

standard, which was effective June 2005. See 40 CFR §51.905(a)(1) and §51.900(f). 

In particular, an area that was classified as Extreme for the 1-hour standard would 

remain subject to the VMT emissions offset requirement even if it would not otherwise 

have been subject to that requirement based on its classification under the new 8-hour 

ozone standard [40 
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CFR §51.900(f)(11)]. The U.S. EPA’s Phase 2 rule, issued on November 29, 2005 (70 

Fed. Reg. 71612) required that areas classified as Severe or Extreme under the new 8-

hour standard would also be subject to the VMT offset requirement.

SOUTH COAST SIP SUBMISSIONS FOR VMT OFFSET REQUIREMENT

In 1994, the District, SCAG, and CARB submitted the South Coast 1-hour ozone plan, 

as required by the 1990 Amendments to the Clean Air Act. The plan included 

transportation control measures (TCMs). In 1997, U.S. EPA approved this plan [62 

Fed. Reg. 1150 (January 8, 1997)]. In 1997 and 1999, the District, SCAG, and CARB 

submitted revisions to the plan, which were approved as amended in 2000 [65 Fed. 

Reg.18903 (April 10, 2000)]. In 2004, the state agencies submitted the 2003 South 

Coast 1-hour ozone SIP. In 2008, the District submitted a VMT offset demonstration to 

comply with the VMT offset requirement by showing that there would be no increase in 

motor vehicle emissions between the area’s base year for the attainment demonstration 

and the area’s attainment year. The U.S. EPA approved the VMT offset demonstration 

[74 Fed. Reg. 10176 (March 10, 2009)]. At the same time, U.S. EPA disapproved the 

overall attainment demonstration which had relied on CARB measures that were 

subsequently withdrawn. (See 1-hour ozone attainment demonstration Appendix).

The U.S. EPA initially designated the South Coast Air Basin as Severe-17 for the 8-hour

ozone standard, but later granted the State’s request to reclassify the area as “Extreme” 

[69 Fed.Reg. 23858 (April 30, 2004)] and [75 Fed.Reg. 24409 (May 5, 2010)]. In 2007, 

the state submitted a SIP revision to address the 8-hour ozone requirements, including 

a VMT offset demonstration in accordance with U.S. EPA’s prior guidance. In March 

2012, U.S. EPA approved the 2007 South Coast 8-hour ozone SIP, including the VMT 

emission offset demonstration [77 Fed.Reg. 12674 (March 1, 2012)].

LITIGATION OVER VMT OFFSET REQUIREMENT

In approving the 2003 VMT offset demonstration, U.S. EPA used its longstanding 

interpretation that no additional TCMs are necessary if aggregate motor vehicle 

emissions are projected to decline each year from the base year of the plan to the 

attainment year [74 Fed. Reg. 10176, 10179-80 (March 10, 2009)]. Several 

environmental and community groups challenged this approval. In February 2011, the 

Ninth Circuit Court of Appeals ruled against U.S. EPA, holding that additional 

transportation control strategies and transportation control measures are required 

whenever vehicle emissions are projected to be higher than they would have been had 

vehicle miles traveled not increased, even where aggregate vehicle emissions are 

actually decreasing [Association of Irritated Residents v U.S. EPA, 632 F. 3d 584, at 

596-597, 686 F. 3d. 668 (reprinted as amended on January 27, 2012, and further 

amended February 13, 2012)]. The U.S. EPA had filed a petition for panel rehearing in 

May 2011, which was denied on January 27, 2012.
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When the U.S. EPA signed its final approval of the 2007 South Coast 8-hour ozone SIP 

on December 15, 2011, the Court had not yet ruled on U.S. EPA’s petition for rehearing. 

Therefore, U.S EPA took final action and approved the VMT offset requirement 

demonstration based on its long-standing interpretation. The final approval was 

ultimately published on March 1, 2012 (77 Fed.Reg. 12674). Several environmental 

and community groups filed a lawsuit challenging that approval (Communities for a 

Better Environment, et al. v. U.S. EPA, Ninth Circuit No. 12-71340).

U.S. EPA’S WITHDRAWAL OF APPROVAL AND DISAPPROVAL OF 

2003 AND 2007 VMT OFFSET REQUIREMENT DEMONSTRATIONS

In response to the decision in Association of Irritated Residents, U.S. EPA proposed to 

withdraw its approval of, and to disapprove the VMT offset demonstrations in the 2003 

and 2007 South Coast 1-hour and 8-hour ozone SIPs (77 Fed, Reg. 58067, September 

19, 2012). In the proposed action U.S. EPA explains that the demonstrations “are not 

consistent with the court’s ruling ...because they fail to identify, compared to a baseline 

assuming no VMT growth, the level of increased emissions resulting solely from VMT 

growth and to show how such increased emissions have been offset through adoption 

and implementation of transportation control strategies and transportation control 

measures.”

On March 28, 2013, U.S. EPA finalized its proposed action (See 78 Fed. Reg. 18849). 

As a result of the final withdrawal and disapproval, the offset sanction in CAA Section 

179(b)(2) applies in the South Coast Air Basin 18 months after the effective date of the 

final disapproval, and highway funding sanctions six months after that. In order to stop 

the sanctions from taking effect, U.S. EPA must take final approval action on a SIP 

submission that corrects the deficiency before the effective dates of the sanctions. A 

federal implementation plan (FIP) would also be required 24 months after the effective 

date of the final disapproval unless the deficiency has been corrected.

U.S. EPA GUIDANCE ON VMT OFFSET REQUIREMENT

In August 2012, U.S. EPA issued guidance entitled “Implementing Clean Air Act Section 

182(d)(1)(A): Transportation Control Measures and Transportation Control Strategies to 

Offset Growth in Emissions Due to Growth in Vehicle Miles Travelled”. Among other 

things, U.S. EPA’s guidance points out that the Court in Association of Irritated 

Residents omitted any reference to “transportation control strategies” which are not 

defined in the CAA or U.S. EPA regulation, but which are eligible to offset growth in 

emissions due to growth in VMT. The U.S. EPA’s new guidance indicates that 

technology improvements such as vehicle technology improvements, motor vehicle 

fuels, and other control strategies that are transportation-related could be used to offset 

increases in emissions due to VMT. EPA’s revised guidance sets forth a method of 

calculating the actual growth in emissions due to growth in VMT. Essentially, the state 

would compare projected attainment year emissions assuming no new control 

measures and no VMT growth with projected actual attainment year emissions 
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(including new control measures and VMT growth). If the latter number is smaller than 

the former, no additional transportation control measures or strategies would be 

required. If additional transportation control measures and transportation control 

strategies are required, they should be clearly identified and distinguished from the 

measures included in the initial calculations for the base year and the three scenarios 

identified for the attainment year.

In addition, the guidance recommends that the base year to be used in the 

demonstration be the base year used in the attainment demonstration for the ozone 

ambient air quality standard. This Appendix includes VMT offset demonstrations in 

accordance with U.S. EPA’s new guidance for both the 8-hour and 1-hour ozone 

requirements. To address U.S. EPA’s guidance on the base year, two base years are 

used: 2002 for the 1997 8-hour standard and 1990 for the 1-hour ozone standard.

TRANSPORTATION CONTROL STRATEGIES AND TRANSPORTATION 

CONTROL MEASURES

By listing them separately, the Clean Air Act [CAA §182(d)(1)(A)] differentiates between 

transportation control strategies (TCS) and transportation control measures (TCM), and 

thus provides for a wide range of strategies and measures as options to offset growth in 

emissions from vehicle miles traveled (VMT) growth. In addition, the example TCMs 

listed in Section 108(f)(1)(A) of the CAA include measures that reduce emissions by 

reducing VMT, reducing tailpipe emissions, and removing dirtier vehicles from the fleet. 

California’s motor vehicle control program includes a variety of strategies and measures 

including new engine standards and in-use programs (e.g., smog check, vehicle scrap, 

fleet rules, idling restrictions). TCMs developed by SCAG provide additional reductions. 

In addition, SCAG prepares a report every two years that reports on the status of 

implementation of TCMs.

Based on the provisions in Section 182(d)(1)(A) and the clarifications provided in the 

U.S. EPA guidance, any combination of transportation control strategies and TCMs may 

be used to meet the requirement to offset growth in emissions resulting from VMT 

growth. Since 1990 when this requirement was established, California has adopted 

more than sufficient enforceable transportation strategies and measures to meet the 

requirement to offset the growth in emissions from VMT growth. For the 1997 8-hour 

standard offset demonstration, 2002 controls are used as the base case control level 

since 2002 is the base year of the SIP. For the 1-hour ozone standard demonstration, 

1990 level controls are used as the base year control level since 1990 is the base year 

for the 1-hour SIP. 

A list of the state’s mobile source control program adopted since 1990 is provided in 

Attachment 1. In addition, a list of TCMs implemented in the South Coast Air Basin is 

provided in Attachment 2.
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EMISSIONS DUE TO VMT GROWTH

As discussed above, the U.S. EPA guidance does provide a recommended calculation 

methodology that could be done to determine if sufficient transportation control 

strategies and TCMs have been adopted and implemented to offset the growth in 

emissions due solely to growth in VMT. As such, any increase in emissions solely from 

VMT increases in the future attainment year from the base year (assuming that there 

are no further motor vehicle control programs implemented after the base year) would 

need to be offset. In addition, a calculation is needed to show the emissions levels if 

VMT had remained constant from the base year to the future attainment year. As 

discussed earlier, a comparison of the projected attainment year emissions assuming 

no new control measures and no VMT growth with projected actual attainment year 

emissions (including new control measures and VMT growth) would be made. If the 

latter number is smaller than the former, no additional transportation control measures 

or strategies would be required.

METHODOLOGY

The following calculations are based on the U.S. EPA guidance recommended 

calculation methodology. As provided in the 2007 SIP for the 8-hour ozone standard, 

2002 is the base year used for the attainment demonstration and 2023 is the attainment 

year. For the 1-hour ozone national ambient air quality standard demonstration, 1990 

serves as the base year and 2022 is the projected attainment year. 

Analysis Tool

This analysis uses California’s approved motor vehicle emissions model, EMFAC. 

The EMFAC model estimates the emissions from two combustion processes: running 

exhaust and start exhaust, and four evaporative processes: hot soak, running losses, 

diurnal, and resting losses.

Emissions from running exhaust, start exhaust, hot soak, and running losses are a 

function of how much a vehicle is driven. Emissions from these processes are directly 

related to VMT, trips, and starts. These processes are included in the calculation of the 

emissions levels used in the VMT offset demonstration. Emissions from resting loss 

and diurnal loss processes are not related to VMT, trips or vehicle starts and are not 

included in the analysis because these emissions occur regardless if the vehicle makes 

a trip (i.e. a start) or not.

EMFAC combines trip-based VMT from the regional transportation planning agencies, 

starts data based on household travel surveys, and vehicle population data from the 

Department of Motor Vehicles with corresponding emission rates to calculate 

emissions. 1

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 More information on data sources can be found in the EMFAC technical document which is located on 

the web at: http://www.arb.ca.gov/msei/emfac2011-technical-documentation-final-updated-0712-v03.pdf
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With the EMFAC model, the calculation of emissions growth and whether it is offset is 

simplified to a comparison of future year emissions with “no growth” in VMT or new 

control strategies to future emissions with VMT growth and new control strategies. This 

follows EPA’s 2012 guidance and is consistent with the Ninth Circuit’s interpretation of 

CAA section 182(d)(1)(A).

The Advanced Clean Cars (ACC) regulations adopted by ARB in 2012 are not included 

in EMFAC 2011. The output from EMFAC 2011 was adjusted off-model to estimate the 

benefits of the ACC regulations. No other off-model adjustments were taken. 

Analysis Using 2002 as the Base Year for the 1997 8-hour Ozone Standard with 

Attainment Year of 2023.

Step 1. Provide the emissions level for the base year.

The following table shows the VOC emissions, VMT, starts, and vehicle population for 

calendar year 2002 from the EMFAC2011 model.

Summary of 2002 Base Year

VMT

(thousand miles/day)

Starts 

(thousands/ day)

Vehicle 

Population

(thousands)

VOC 

Emissions

(tons/day)

2002 Base Year 330,268 58,039 9,108 280

Step 2. Calculate three emissions levels in the attainment year.

For the attainment year,

(1) Calculate emissions level with the motor vehicle control program frozen at 

2002 levels and with projected VMT, starts, and vehicle population for the 

attainment year. This represents what the emissions in the attainment year 

would have been if transportation control strategies and TCMs had not 

been implemented after 2002;

(2) Calculate emissions level with the motor vehicle control program frozen at 

2002 levels and assuming VMT, starts, and vehicle population do not 

increase from 2002 levels; and

(3) Calculate an emissions level that represents emissions with full 

implementation of all transportation control strategies and TCMs since 

2002 and which represents the projected future year baseline emissions 
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inventory using the VMT, starts, and vehicle population for the attainment 

year.

Calculation 1. Calculate the emissions in the attainment year 

assuming no new measures since the base year, and including 

growth in VMT, starts, and vehicle population.

To perform this calculation, the California Air Resources Board (CARB) staff 

identified the on-road motor vehicle control programs adopted since 2002 and 

adjusted the EMFAC2011 to reflect the VOC emissions levels in 2023 without the 

benefits of the post-2002 control programs. The projected VOC emissions are 

115 tons/day.

Calculation 2. Calculate the emissions with no growth in VMT, starts, 

or vehicle population.

EMFAC2011 allows the user to input different VMT, starts, and vehicle population 

than default. As such, for this calculation, the EMFAC 2011 model was run 

without the benefit of the post 2002 control program for calendar year 2023 with 

the 2002 level of VMT of 330,267,528 miles per day, the 2002 level of starts at 

58,039,006 per day, and the 2002 level of population at 9,108,417. The VOC 

emissions associated with 2002 VMT, starts, and vehicle population are 89 

tons/day.

Calculation 3. Calculate emission reductions with full 

Implementation of Transportation Control Strategies & TCMs.

The VOC emission levels for 2023 assuming the benefits of the post-2002 motor 

vehicle control program and the projected VMT, starts, and vehicle population in 

2023 are calculated using EMFAC2011. The projected VOC emissions level is 

62 tons/day. VOC emissions for the three sets of calculations described above 

are provided in the following table.
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Summary of 2023 Attainment Year Emissions Levels

* CY 2023 VMT based on SCAG 2012 Regional Transportation Plan

** Does not include diurnal or resting loss emissions.

As provided in the U.S. EPA guidance, to determine compliance with the provisions of 

Section 182(d)(1)(A) of the federal Clean Air Act, the emissions levels calculated in 

Calculation 3 should be less than the emissions levels in Calculation 2:

VOC: 62 < 89 tons/day

Description

VMT* 

(miles/day, 

thousands)

Starts

(thousands/ day)

Vehicle 

Population

(thousands)

VOC 

Emissions** 

(tons/day)

(1)

Emissions with 

Motor Vehicle 

Control Program 

Frozen at 2002 

Levels. 

(VMT, starts and vehicle 

population at 2023 levels.)

395,750 72,730 11,311 115

(2)

Emissions with 

Motor Vehicle 

Control Program 

Frozen at 2002 

Levels. 

(VMT, starts, and vehicle 

population at 2002 levels)

330,268 58,039 9,108 89

(3)

Emissions with Full 

Motor Vehicle

Control Program in 

Place

(VMT, starts and vehicle 

population at 2023 levels)

395,750 72,730 11,311 62
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Analysis Using 1990 as the Base Year for the 1-hour Ozone Standard with 

Attainment Year of 2022.

Step 1. Provide the emissions levels for the base year.

The following table shows the VOC emissions, VMT, starts, and vehicle population for 

calendar year 1990 from the EMFAC2011 model.

Summary of 1990 Base Year

VMT

(thousand miles/day)

Starts 

(thousands/ day)

Vehicle 

Population

(thousands)

VOC 

Emissions

(tons/day)

1990 Base Year 257,490 46,060 7,335 873

Step 2. Calculate three emissions levels in the attainment year.

For the attainment year,

(1) Calculate emissions level with the motor vehicle control program frozen at 

1990 levels and with projected VMT, starts, and vehicle population for the 

attainment year. This represents what the emissions in the attainment 

year would have been if transportation control strategies and TCMs had 

not been implemented after 1990;

(2) Calculate emissions level with the motor vehicle control program frozen at 

1990 levels and assuming VMT, starts, and vehicle population do not 

increase from 1990 levels; and 

(3) Calculate an emissions level that represents emissions with full 

implementation of all transportation control strategies and TCMs since 

1990 and which represents the projected future year baseline emissions 

inventory using the VMT, starts, and vehicle population for the attainment 

year.

Calculation 1. Calculate the emissions in the attainment year 

assuming no new measures since the base year, and including 

growth in VMT, starts, and vehicle population.

To perform this calculation, staff identified the on-road motor vehicle control 

programs adopted since 1990 and adjusted the EMFAC2011 model to reflect the 

VOC emissions 
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levels in 2022 without the benefits of the post-1990 control programs. The 

projected VOC emissions are 488 tons per day.

Calculation 2. Calculate the emissions with no growth in VMT, starts, 

or vehicle population.

EMFAC2011 allows the user to input different VMT, starts, and vehicle population 

than default. As such, for this calculation, the EMFAC 2011 model was run 

without the benefit of the post 1990 control program for calendar year 2022 with 

the 1990 level of VMT of 257,490,447 miles per day, the 1990 level of starts at 

46,059,760 per day, and the 1990 level of population at 7,334,677. The VOC 

emissions associated with 1990 VMT, starts, and vehicle population are 312 

tons/day. 

Calculation 3. Calculate emission reductions with full 

implementation of Transportation Control Strategies and TCMs.

The VOC emission levels for 2022 assuming the benefits of the post-1990 motor 

vehicle control program and the projected VMT, starts, and vehicle population in 

2022 are calculated using EMFAC2011. The projected VOC emissions level is 

65 tons/day. VOC emissions for the three sets of calculations described above 

are provided in the following table.
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Summary of 2022 Attainment Year Emissions Levels

* CY 2022 VMT based on SCAG 2012 Regional Transportation Plan

** Does not include resting or diurnal loss emissions.

As provided in the U.S. EPA guidance, to determine compliance with the 

provisions of Section 182(d)(1)(A) of the federal Clean Air Act, the emissions 

levels calculated in Calculation 3 should be less than the emissions levels in 

Calculation 2:

VOC: 65 < 312 tons/day

Description

VMT* 

(miles/day, 

thousands)

Starts

(thousands/ day)

Vehicle 

Population

(thousands)

VOC 

Emissions** 

(tons/day)

(1)

Emissions with 

Motor Vehicle 

Control Program 

Frozen at 1990 

Levels.

(VMT, starts, and vehicle 

population at 2022 levels.)

394,838 72,531 11,280 488

(2)

Emissions with 

Motor Vehicle 

Control Program 

Frozen at 1990 

Levels.

(VMT, starts, and vehicle 

population at 1990 Levels)

257,490 46,060 7,335 312

(3)

Emissions with Full 

Motor Vehicle

Control Program in 

Place

(VMT, starts, and vehicle 

population at 2022 Levels)

394,838 72,531 11,280 65
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SUMMARY

The previous sections provide an analysis to demonstrate compliance with the 

provisions of Section 182(d)(1)(A) of the federal Clean Air Act. To further illustrate the 

demonstration, Figures 1 and 2 below show graphically the emissions benefits of the 

motor vehicle control programs in offsetting VOC emissions due to increased VMT, 

starts, and vehicle population in the South Coast Air Basin for both the 1997 8-hour 

ozone standard (2002 base year) and the 1-hour ozone standard (1990 base year). The 

left bar (in purple) shows the emissions in the base year with base year controls. The 

three bars on the right in each figure show the emissions levels in the attainment year 

for the three calculations identified above: the red bar shows attainment year emissions 

with base year controls and attainment year VMT, starts, and vehicle population, the 

green bar shows attainment year emissions with base year controls, VMT, starts, and 

vehicle population, and the blue bar shows attainment year emissions with attainment 

year controls, VMT, starts, and vehicle population. Based on the U.S. EPA guidance, if 

the blue bar is lower than the green bar, then the identified transportation control 

strategies and TCMs are sufficient to offset the growth in emissions.
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Figure 1. VOC Emissions* from On-Road Mobile Sources in 

the South Coast Air Basin (2002 Base Year)

* Does not include resting or diurnal loss emissions
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Figure 2. VOC Emissions* from On-Road Mobile Sources in the

South Coast Air Basin for 1990 Base Year

* Does not include resting or diurnal loss emissions