1

Sacramento 

Federal Ozone Nonattainment Area

July 24, 2014
2

INTRODUCTION

The requirement to offset growth in vehicle miles traveled (VMT) is contained in the 

federal Clean Air Act (section 182(d)(1)(A)). This section applies to areas classified as 

severe or extreme nonattainment of the National Ambient Air Quality Standard 

(NAAQS) for ozone. The Sacramento Federal Ozone Nonattainment Area is currently 

designated as Severe-15 for the 8-hour ozone NAAQS, and is subject to this 

requirement. This demonstration is included as part of the 2013 update to the 

Sacramento Regional 8-Hour Ozone Attainment and Reasonable Further Progress Plan 

to address the requirements of Section 182(d)(1)(A).

BACKGROUND

In 1997, U.S. EPA replaced the 1-hour ozone standard set in 1979 with an 8-hour 

standard of 0.08 ppm [ see 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 in place as a result of not attaining the 1-hour 

NAAQA remained applicable even after the revocation of that 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 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.

SACRAMENTO REGION SIP SUBMISSIONS FOR VMT OFFSET 

REQUIREMENT AND LITAGATION

In 2009, the 8-hour ozone reasonable further progress plan for the Sacramento region

was submitted to U.S. EPA as required by the 1990 Amendments to the Clean Air Act. 

The 2009 plan addressed the requirements of Section 182(d)(1)(A) according to EPA’s 

longstanding interpretation that “no further 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.”

Subsequent to that submission, a decision of the Ninth Circuit Court of Appeals in 

Association of Irritated Residents (AIR) v U.S. EPA (reprinted as amended on January 

27, 2012, 632 F.3d 584, 9th Cir. 2011) rejected EPA’s interpretation of the CAA, which 

had provided the basis for U.S. EPA’s approval of VMT offset demonstrations in district 

plans. With the AIR decision, the Court determined that the CAA requires additional 

transportation control strategies and measures whenever vehicle emissions are 

projected to be higher than would have been had VMT not grown, even if aggregate 

vehicle emissions are decreasing. 
3

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 the AIR decision, omitted any 

reference to “transportation control strategies” (TCS). TCSs, which are not defined in 

the CAA or U.S. EPA regulation, 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. 

U.S. 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 (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 a VMT offset demonstration in 

accordance with U.S. EPA’s new guidance for the 8-hour ozone requirements. To 

address U.S. EPA’s guidance on the base year, 2002 is used as the base year for the 

1997 8-hour 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 

increased emissions from growth in vehicle miles traveled (VMT). 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, and idling restrictions). The Sacramento Area Council of

Governments (SACOG) has also developed a number of TCMs to provide emission 

reductions. 
4

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 a 

substantial number of enforceable transportation strategies – more than enough 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. 

A listing of the mobile source control programs adopted by the State since 1990 is 

provided in Attachment 1. A list of SACOG TCMs implemented in the Sacramento 

Ozone Nonattainment Area is provided in Attachment 2.

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. For the 8-hour ozone standard, 2002 is the base year used 

for the attainment demonstration and 2018 is the 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 
5

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

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 2018.

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 52,595 7,935 1,270 45

_____________________________

1

 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

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

For the attainment year,
6

(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 

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 2018 without the 

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

28 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 2018 with 

the 2002 level of VMT of 52,594,743 miles per day, the 2002 level of starts at 

7,934,624 per day, and the 2002 level of population at 1,270,259 vehicles. The 

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

19 tons/day.

Calculation 3. Calculate emission reductions with full 

Implementation of Transportation Control Strategies & TCMs.
7

The VOC emission levels for 2018 assuming the benefits of the post-2002 motor 

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

2018 are calculated using EMFAC2011. The projected VOC emissions level is 

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

are provided in the following table.

Summary of 2018 Attainment Year Emissions Levels

* 2018 VMT based on SACOG’s 2013 MTIP

** 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:

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 2018 levels.)

64,709 10,640 1,650 28

(2)

Emissions with 

Motor Vehicle 

Control Program 

Frozen at 2002 

Levels. 

(VMT, starts, and vehicle 

population at 2002 levels)

52,595 7,935 1,270 19

(3)

Emissions with Full 

Motor Vehicle

Control Program in 

Place

(VMT, starts and vehicle 

population at 2018 levels)

64,709 10,640 1,650 14
8

VOC: 14 < 19 tons/day

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, Figure 1 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 Sacramento region for the 1997 8-hour ozone standard (2002 

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.
9

Figure 1. VOC Emissions* from On-Road Mobile Sources in 

the Sacramento Ozone Nonattainment Area

(2002 Base Year)

* Does not include resting or diurnal loss emissions

X:\VMT Offset Demonstration\FINAL CORRECTED Sac VMT Offset Doc for EPA Reformated.docx