Appendix E. Development of the Base- and Future-Year 

Mobile Source Emissions Inventory for the Treasure Valley, 

Idaho
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Development of the Base- and Future-Year Mobile Source 

Emissions Inventory for the Treasure Valley, Idaho

July 2012

Prepared by

Idaho Department of Environmental Quality

Technical Services Division

1410 N. Hilton

Boise, Idaho 83706
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Base- and Future-Year Emissions Inventory

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Table of Contents

List of Figures................................................................................................................................ iv

List of Tables ................................................................................................................................. iv

Abbreviations, Acronyms, and Symbols ........................................................................................ v

1 Introduction.............................................................................................................................. 1

2 Methodology: MOVES Input Database Development............................................................ 2

2.1 VMT-Related Inputs......................................................................................................... 3

2.1.1 Road Type................................................................................................................. 3

2.1.2 Annual VMT............................................................................................................. 4

2.1.3 Monthly, Daily, and Hourly VMT............................................................................ 4

2.2 Source-Related Inputs ...................................................................................................... 5

2.2.1 Source Type Population............................................................................................ 6

2.2.2 Age Distribution........................................................................................................ 6

2.3 VHT-Related Inputs ......................................................................................................... 6

2.3.1 Ramp Fractions......................................................................................................... 7

2.3.2 Average Speed Distribution...................................................................................... 7

2.4 Fuel-Related Inputs .......................................................................................................... 8

2.4.1 Alternative Vehicle Fuels and Technology (AVFT) ................................................ 8

2.4.2 Fuel Supply ............................................................................................................... 8

2.4.3 Fuel Formulation....................................................................................................... 8

2.5 Meteorology ..................................................................................................................... 9

2.6 Inspection and Maintenance (I/M) Programs................................................................. 10

2.7 On-Road Retrofits .......................................................................................................... 10

3 Methodology: Paved and Unpaved Road Dust...................................................................... 10

3.1 Road Dust Emission Factor............................................................................................ 11

3.2 Vehicle Miles Traveled .................................................................................................. 11

3.3 Precipitation Data ........................................................................................................... 12

3.4 Average Vehicle Weight by Roadway Type .................................................................. 12

3.5 Silt Loading .................................................................................................................... 13

3.6 Unpaved Road Dust Emissions...................................................................................... 14

4 Results.................................................................................................................................... 14

4.1 On-Road Mobile Source Emission Estimates ................................................................ 14

4.2 Paved and Unpaved Road Dust Emission Estimates...................................................... 15

5 Quality Control and Quality Assurance................................................................................. 16

6 Conclusion ............................................................................................................................. 17

References..................................................................................................................................... 18
Base- and Future-Year Emissions Inventory

iv

Appendix A. Crosswalk between COMPASS TDM Road Types, FHWA Roadway Types, and 

MOVES Roadway Types.............................................................................................................. 19

Appendix B Crosswalk between ATR Length Bins, FHWA Vehicle Classes and MOVES 

Source Types................................................................................................................................. 21

Appendix C ITD Statewide Vehicle Classification Data............................................................. 23

List of Figures

Figure 1. MOVES input files and groups. ...................................................................................... 2

Figure 2. Meteorological and precipitation observation sites in Ada and Canyon Counties.......... 9

List of Tables

Table 1. MOVES road type descriptions. ....................................................................................... 3

Table 2. MOVES source type descriptions..................................................................................... 3

Table 3. TDM-based annual vehicle miles traveled.a..................................................................... 4

Table 4. Crosswalk between MOVES source types and data sources for source-related 

MOVES input parameters................................................................................................... 5

Table 5. MOVES speed bins........................................................................................................... 7

Table 6. E10 market share. ............................................................................................................. 9

Table 7. Days with at least 0.01 inches of rain in Ada and Canyon Counties.............................. 12

Table 8. Average vehicle weight by vehicle type. ........................................................................ 13

Table 9. Silt loadings used for paved road emission factor calculation........................................ 13

Table 10. Annual on-road emissions in the Treasure Valley........................................................ 14

Table 11. Daily average on-road emissions in the Treasure Valley during the ozone season 

(summer)........................................................................................................................... 15

Table 12. Daily average on-road emissions in the Treasure Valley during the PM season 

(winter).............................................................................................................................. 15

Table 13. Annual paved road dust emissions in the Treasure Valley........................................... 15

Table 14. Daily average paved road dust emissions in the Treasure Valley during the ozone 

season (summer). .............................................................................................................. 16

Table 15. Daily average paved road dust emissions in the Treasure Valley during the PM 

season (winter).................................................................................................................. 16

Table 16. Annual unpaved road dust emissions in the Treasure Valley....................................... 16
Base- and Future-Year Emissions Inventory

v

Abbreviations, Acronyms, and Symbols

ATR automatic traffic recorder

AVFT alternative vehicle fuels and technology

BPR Bureau of Public Roadways

CBD central business district

CNG compressed natural gas

CO carbon monoxide

COMPASS Community Planning Association of Southwest Idaho

DEQ Idaho Department of Environmental Quality

DMV Idaho Department of Motor Vehicles

E10 fuel blend of 10% ethanol and 90% gasoline

EI emissions inventory

EPA US Environmental Protection Agency

ERG Eastern Research Group

FHWA Federal Highway Administration

HOV high-occupancy vehicle

HPMS Highway Performance Monitoring System

I/M inspection and maintenance

ITD Idaho Transportation Department

MOVES Motor Vehicle Emissions Simulator

mph miles per hour

NH3 ammonia

NOx nitrogen oxides

PM particulate matter

PM10 particulate matter with a diameter less than 10 microns

PM2.5 particulate matter with a diameter less than 2.5 microns

SH state highway

SIP state implementation plan

SO2 sulfur dioxide

TDM travel demand model

TVRDS Treasure Valley road dust study

US United States

VHT vehicle hours traveled

VIN vehicle identification number

VMT vehicle miles traveled

VOC volatile organic compound
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Base- and Future-Year Emissions Inventory

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1 Introduction

The State of Idaho contracted with Eastern Research Group (ERG) and Environ International 

Corporation (Environ) to complete state implementation plan (SIP)-quality emissions inventories

(EIs) for the Treasure Valley of Idaho. ERG and Environ jointly completed the 2008, 2015, and 

2023 EIs (ERG and Environ 2010) using the MOBILE6.2 model for on-road mobile source 

emissions. The Idaho Department of Environmental Quality (DEQ) contracted with ERG and 

Environ to prepare these EIs to support renewal of the northern Ada County PM10 maintenance 

plan and other projects.1 As a result, Elmore and Canyon Counties are included in the EIs.

Since completing these EI estimates, the US Environmental Protection Agency (EPA) has 

released a new mobile source emissions model, the Motor Vehicle Emissions Simulator 

(MOVES2010a) (EPA 2011a). In an effort to ensure that the State of Idaho is using the most 

recent emissions models for SIP and maintenance plan development and for developing motor 

vehicle emission budgets for subsequent conformity determinations, the mobile source portion of

each EI was redone using the MOVES2010a emissions model (hereafter referred to simply as 

MOVES), and the paved road dust emissions were developed using the latest EPA-recommended 

(AP-42) method (EPA 2011b). Unpaved road dust emission estimates included in the 2010 EI

were used as reported (ERG and Environ 2010). This report details the methodologies and results

for the MOVES on-road emissions modeling and road dust computations.

MOVES is the EPA-designated model for on-road mobile EI development for SIPs and

maintenance plans and for Federal Highway Administration (FHWA) transportation conformity 

determinations. The on-road mobile source EI was developed using MOVES according to the 

Technical Guidance on the Use of MOVES2010 for Emission Inventory Preparation in State 

Implementation Plans and Transportation Conformity (EPA 2010). The results will be reviewed 

through an interagency consultation process.

Paved and unpaved road dust emission estimates were included in the 2008, 2015, and 2023 

Treasure Valley EIs (ERG and Environ 2010). For the PM10 maintenance plan update, new 

paved road dust emissions estimates were developed by DEQ; however, unpaved road dust 

emissions estimates included in this update were not changed from those presented in the 2008, 

2015, and 2023 Treasure Valley EIs (ERG and Environ 2010).

The Northern Ada County PM10 Maintenance Area only includes the northern, populated portion 

of Ada County. The southern portion is largely unpopulated, but all emission estimates represent 

the entire county. The EI for the entire county provides a conservative surrogate for emissions in 

the Northern Ada County Maintenance Area. In addition, the adjacent Canyon County is not 

within the PM10 maintenance area. However, since this mobile source EI must meet other 

purposes, such as photochemical modeling, and since the airshed today includes Canyon County, 

the Canyon County emissions are retained in this report.

 

1

PM10 is particulate matter with an aerodynamic diameter less than 10 microns. PM2.5, discussed later in this report, 

has an aerodynamic diameter less than 2.5 microns. 
Base- and Future-Year Emissions Inventory

2

2 Methodology: MOVES Input Database Development

To operate the MOVES model at the county-level as required by EPA for SIP-level EIs, DEQ 

developed an input database for each specific combination of inputs. This section discusses the 

assumptions, sources of input information, and calculation methodologies involved in developing 

SIP-level MOVES input databases.

Figure 1 describes the required MOVES input databases, grouped by common data source. For 

example, inputs related to vehicle miles traveled (VMT) (top box in Figure 1)—such as road type 

distribution and monthly, daily, and hourly traffic profiles—require detailed information on the 

VMT within the modeling domain. DEQ prepared input files for each group using a combination 

of (primarily) local data and national default values where good local data were not available.

This section discusses the creation of each input. The input data file format under discussion is 

provided after each section heading for clarification.

Figure 1. MOVES input files and groups.
Base- and Future-Year Emissions Inventory

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2.1 VMT-Related Inputs

VMT inputs describe the distance traveled on different roadways by the various source types 

(vehicles). VMT-related inputs include road type distribution and VMT (annual, monthly, daily, 

and hourly estimates). The road type VMT distribution data set was developed from the 

Ada/Canyon County travel demand model (TDM), and the monthly, weekday/weekend, and 

hourly VMT profiles were developed from permanent automatic traffic recorders (ATRs). The 

TDM results for the 2008 base year were prepared by the Community Planning Association of 

Southwest Idaho (COMPASS) in 2009 and updated in early 2011 to reflect the recently released 

2010 census data. This 2011 update allowed COMPASS to make some minor adjustments in the 

2008 model to reflect real variations in population shifts in some areas reflected in the first new 

census tract data in 10 years (M. Waldinger, COMPASS, personal communication, 2011).

MOVES road types and source (vehicle) types are defined in Table 1 and Table 2. 

Table 1. MOVES road type descriptions.

Road Type Description

1 Off-Network

2 Rural Restricted Access

3 Rural Unrestricted Access

4 Urban Restricted Access

5 Urban Unrestricted Access

Table 2. MOVES source type descriptions.

MOVES 

Source Type Description

11 Motorcycle

21 Passenger Car

31 Passenger Truck

32 Light Commercial Truck

41 Intercity Bus

42 Transit Bus

43 School Bus

51 Refuse Truck

52 Single Unit Short-haul Truck

53 Single Unit Long-haul Truck

54 Motor Home

61 Combination Short-haul Truck

62 Combination Long-haul Truck

2.1.1 Road Type

RoadtypeDistribution(sourceTypeID, roadTypeID, roadTypeVMTFraction)

The road type distribution describes the fraction of fleet miles driven on each of the four 

applicable MOVES road types (rural restricted, rural unrestricted, urban restricted, and urban 

unrestricted) within the modeling domain for each source (vehicle) type. Road type distribution 
Base- and Future-Year Emissions Inventory

4

inputs were derived from TDM outputs provided by COMPASS, annual Highway Performance 

Monitoring System (HPMS) VMT by FHWA road type, and link-level Idaho Transportation 

Department (ITD) ATR data. A crosswalk table in Appendix A shows the relationships between 

COMPASS TDM road types, the HPMS/FHWA road types, and the MOVES roadway types. 

ITD ATR data were aggregated by county and used to allocate the annual VMT for each source 

type to road types. When the annual road distributions were complete for the FHWA road types, 

the distributions were aggregated into the four MOVES road types.

2.1.2 Annual VMT

HPMSVTypeYear(HPMSVtypeID, yearID, HPMSBaseYearVMT, baseYearOffNetVMT)

Annual VMT is the yearly VMT for each HPMS vehicle type for each county in the domain. ITD 

ATR data were used to generate a weekday/weekend ratio and fleet mix for each road type,

which were then applied to COMPASS TDM annual average weekday VMT outputs to estimate 

annual VMT for base and future years.

Comparing annual VMT attributed to local roads from the TDM output to annual local road 

VMT from ITD fuels sales data indicated that the TDM underestimates VMT from local roads, a 

common trait of TDM models nationwide. To compensate for this, DEQ scaled up TDM local 

road VMT to match ITD HPMS estimates. For base and future years, annual local road VMT 

estimates were adjusted up using the same scaling factor. The final TDM-based VMT after the 

local road reconciliation are summarized in Table 3. 

Table 3. TDM-based annual vehicle miles traveled.a

County Year Vehicle Miles Traveled

Ada 2008 3,203,969,729

Ada 2015 3,887,965,879

Ada 2023 5,533,619,325

Canyon 2008 1,574,168,589

Canyon 2015 1,992,190,855

Canyon 2023 3,164,270,684

a After adjustment of local road vehicle miles traveled to match HPMS

2.1.3 Monthly, Daily, and Hourly VMT

MonthVMTFraction(sourcetypeID, isLeapYear, monthID, monthVMTFraction)

DayVMTFraction(sourceTypeID, monthID, roadTypeID, dayID, dayVMTFraction)

HourVMTFraction(sourceTypeID, roadTypeID, dayID, hourID, hourVMTFraction)

Temporal distribution profiles further divided the source type annual VMT into finer time 

increments. Temporal profiles were derived from ATR data and annual VMT by FHWA road 

type, the latter of which were calculated from TDM outputs provided by COMPASS.

ATR data contain hourly vehicle counts for each of 5 length categories or “bins.” Counts for 

each length bin were converted to temporal distributions for each MOVES vehicle type and 

roadway type using a crosswalk scheme developed based on discussions with ITD and 2007–

2009 Idaho statewide vehicle classification data (Scott Fugit, ITD, personal communication). 

The final crosswalk table, which maps ATR length bins to MOVES vehicle types, is provided in 

Appendix B, and the 2007–2009 classification data are provided in Appendix C. Neither ATR 
Base- and Future-Year Emissions Inventory

5

data nor FHWA vehicle classification data distinguish between personal or commercial trips, and 

long or short-haul truck trips, so it was necessary to use national default fractions available in the 

MOVES model to make the final splits from FHWA classes to MOVES vehicle types in these 

areas. For each ATR site, a full year of ATR data were processed. Hourly, weekday/weekend, 

and monthly statistics were calculated for each vehicle type. Finally, ATR sites were grouped 

based on MOVES road types, and each site was weighted equally in constructing the final 

temporal profiles. For the purposes of the MOVES modeling and paved road dust calculations, 

winter (particulate matter, or PM) season is defined as November 1–February 29, and the 

summer or “nonwinter” season is defined as April 1–October 31.

Future-year temporal profiles for each road type were developed from the TDM output using the 

base-year ATR-based temporal profiles along with future-year VMT.

2.2 Source-Related Inputs

This group of inputs includes source type population and age distribution. Source-related inputs

describe and group the vehicles in the modeling domain and are compiled using a variety of data 

sources (Table 4). The fleet mix distribution is a key component of on-road mobile source 

emissions.

In March 2011, DEQ decoded individual Idaho Department of Motor Vehicles (DMV)

registration records of vehicles registered in the Treasure Valley using the Polk vehicle 

identification number (VIN) decoding system. The decoded VINs provide information regarding 

the vehicle make, model, age, and fuel types. This information was then used to develop the 

MOVES source type population input and fleet age distribution input. An earlier VIN decoding 

project by Sierra Research (Sierra Research 2006) is used to provide source population data for 

the years prior to 1981.

Table 4. Crosswalk between MOVES source types and data sources for source-related MOVES 

input parameters.

MOVES Source Type Source-Related Input Data Source

Motorcycle ITD—DMV Registration Database (2011), Sierra Research (2006)

Passenger Car ITD—DMV Registration Database (2011), Sierra Research (2006)

Passenger Truck ITD—DMV Registration Database (2011), Sierra Research (2006)

Light Commercial Truck ITD—DMV Registration Database (2011), Sierra Research (2006)

Intercity Bus MOVES Default Database, Annual Local VMT, Sierra Research (2006)

Transit Bus ValleyRide

School Bus Idaho Department of Education

Refuse Truck MOVES Default Database, Annual Local VMT, Sierra Research (2006)

Single Unit Short-haul Truck MOVES Default Database, Annual Local VMT, Sierra Research (2006)

Single Unit Long-haul Truck MOVES Default Database, Annual Local VMT, Sierra Research (2006)

Motorhome ITD—DMV Registration Database (2011), Sierra Research (2006)

Combination Short-haul Truck MOVES Default Database, Annual Local VMT, Sierra Research (2006)

Combination Long-haul Truck MOVES Default Database, Annual Local VMT, Sierra Research (2006)
Base- and Future-Year Emissions Inventory

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2.2.1 Source Type Population

SourceTypeYear(yearID, sourceTypeID, sourceTypePopulation)

The source type population input describes the types and numbers of vehicles that make up the 

fleet. Five major data sources were used to develop the source type population inputs: VINdecoded ITD registration data, MOVES national default population and activity data, local 

activity data derived from TDM output, ValleyRide transit bus fleet data, and Idaho Department 

of Education school bus fleet data (Table 4).

Direct population data were available for transit buses from ValleyRide and for school buses 

from the Idaho Department of Education. For motorcycle, passenger car, passenger truck, light

commercial truck, and motorhome source types, VIN-decoded registration data were used to 

determine vehicle populations. 

For all other heavy-duty source types, a factor was used to estimate the local source type 

populations using local activity data, MOVES national default activity data, and MOVES 

national default source type populations (Equation 1).

 

  (

 

 

 

 )

Equation 1. Estimate of vehicle population for 

source types without local data available.

Equation 1 was used to estimate the MOVES source type population inputs for most heavy-duty 

vehicle types for Ada and Canyon Counties, except transit buses, school buses, and motorhomes.

Future-year source type populations were estimated using Equation 1 by substituting local 

future-year annual VMT for base-year annual VMT, future-year national default population for 

base-year national default population, and future-year national default VMT for base-year 

national default VMT. 

2.2.2 Age Distribution

AgeDistribution(SourceTypeID, YearID, AgeID, AgeFraction)

This input provides an age profile of the fleet. Separate age distributions were developed for Ada 

and Canyon Counties using VIN-decoded vehicle registration data, pre-1981 vehicle population 

data from an earlier VIN-decoder study (Sierra Research 2006), transit bus fleet age data from 

ValleyRide, and school bus fleet age data from the Idaho Department of Education. The same 

age distribution inputs were used for base and future years.

2.3 VHT-Related Inputs

Vehicle hours traveled (VHT) inputs capture the time spent on roads by vehicles. This group of 

inputs includes ramp fractions and average speed distribution. 
Base- and Future-Year Emissions Inventory

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2.3.1 Ramp Fractions

RampFraction(roadTypeID, rampFraction)

Ramp fraction defines the portion of VHT on roadways that contain entrance and exit ramps for 

restricted access roadways. Base- and future-year ramp fractions for urban and rural freeways 

were calculated by aggregating VHT on ramps and restricted access roadways using TDM output 

from COMPASS, then dividing ramp VHT by total restricted access roadway VHT to get the 

fraction of restricted access VHT attributed to ramps. Urban ramp fractions are 0.085 for winter 

season and 0.086 for summer season, while rural ramp fractions are 0.010 for both seasons in 

Ada and Canyon Counties.

2.3.2 Average Speed Distribution

AverageSpeedDistribution(sourceTypeID, roadTypeID, hourDayID, avgSpeedBinID, 

avgSpeedFraction)

The average speed distribution allocates the different source types (vehicles) for each roadway 

type to 16 speed bins ranging from 0 to >75 miles per hour (mph) (Table 5). This input reflects 

levels of congestion on roadways. Average speed distributions were developed from TDM 

average daily traffic counts for each roadway segment and hourly traffic count statistics 

developed from detailed ATR traffic count data provided by ITD. 

Table 5. MOVES speed bins.

avgSpeedBinID avgBinSpeed avgSpeedBinDesc

1 2.5 speed < 2.5 mph

2 5 2.5 mph ≤ speed < 7.5 mph

3 10 7.5 mph ≤ speed < 12.5 mph

4 15 12.5 mph ≤ speed < 17.5 mph

5 20 17.5 mph ≤ speed <22.5 mph

6 25 22.5 mph ≤ speed < 27.5 mph

7 30 27.5 mph ≤ speed < 32.5 mph

8 35 32.5 mph ≤ speed < 37.5 mph

9 40 37.5 mph ≤ speed < 42.5 mph

10 45 42.5 mph ≤ speed < 47.5 mph

11 50 47.5 mph ≤ speed < 52.5 mph

12 55 52.5 mph ≤ speed < 57.5 mph

13 60 57.5 mph ≤ speed < 62.5 mph

14 65 62.5 mph ≤ speed < 67.5 mph

15 70 67.5 mph ≤ speed < 72.5 mph

16 75 72.5 mph ≤ speed

The hourly ATR-based traffic count profiles for each roadway type were used to estimated

hourly volume on each segment, and the modified Bureau of Public Roadways (BPR) 

volume/capacity curve (Equation 2) was then used to develop the average speed distribution 

database for each hour.

 ( (

 

 )

 

) Equation 2. BPR Curve
Base- and Future-Year Emissions Inventory

8

Where A and B are local coefficients used in the TDM as provided by COMPASS.

Base- and future-year average speed distributions were developed for all four MOVES road 

types using TDM base and future-year outputs developed by COMPASS for the Treasure Valley

and detailed ATR data provided by ITD.

2.4 Fuel-Related Inputs

This group of inputs includes data regarding alternative vehicle fuels and technology (AVFT), 

fuel supply, and fuel formulation. 

2.4.1 Alternative Vehicle Fuels and Technology (AVFT)

AVFT(sourceTypeID, modelYearID, fuelTypeID, engTechID, fuelEngFraction)

AVFT input files in MOVES allow the user to assign source type activity by model year to 

vehicles with different fuel and/or engine technologies. Ada and Canyon Counties were modeled 

using a custom AVFT input file derived from VIN-decoded registration data. The same AVFT 

input was used for base and future years.

2.4.2 Fuel Supply

FuelSupply(countyID, fuelYearID, monthgroupid, fuelformulationid, marketshare, 

marketsharecv)

National default fuel supply inputs were used for all source types except transit buses. A large 

portion of the transit bus fleet in the Treasure Valley operates on compressed natural gas (CNG). 

For this reason, CNG fuels were included in base- and future-year modeling.

2.4.3 Fuel Formulation

FuelFormulation(fuelformulationid, fuelSubtypeID, RVP, sulfurLevel, ETOHVolume, 

MTBEVolume, ETBEVolume, TAMEVolume, aromaticContent, olefinContent, benzeneContent, 

e200, e300, volToWtPercentOxy, BioDieselEsterVolume, CetaneIndex, PAHContent)

With the exception of 10% ethanol in gasoline (E10), MOVES national default fuel formulations 

were used as base-year inputs for each county. These default values were judged to be reasonable 

based on local knowledge, except for the E10 market share. The base-year E10 market share was

updated with information provided by fuel suppliers (Table 6). Future-year runs used default 

values since the E10 market share is known to be nearly 100% at the present (late 2011) and is

expected to remain so in the foreseeable future, consistent with the MOVES default database.
Base- and Future-Year Emissions Inventory

9

Table 6. E10 market share.

Year Market Share

2008 0.68

2015 1.00 (National Default)

2023 1.00 (National Default)

2.5 Meteorology

ZoneMonthHour(monthID, zoneID, HourID, temperature, relHumidity)

The meteorology input compiles the average hourly temperature and relative humidity data for 

each county. Base- and future-year inventories were modeled using average hourly temperature 

and relative humidity data by county for each month from a representative weather station for 

each county. Ada County is represented by the National Weather Service station at the Boise Air 

Terminal and Canyon County is represented by the data set from the Caldwell Industrial Airport

(Figure 2).

Figure 2. Meteorological and precipitation observation sites in Ada and Canyon Counties.
Base- and Future-Year Emissions Inventory

10

2.6 Inspection and Maintenance (I/M) Programs

IMCoverage(polProcessID, stateID, countyID, yearID, sourceTypeID, fuelTypeID, 

IMProgramID, inspectFreq, testStandardsID, begModelYearID, endModelYearID, useIMyn, 

complianceFactor)

Inspection and maintenance (I/M) programs require registered vehicles to undergo periodic 

emissions tests. During the 2008 base year, Ada County had an active I/M program and was 

modeled as such. Canyon County did not have an active I/M program during 2008 and was

modeled without any I/M program active in MOVES.

For 2015 and 2023 future-year modeling, Ada and Canyon Counties were both modeled with 

active I/M programs to reflect the implementation of an I/M program in 2010 in Canyon County 

along with accompanying changes to the Ada County I/M program during the same time frame. 

2.7 On-Road Retrofits

On-roadRetrofits(Pollutant, Process, Fuel, Source, InitialCalendarYear, FinalCalendarYear, 

InitialModelYear, FinalModelYear, Fraction/Year, FractionEffective)

Neither Ada nor Canyon County use on-road retrofits. Therefore, both counties were modeled 

without on-road retrofits specified in the MOVES input database.

3 Methodology: Paved and Unpaved Road Dust

Fugitive dust from paved and unpaved roads is a significant source of PM emissions. In general, 

the factors that affect paved road dust emissions include the weight of the vehicles that drive on 

the roadway surface, vehicle speed, fine particle (silt) loading on the roadway surface available 

for entrainment, and precipitation on the roadway that decreases road dust emissions. Unpaved 

road dust emission estimates are used directly as reported in the area sources category of the

Treasure Valley base-year and future-year EIs (ERG and Environ 2010). Paved road dust 

emission estimates were developed by DEQ in 2011 to replace the ERG/Environ (2010) 

estimates, as described in the following sections. 

In 2010, the 2008, 2015, and 2023 paved road dust emission estimates were completed for the 

Treasure Valley as part of the 2008 SIP-level EI (ERG and Environ 2010). The paved road dust 

EIs were completed using an emission factor methodology and local data developed during the 

Treasure Valley road dust study (TVRDS) (Etyemezian et al. 2002). DEQ subsequently learned 

that the TVRDS paved road dust emissions estimates were based on a calibration originally 

established for unpaved roads, not paved roads. In subsequent studies when Etyemezian et al.

recalibrated the system for paved roads, they determined that the 2002 TVRDS emission factor 

measurements in the Treasure Valley were “unreasonably high” (Langston et al. 2008). As a 

result, DEQ abandoned the 2002 TVRDS emission factors and recalculated Ada and Canyon

County paved road dust emissions using a new emission factor equation published in January 

2011 by EPA as the agency’s recommended method in AP-42 Compilation of Air Pollutant 

Emission Factors (EPA 2011b, section 13.2.1). However, since the silt loadings measured by 

Etyemezian et al. in the TVRDS are based on local conditions and are somewhat more 

conservative than default silt loadings published in the January 2011 AP-42 method, the local silt 
Base- and Future-Year Emissions Inventory

11

loadings were retained rather than using the default loadings. This approach is described in the 

following sections.

3.1 Road Dust Emission Factor 

Paved road dust emissions were computed on a monthly basis using Equation 3 from the January 

2011 AP-42 Compilation of Air Pollutant Emission Factors (EPA 2011b, section 13.2.1). This 

form of the equation accounts for the dust suppression effect of precipitation that occurs during 

each month.

Eext = [ k (sL)0.91 x (W)1.02 ] (1 – P/4N) Equation 3. Paved road dust 

emissions.

where

Eext = PM10 or PM2.5 emission factor in the same units as k

k = particle size multiplier (1.0 for PM10) (grams/VMT)

sL = road surface silt loading (grams per square meter)

W = average weight of the vehicles traveling the road (tons)

P = number of “wet” days with at least 0.254 millimeters (0.01 inches) of precipitation 

during the averaging period (daily)

N = number of days in the averaging period (e.g., 28, 29, 30, or 31 for monthly)

The emissions for each county and each roadway type are computed as the product of the 

emission factor and the VMT on each roadway type and in each county. Therefore, for each 

roadway type, each county, and each month in the modeling period, VMT, road surface silt 

loading, average weight of the vehicles traveling the road, and the number of days with at least 

0.254 millimeters (0.01 inches) of precipitation must be determined.

3.2 Vehicle Miles Traveled

To generate paved road mobile emissions estimates for each month, daily averaged VMT is

required. The VMT was generated from the TDM outputs provided by COMPASS in spring 

2011. The TDM for the base year 2008 was updated by COMPASS based on the newly released 

2010 census information just prior to use in these mobile source emissions calculations. Prior to 

the 2010 census update, the areas of growth in the TDM model were based on projections from 

the 2000 census, so the 2010 census update provided verification and some minor corrections to 

the demographic basis of the TDM model (M. Waldinger, COMPASS, personal communication, 

2011). 

Since local roads are not captured in detail by the TDM, the local road VMT were adjusted to be 

consistent with HPMS-based local road VMT estimates. The adjustment to the HPMS VMT is 

normally applied in SIP inventories to ensure that the most accurate basis is used for total 

VMT—the total fuel sales volume at the county level. The annual VMT totals used in the road 

dust calculations are the same as those used in the MOVES modeling, summarized in Table 3,

section 2.1.2.
Base- and Future-Year Emissions Inventory

12

3.3 Precipitation Data

Precipitation is used in Equation 3 to adjust the road dust emissions for rainy days when enough 

precipitation (≥0.01 inches) falls to suppress road dust emissions. The number of days in each 

month with at least a trace of precipitation were reported in the ERG and Environ (2010) report 

and originated from the 2008 precipitation data from the Western Regional Climate Center 

(WRCC 2009). Data from the Boise Airport Weather Service Forecast Office (WSFO

No. 1001022) were used for Ada County, while data from the Caldwell Airport Station 

(No. 101380) were used for Canyon County. The precipitation data inputs are presented in Table 

7.

Table 7. Days with at least 0.01 inches of rain in Ada and Canyon Counties.

Month Ada County Canyon County

January 12 10

February 10 8

March 10 8

April 8 7

May 8 6

June 6 5

July 2 2

August 2 2

September 4 3

October 6 5

November 10 9

December 11 10

3.4 Average Vehicle Weight by Roadway Type

Average vehicle weight for each roadway type is derived from the vehicle type fraction on each 

roadway type and average vehicle weight by vehicle type.

Permanent ATR data for Ada and Canyon Counties were provided by ITD and combined with 

statewide FHWA vehicle classification data from ITD to determine the vehicle type fractions 

traveling on each roadway type in the modeling domain. The ATR data identifies motorcycles, 

passenger vehicles, and two classes of heavy-duty vehicles by length measurement; however, the 

FHWA vehicle classification statistics by roadway type are needed to provide greater detail in 

vehicle classification. The average vehicle weight for each vehicle type was obtained from the 

MOVES default database (EPA 2010) as shown in Table 8. Source type IDs are provided in 

Table 2.
Base- and Future-Year Emissions Inventory

13

Table 8. Average vehicle weight by vehicle type.

SourceType 

ID HPMS Vtype ID SourceType Name Source Mass (Metric Tons)

11 10 Motorcycle 0.285

21 20 Passenger Car 1.479

31 30 Passenger Truck 1.867

32 30 Light Commercial Truck 2.060

41 40 Intercity Bus 19.594

42 40 Transit Bus 16.556

43 40 School Bus 9.070

51 50 Refuse Truck 20.684

52 50 Single Unit Short-haul Truck 7.642

53 50 Single Unit Long-haul Truck 6.250

54 50 Motor Home 6.735

61 60 Combination Short-haul Truck 29.328

62 60 Combination Long-haul Truck 31.404

3.5 Silt Loading

General default silt loadings are available in the January 2011 emission factor methodology for 

paved roads (EPA 2011b); however, if local data are available, they are preferred. During the 

2002 TVRDS, silt loading measurements were taken at numerous locations in Ada and Canyon 

Counties. An inquiry to the Ada County Highway District revealed that while there is some 

recent movement toward using more salt and less sand, there had not been any large shift away 

from sanding as of 2008, the year of the base inventory. This information confirmed that while 

sanding varies with the number of storms each year, the 2008 winter sanding practices and sand

consumption were approximately the same as those observed in the 2002 period when the 

TVRDS was conducted. As a result, the local silt loading measurements made in the TVRDS on 

local and arterial roadways (Etyemezian et al. 2002) were determined to still be applicable and 

were used for silt loadings. These loadings are somewhat higher than the default loadings in the 

January 2011 EPA road dust method but were used as the best available representation of local 

conditions and to ensure that the results are conservative. For safety reasons, Etyemezian et al. 

(2002) did not make road dust measurements on the interstate, so the default values from the 

EPA methodology (EPA 2011b) are used in calculations for interstates. Silt loadings used for 

this mobile source inventory are shown in Table 9 below.

Table 9. Silt loadings used for paved road emission factor calculation.

Road 

Type

Winter Silt Loading 

(g/m2)

Summer Silt Loading

(g/m2)

Source

Interstate 0.015 0.015 EPA 2011b

Arterial 1.9 0.5 Etyemezian et al. 2002

Local 4 0.4 Etyemezian et al. 2002

Note: grams per square meter (g/m2)
Base- and Future-Year Emissions Inventory

14

3.6 Unpaved Road Dust Emissions 

For the PM10 maintenance plan update, unpaved road dust emission estimates from the 2008, 

2015, and 2023 EIs for the Treasure Valley airshed (ERG and Environ 2010) were used without 

adjustment. ERG and Environ obtained unpaved road traffic counts and road segment lengths for 

the 2008 base year from a survey of the city and county road departments in Ada and Canyon 

Counties and estimated annual VMT from that survey. An average speed of 25 mph was 

assumed for all unpaved roads. Then the unpaved road dust emission estimation methodology 

used in the TVRDS was adopted, as detailed in the EI report (ERG and Environ 2010). The 

unpaved road dust emission factor method used by Etyemezian et al. in the TVRDS was based 

on unpaved road dust calibrations conduced at Fort Bliss, Texas, prior to the TVRDS. As a 

result, DEQ believes that although there were problems with the paved road emission factors

(discussed in Section 3, 2nd paragraph), the unpaved road dust emission factors from the TVRDS

should be accurate and reflect the best local data available. Therefore, the unpaved road dust 

emission estimates developed by ERG and Environ using the TVRDS emission factors were 

used as reported. 

4 Results

On-road mobile source, paved road dust, and unpaved road dust emissions estimate results are 

presented in this section.

4.1 On-Road Mobile Source Emission Estimates

On-road mobile source emissions are reported annually, as an average ozone season day, and as 

an average PM season day. The ozone season is defined as April 1–October 31, and the PM

season is November 1–February 29. This definition matches other emission sources developed 

by ERG and Environ in the EI. On-road mobile source emissions results from the MOVES 

model are shown in Table 10, Table 11, and Table 12 as annual total, average ozone season day, 

and average PM season day, respectively. These emission results include estimates for nitrogen 

oxides (NOx), sulfur dioxide (SO2), volatile organic compounds (VOC), carbon monoxide (CO), 

particular matter, and ammonia (NH3). The PM10 and PM2.5 emission estimates in these tables 

include particulate matter from direct exhaust, brake wear, and tire wear and do not include 

paved and unpaved road dust. 

Table 10. Annual on-road emissions in the Treasure Valley.

Year County

NOx SO2 VOC CO PM10 PM2.5 NH3

(tons per year)

2008

Ada

9775.4 67.4 4182.3 47168.3 412.7 330.4 152.9

2015 5856.6 33.4 2939.7 39263.4 283.0 193.0 126.4

2023 4306.4 42.0 2396.8 38771.8 285.2 157.1 146.6

2008

Canyon

5847.8 35.0 3202.1 36404.4 258.2 212.1 81.8

2015 3870.5 17.7 2177.2 27974.3 178.4 127.6 74.4

2023 3273.6 25.1 1935.0 27684.6 191.5 112.3 95.0
Base- and Future-Year Emissions Inventory

15

Table 11. Daily average on-road emissions in the Treasure Valley during the ozone season 

(summer).

Year County

NOX SO2 VOC CO PM10 PM2.5 NH3

(tons per day)

2008

Ada

28.5 0.20 10.9 110.9 1.09 0.86 0.45

2015 16.9 0.10 7.7 83.8 0.74 0.48 0.37

2023 12.4 0.12 6.6 78.5 0.75 0.37 0.43

2008

Canyon

16.7 0.10 8.4 87.9 0.65 0.52 0.24

2015 11.0 0.05 5.7 61.1 0.45 0.31 0.22

2023 9.2 0.07 5.2 57.2 0.49 0.26 0.28

Table 12. Daily average on-road emissions in the Treasure Valley during the PM season (winter).

Year County

NOX SO2 VOC CO PM10 PM2.5 NH3

(tons per day)

2008

Ada

24.0 0.16 12.30 156.4 1.19 0.98 0.38

2015 14.7 0.08 8.69 144.3 0.83 0.61 0.31

2023 10.9 0.10 6.57 149.7 0.84 0.53 0.36

2008

Canyon

14.8 0.09 9.28 117.1 0.80 0.67 0.20

2015 9.9 0.05 6.45 100.6 0.55 0.42 0.19

2023 8.5 0.06 5.46 105.3 0.59 0.39 0.24

4.2 Paved and Unpaved Road Dust Emission Estimates

Paved road dust emissions are reported annually, as an average ozone season day, and as an 

average PM season day (Table 13 through Table 15). Unpaved road dust is reported only as an 

annual total (Table 16).

Table 13. Annual paved road dust emissions in the Treasure Valley.

Year County

PM10 PM2.5

(tons per year)

2008

Ada

7501 428

2015 9164 522

2023 13243 755

2008

Canyon

4154 237

2015 5211 297

2023 8417 480
Base- and Future-Year Emissions Inventory

16

Table 14. Daily average paved road dust emissions in the Treasure Valley during the ozone 

season (summer).

Year County

PM10 PM2.5

(tons per day)

2008

Ada

10.4 0.6

2015 12.4 0.7

2023 17.9 1.0

2008

Canyon

5.2 0.3

2015 6.6 0.4

2023 10.6 0.6

Table 15. Daily average paved road dust emissions in the Treasure Valley during the PM season 

(winter).

Year County

PM10 PM2.5

(tons per day)

2008

Ada

34.8 2.0

2015 43.1 2.5

2023 62.4 3.6

2008

Canyon

19.9 1.1

2015 25.2 1.4

2023 40.7 2.3

Table 16. Annual unpaved road dust emissions in the Treasure Valley.

Year County

PM10 PM2.5

(tons per year)

2008

Ada

965.8 55.1

2015 841.5 48.0

2023 718.8 41.0

2008

Canyon

165.4 9.4

2015 165.4 9.4

2023 165.4 9.4

5 Quality Control and Quality Assurance

Quality control was achieved by a quality assurance check of each set of inputs by a team 

member not directly involved with developing the input. In general, each input was checked for 

internal consistency, compared with national defaults, and assessed for reasonableness. Input 

data and outputs were graphed and analyzed to ensure that the expected vehicle population, 

roadway activity, and seasonal patterns were obtained and that differences between these inputs 

and those of the national default data set for these counties were understood and justified. Paved 

road dust inputs and computations were checked for accuracy and reasonableness.
Base- and Future-Year Emissions Inventory

17

6 Conclusion

On-road mobile sources of particulate matter and secondary aerosol precursors represent the 

most significant source of particulate matter pollution in the Treasure Valley. Both the on-road 

mobile emissions and paved road dust components of the Treasure Valley EI were revised by 

DEQ in 2011 to ensure that the northern Ada County PM10 maintenance plan renewal would be 

as up-to-date and accurate as possible. This revision was made necessary by the advent of new 

EPA-recommended models for on-road emissions (MOVES2010a) and for paved road dust 

(EPA 2011a). By completing these updates for the northern Ada County PM10 maintenance plan 

renewal, DEQ ensured that the conformity determinations for years to come will be made on the 

same basis as the motor vehicle emission budgets and that artificial method-caused differences 

will be minimized.

Trends indicate that motor vehicle exhaust and evaporative emissions will decrease in the 

foreseeable future even though the VMT is projected to increase. On the other hand, paved road 

dust emissions are projected to increase roughly proportionally to VMT.
Base- and Future-Year Emissions Inventory

18

References

EPA (US Environmental Protection Agency). 2010. Technical Guidance on the Use of 

MOVES2010 for Emission Inventory Preparation in State Implementation Plans and 

Transportation Conformity. Washington, DC: EPA, Office of Transportation and Air 

Quality, Transportation and Regional Programs Division. EPA-420-B-10-023.

EPA (US Environmental Protection Agency). 2011a. "MOVES (Motor Vehicle Emission 

Simulator)." Available at: http://www.epa.gov/otaq/models/moves/index.htm.

EPA (US Environmental Protection Agency). 2011b. AP-42 Compilation of Air Pollutant 

Emission Factors. Accessed October 2011 at:

http://www.epa.gov/ttn/chief/ap42/index.html. 

ERG and Environ (Eastern Research Group and Environ International Corporation). 2010.

“2008, 2015, and 2023 Emission Inventories for the Treasure Valley Airshed.” Cordova, 

CA: ERG and Environ.

Etyemezian V., H. Kuhns, J. Gillies, M. Green, J. Chow, S. Kohl, and J. Watson. 2002. The

Treasure Valley Road Dust Study. Las Vegas, NV: Desert Research Institute. 

Langston R., S. Russell, R.S. Merle, D. Hart, V. Etyemezian, H. Kuhns, J. Gillies, D. Fritz, 

K. Bumiller, and D. James. 2008. The Preferred Alternative Method for Measuring 

Paved Road Dust Emissions for Emissions Inventories: Mobile Technologies vs. The 

Traditional AP-42 Methodology. In: 16th Annual International Emissions Inventory 

Conference. 

Sierra Research. 2006. Development of Fleet Characteristics Data for the Idaho On-Road Motor 

Vehicle Fleet. Sacramento, CA: Sierra Research. SR2006-01-01.

WRCC (Western Regional Climate Center). 2009. Accessed from http://www.wrcc.dri.edu/ in 

2009.
Base- and Future-Year Emissions Inventory

19

Appendix A. Crosswalk between COMPASS TDM Road 

Types, FHWA Roadway Types, and MOVES Roadway Types

COMPASS 

Road Type 

ID

COMPASS Road Type 

Descriptions

Area 

Type

FHWA 

Road 

Type 

Code

FHWA Road 

Type 

Description

MOVES 

Road 

Type

MOVES 

Road Type 

Description

Parked vehicles and extended idle are not included in COMPASS or FHWA road types 1 Off Network

1

Interstate or expressway with urban 

interchanges (e.g., SH 16)a Rural

01

Rural Principal 

Arterial—

Interstate

2

Rural 

Restricted 

Access 2 HOVb

(not currently in use) Rural

19 Interstate ramps Rural

3

Principal arterials in CBDcand/or 

are one-way

Rural

02 Rural Principal 

Arterial—Other

3

Rural 

Unrestricted 

Access

4

Expressway with at-grade 

intersections (e.g., Chinden Blvd) Rural

5

Principal arterials in urban areas 

(use area of impact) Rural

6

Principal arterial in rural areas (use 

area of impact) Rural

7

Minor arterials in CBD and/or are 

one-way

Rural

06 Rural Minor 

Arterial

8 Minor arterials (not currently in use) Rural

9

Minor arterials in urban areas (use 

area of impact) Rural

10 Minor arterials in rural areas (use 

area of impact) Rural

11 Rural minor arterials Rural

12 Collectors in CBD and/or are onewayRural

07 Rural Major 

Collector 13

Collectors in urban areas (use area 

of impact) and for subdivision 

access which allow through travel

Rural

14 Collectors in rural areas (use area of 

impact) Rural

15 Collectors for subdivision access to 

local roads and no through travel Rural 08 Rural Minor Collector

16 Local roads for subdivision access Rural

09 Rural Local

17 Local roads in urban areas added 

for circulation Rural

18 Local roads in rural areas added for 

circulation Rural

20 Centroid connector Rural

1

Interstate or expressway with urban 

interchanges (e.g., SH 16) Urban

11

Urban Principal 

Arterial—

Interstate

4

Urban 

Restricted 

Access

19 Interstate ramps Urban

2

HOV (not currently in use)

Urban 12

Urban Principal 

Arterial—Other 

Freeways or 

Expressways
Base- and Future-Year Emissions Inventory

20

COMPASS 

Road Type 

ID

COMPASS Road Type 

Descriptions

Area 

Type

FHWA 

Road 

Type 

Code

FHWA Road 

Type 

Description

MOVES 

Road 

Type

MOVES 

Road Type 

Description

3

Principal arterials in CBD and/or are 

one-way

Urban

14 Urban Principal 

Arterial—Other

5

Urban 

Unrestricted 

Access

4

Expressway with at-grade 

intersections (e.g., Chinden Blvd) Urban

5

Principal arterials in urban areas 

(use area of impact) Urban

6

Principal arterial in rural areas (use 

area of impact) Urban

7

Minor arterials in CBD and/or are 

one-way

Urban

16 Urban Minor 

Arterial

8 Minor arterials (not currently in use) Urban

9

Minor arterials in urban areas (use 

area of impact) Urban

10 Minor arterials in rural areas (use 

area of impact) Urban

11 Rural minor arterials Urban

12 Collectors in CBD and/or are onewayUrban

17 Urban Collector

13

Collectors in urban areas (use area 

of impact) and for subdivision 

access which allow through travel

Urban

14 Collectors in rural areas (use area of 

impact) Urban

15 Collectors for subdivision access to 

local roads and no through travel Urban

16 Local roads for subdivision access Urban

19 Urban Local

17 Local roads in urban areas added 

for circulation Urban

18 Local roads in rural areas added for 

circulation Urban

20 Centroid connector Urban

a

SH = state highway

b HOV = high-occupancy vehicle

c CBD = central business district
Base- and Future-Year Emissions Inventory

21

Appendix B. Crosswalk between ATR Length Bins, FHWA 

Vehicle Classes, and MOVES Source Types 

ATR 

Length 

Bin

ATR Length Bin 

Range

FHWA 

Vehicle 

Class

FHWA Vehicle Class 

Description

MOVES 

Source 

Type ID

MOVES Source Types

1 0–5.9 ft 1 Motorcycles 11 Motorcycle 

2 6–22.9 ft

2 Passenger Cars 21 Passenger Car 

3

Other Two-Axle, FourTire, Single-Unit 

Vehicles

31 Passenger Truck 

32 Light Commercial Truck 

3 23–39.9 ft

4 Buses

41 Intercity Bus 

42 Transit Bus 

43 School Bus 

5

Two-Axle, Six-Tire, 

Single-Unit Trucks

51 Refuse Truck 

52 Single Unit Short-haul Truck 

53 Single Unit Long-haul Truck 

54 Motor Home 

6

Three-Axle, SingleUnit Trucks

51 Refuse Truck 

52 Single Unit Short-haul Truck 

53 Single Unit Long-haul Truck 

54 Motor Home 

7

Four-or-More Axle, 

Single-Unit Trucks

51 Refuse Truck 

52 Single Unit Short-haul Truck 

53 Single Unit Long-haul Truck 

54 Motor Home 

4 40–69.9 ft

8

Four-or-Less Axle, 

Single-Trailer Trucks

61 Combination Short-haul Truck 

62 Combination Long-haul Truck 

9

Five-Axle, SingleTrailer Trucks

61 Combination Short-haul Truck 

62 Combination Long-haul Truck 

10 Six-or-More Axle, 

Single-Trailer Trucks

61 Combination Short-haul Truck 

62 Combination Long-haul Truck 

5 >70 ft

11 Five-or-Less Axle, 

Multi-Trailer Trucks

61 Combination Short-haul Truck 

62 Combination Long-haul Truck 

12 Six-Axle, Multi-Trailer 

Trucks

61 Combination Short-haul Truck 

62 Combination Long-haul Truck 

13 Seven-or-More Axle, 

Multi-Trailer Trucks

61 Combination Short-haul Truck 

62 Combination Long-haul Truck 
Base- and Future-Year Emissions Inventory

22

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Base- and Future-Year Emissions Inventory

23

Appendix C. ITD Statewide Vehicle Classification Data
Base- and Future-Year Emissions Inventory

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