TRANSPORTATION

Air Quality

Selected Facts and Figures

FEBRUARY 2016
Quality Assurance Statement

The Federal Highway Administration provides high-qualityinformation

toserveGovernment,industry,andthepublicinamannerthat

promotespublicunderstanding.Standardsand policiesareusedto

ensure and maximize the quality, objectivity, utility, and integrity of its 

information.FHWAperiodicallyreviewsqualityissuesandadjustsits

programs and processesto ensure continuous quality improvement.

Notice

This documentis disseminated underthe sponsorship ofthe U.S.

Department of Transportation in the interest ofinformation exchange.

TheU.S.Governmentassumesnoliabilityforuseoftheinformation

contained in thisdocument.

This report does not constitute a standard,specification, or regulation.
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CONTENTS

Purpose and Terms.........................................................4

Terms...............................................................................4

Overview.........................................................................6

Air Pollutants...................................................................7

Air Quality Standards......................................................9

Nonattainment and Maintenance Areas......................10

Progress in Reducing Motor Vehicle Emissions............13

Trends in Travel.............................................................14

Roads and Spending......................................................20

Traffic Congestion.........................................................21

Emissions Sources.........................................................23

Emissions Trends...........................................................24

Air Quality Trends.........................................................30

Mobile Source Air Toxics...............................................31

Greenhouse Gas Emissions...........................................34

Emissions Standards .....................................................37

Vehicle Emissions..........................................................41

Policy Responses...........................................................45

Where Can I Get More Information?............................54

TABLE OF CONTENTS
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PURPOSE AND TERMS

Purpose and Terms

What’s It All About?

Thisbrochureprovides an overview offacts and figuresregarding the linkages

betweentransportationandairquality.Thefocusofthisbrochureisprimarilyon

transportation-related emissions trends, policies, technologies, and standards that

effect on-road mobile sources, including automobiles, light-duty trucks, and heavydutytrucks.

Terms

AFV Alternative-fuel vehicle.

AVCV Automated vehicle/connected vehicle.

CAA The Clean Air Act.

CAAA TheCleanAirActAmendmentsof1990.

CMAQ Congestion Mitigation and Air Quality Improvement

Program.

CNG Compressed natural gas.

CO Carbon monoxide, a product ofincomplete

combustion.

CO2 Carbondioxide,notacriteriapollutantbuta

greenhouse gas.

Condensables PM2.5 formed in the atmosphere from precursor gases,

such as sulfur dioxide and oxides ofnitrogen.

EPA U.S. Environmental Protection Agency.

GHG Greenhouse gas.

HC Hydrocarbons, gaseous compounds made of carbon

and hydrogen; the term is used interchangeably with

VOCs.

I/M Inspection andmaintenance.

LNG Liquefied natural gas.

LPG Liquefied petroleumgas.

Maintenance area Any geographic region previously designated as a

nonattainment area and subsequently redesignated

attainmentsubject to a maintenance plan.

MOVES Motor Vehicle Emission Simulator.

MSAT Mobile source airtoxics.

NAAQS National Ambient Air Quality Standards, federally

established standards for pollutant concentrations 

that states, cities, and towns must meet by specified

deadlines.
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NEI NationalEmissionsInventory.

Nonattainment area A designated geographic area that has failed to meet

the National Ambient Air Quality Standards.

Non-road engines Aircraft, trains, boats, off-road recreational vehicles,

farm and construction equipment, and yard tools.

NOX Oxides of nitrogen, a collective term for all

compounds of nitrogen and oxygen, including

nitrogen monoxide and nitrogen dioxide.

NO2 Nitrogen Dioxide.

O3 Ozone, an oxygen compound that can develop when

oxidesofnitrogen, volatileorganic compounds,and

sunlightinteract in the lower atmosphere; the primary

constituent ofsmog.

On-road vehicles Cars, vans, buses, light-duty and heavy-duty trucks,

and motorcycles.

PM Particulate matter.

PM10 Particulate matter with a diameter less than10

micrometers.

PM2.5 Particulate matter with a diameter less than2.5

micrometers.

Point and area sources Stationary sources of emissions, including electric

utilities,factories, petroleumrefineries, dry cleaners,

and others.

ppm Parts permillion.

Precursors Pollutants that contribute to the formation of other

pollutants; hydrocarbons and oxides of nitrogen are

precursors of ozone and particulate matter.

SIP StateImplementationPlan.

SO2 Sulfur dioxide.

TCM Transportation control measure.

ULSD Ultra low-sulfurdiesel.

VMT Vehiclemilestraveled.

VOCs Volatile organic compounds, gaseous compounds 

made of carbon and hydrogen;the termis used

interchangeably with HC.
OVERVIEW

6

Overview

What Is Transportation and Air Quality?

InresponsetotheCleanAirAct(CAA),theU.S. EnvironmentalProtectionAgency

(EPA) establishedNational AmbientAirQuality Standards(NAAQS)for various

pollutants—known as criteria pollutants—that adversely affect human health and

welfare.Thisbrochurefocusesonthefourmajortransportation-relatedcriteria

pollutants:

• Ozone (O3) and its precursors: volatile organic compounds (VOCs) and oxides of

nitrogen (NOX)

• Particulate matter (PM)

• Carbon monoxide (CO)

• Nitrogen Dioxide (NO2)

Other criteria pollutants include sulfur dioxide (SO2) and lead. In the past, motor

vehicleswereamajorsourceofleademissions,howeverthesewerevirtually

eliminated when leaded gasoline was phased out.

Althoughnot criteriaairpollutants,toxicairpollutants,orairtoxics,arepollutants

that cause or may cause cancer or other serious health effects,such as reproductive

effects or birth defects. Airtoxics may also cause adverse environmental and

ecological effects. The CAA identifies 188 air toxics. EPA has identified 21

pollutants as mobile source air toxics including diesel particulate matter, benzene,

metals, and organicmaterials.

Fuel combustion by motor vehicles and othersources releases carbon dioxide (CO2),

which is a greenhouse gas that traps heat within the earth’s atmosphere.

Significantprogresshasbeenmadeinreducingcriteriapollutantemissionsfrom

motor vehicles and improving air quality since the 1970s, even as vehicle travel has 

increased. The airisnoticeably cleanerthan in 1970,and total criteria-pollutant

emissionsfrommotorvehiclesarelessthantheywerein1970despiteanear

tripling of vehicle miles of travel. With the reduction in criteria pollutants, many air

toxics have also beenreduced.

Still,challengesremain.Inrecent years,EPAhasissuedrevisedstandardsforO3 and

PM that reflect improved understanding of the health effects of these pollutants.

Based on monitoring data, more than 150 million people in the United States reside

incountiesthatdonotmeettheair-qualitystandardsforatleastoneNAAQS

pollutant. Formore detailed data onmany ofthe subjects covered,referto the

sources and websiteslisted in this publication.
AIR POLLUTANTS

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Air Pollutants

How Do These Pollutants Affect Us?

Above certain concentrations, ozone, PM, and CO can cause or exacerbate health

problems and/or increase mortality rates, making their control an important goal

undertheCAA.

What Is Ozone (O3)?

Ground-level ozoneisthemajor component ofsmog. While ozonein the upper

atmosphere (the ozone layer) occurs naturally and protectslife on earth from

harmful ultraviolet radiation, ozone at ground level is a noxious pollutant. Groundlevel ozoneisnot directly emitted,butisformed by thereactionofNOX andVOCs

in the presence ofsunlight.

Ozone is a severe irritant,responsible forthe choking, coughing, and stinging

eyes associatedwithsmog.Ozonedamageslung tissue, aggravatesrespiratory

disease, and makes people more susceptible to respiratory infections. Children are

especially vulnerable to ozone’s harmful effects, as are adults with existingdisease.

Evenhealthyindividualsmayexperienceimpairedlungfunctionfrombreathing

ozone-polluted air. In addition to affecting human health, ozone harms vegetation,

resulting in reduced agricultural and commercial forest yields, increased tree and

plantsusceptibility to disease and other environmentalstresses, and potential longterm effects on forests and ecosystems. Peak concentrationstypically occur in

summer.

What Is Particulate Matter (PM)?

PM isthe termused for amixture ofsolid particles and liquid dropletsfound in

the air.These particles comein awide range ofsizes and can remainsuspended

intheairforextendedperiods.PMcanbeemitteddirectlybyasourceorformed

intheatmospherebythetransformationofgaseousemissions,suchasSO2,NOX,

andVOCs.

Fineparticles,fewerthan2.5micronsindiameter(PM2.5),resultfromfuel

combustion bymotor vehicles and othersources, aswell astransformation of

gaseous emissions. Coarser particles up to 10 microns in diameter (PM10) generally

consist of windblown dust and are released fromagriculture, and crushing and

grinding operations.
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Particulatematterirritatesthemembranesoftherespiratorysystem, causing

increased respiratory problems and disease, decreased lung function, alterations 

ofthe body’s defense systems, and premature mortality. Sensitive groupsinclude

the elderly, individuals with cardiopulmonary disease such as asthma, and

children. In addition to health problems, airborne particles cause soiling and

damage to materials and reduce visibility in many parts of the United States.

What Is Carbon Monoxide (CO)?

COis an odorless, colorless gasthatinterferes with the delivery of oxygen to the

body’s organs and tissues. Effects of COinclude dizziness, headaches, fatigue,

visual impairment,reducedwork capacity,reducedmanual dexterity, and poor

learningability.ThehealtheffectsofCOvarydependingonthelengthandintensity

of exposure and the health ofthe individual, and aremostseriousforthose who

sufferfromcardiovascular disease.

TheincompleteburningofcarboninfuelssuchasgasolineproducesCO.High

concentrations of CO occur along side roads with heavy traffic, particularly at major

intersections, and in enclosed areas, such as garages and poorly ventilatedtunnels.

Peak concentrationstypically occur during the coldermonths ofthe year when

vehicular emissions of CO are greater and nighttime inversion conditions are more

frequent.

What Is Nitrogen Dioxide (NO2)?

NO2 is one of a group of highly reactive gasses known as “oxides of nitrogen,” or

“nitrogenoxides(NOX).”NO2 formsquickly fromemissionsfromcars,trucksand

buses,powerplants,andoff-roadequipment.Inadditiontocontributingtothe

formationofground-levelozoneandfineparticlepollution,NO2 islinkedwitha

number of adverse effects on the respiratory system.
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AIR QUALITY STANDARDS

Air Quality Standards

What Are the National Ambient Air Quality Standards (NAAQS)?

TheNAAQSaresetbyEPAtoprotectpublichealthandwelfare.Primarystandardsare

designed to protect against adverse health effects, while secondary standards protect

against welfare effects, such as damage to crops, vegetation, buildings, and visibility.

Primary Air Quality Standards for Transportation-Related Pollutants (2015)

Pollutant Type of Average Concentration

CO 8-

hour

1 h

9 ppm

35 ppm

NO2

1-hour

Annual

100 ppb

53 ppb

O3 8-hour 0.070 ppm

PM2.5

Annual

24-hour

12μg/m3

35μg/m3

PM10 24-hour 150 μg/m3

ppm= parts permillion.

ppb= parts per billion.

μg/m3 = micrograms per cubicmeter

Source: U.S. EPA, http://www3.epa.gov/ttn/naaqs/criteria.html

Anareaisinviolationofastandardifitexceedstheconcentrationlevelforits

evaluation time frames. For example, for an area to attain the 8-hour ozone standard,

the 3-year averageofthe fourth-highest dailymaximum8-hour averageozone

concentrations measured at each monitor within the year must not exceed 0.070 ppm.

What Are Nonattainment Areas?

Areasthat do not meettheNAAQS are designated as nonattainment areas. The

CAArequiresStatestodevelopplans,knownasStateImplementationPlans(SIPs),

toattainthestandardsforeachareadesignatednonattainmentforaNAAQS.Ifthe

areas do not meet these and other requirements, they face CAA-required sanctions 

andotherpenalties,includinglossofhighwayfunds.Metropolitanplanning

organizationsandtheU.S.DepartmentofTransportation(U.S.DOT)mustensure

that transportation plans, programs, and projects conform to these SIPs.

What Are Maintenance Areas?

Amaintenanceareaisanygeographic regionoftheUnitedStatesthatEPA

previously designated as a nonattainment area for one or more pollutants pursuant

totheCleanAirActAmendmentsof1990(CAAA), andsubsequentlyredesignated

asanattainmentareasubjecttotherequirementtodevelopamaintenanceplan

underSection175AoftheCleanAirAct,asamended.Themaintenanceplan

normally contains anemissionsormodelingdemonstrationthatshowshowthe

area willstay in compliance through the 20-year maintenance period.
1

Nonattainment and Maintenance Areas

Am I Living in a Nonattainment Area?

Themaps on the followingpagesshowthe nonattainment areasforthe various

pollutants.AdditionalinformationcanbefoundintheGreenBookonEPA’swebsite

at http://www3.epa.gov/airquality/greenbk/.

Where Are the Ozone Nonattainment Areas?

OnOctober 26, 2015, EPAadopted a more stringent 8-hour ozone health-based

standardof0.070ppm.EPAwilldesignatein2017bothnewnonattainment

classificationrangesandthoseareasthatexceedthe2015standard.Toavoid

backsliding, or losing progress toward attaining the 8-hour ozone standard, specific

control measures for the previous 1-hour standard may be required to stay in place

until an area attains the 8-hourstandard.

Themapbelowshowstheozonenonattainmentareasdesignatedunderthe

older 2008 8-hourstandard of 0.075 ppm. The classification of 8-hour ozone

nonattainmentareas canrangefrommarginalwithan8-hourdesignvalueof

up to 0.086 ppmO3,to extreme with an 8-hour design value equalto or above

0.175 ppm O3.

Ozone Nonattainment Areas (January 2015)

Source: U.S. Environmental Protection Agency, Green Book, January 2015, 

http://www3.epa.gov/airquality/greenbk/

http://www3.epa.gov/oaqps001/greenbk/map8hr_2008.html

NONATTAINMENT AND MAINTENANCE AREAS
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Where Are the PM10 Nonattainment Areas?

ThismapshowsthatamajorityofthePM10nonattainmentareasarelocatedin

the western half of the United States, where a dryer climate contributes to the

formation of greater PM10 pollution.

PM10 Nonattainment Areas (January 2015)

Source: U.S. Environmental Protection Agency, Green Book, January 2015, 

http://www3.epa.gov/airquality/greenbk/mappm10.html
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Where Are the PM2.5 Nonattainment Areas?

ThismapshowsthatamajorityofthePM2.5nonattainmentareasarelocatedinthe

westernhalfoftheUnitedStatesandinindustrialareasintheeast.

Counties Designated Nonattainment for PM2.5

(October 2015)

Nonattainment areas are indicated by color. When only a portion of a county

isshownincolor,itindicatesthatonlythatpartofthecountyiswithina

nonattainmentareaboundary

Source: U.S. Environmental Protection Agency, Green Book, October 2015, 

https://www3.epa.gov/airquality/greenbook/mappm25both.html

Where Are the CO Nonattainment Areas?

AsofSeptember2010,allCOareashavebeenredesignatedtomaintenanceareas. 

GreatprogresshasbeenmadeinreducingCOemissionssince1970.

Where Are the NO2 Nonattainment Areas?

LosAngeles-SouthCoastAir basinwasredesignatedto aNO2maintenanceareain

1998.All otherareasofthe countrypresentlymeetthe currentNO2NAAQS,with

annualconcentrationswellbelowthelevelofthestandard(53ppb).Annualaverage

ambient NO2 concentrations have decreased by more than 40 percent since 1980.
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Progress in Reducing Motor Vehicle Emissions

Have We Made Progress in Reducing Motor Vehicle Emissions?

Air pollution emissionsfrommotor vehicles have dropped considerably since

1970—VOC emissions are down 87 percent,NOX emissions are down 64 percent,

PM10 emissionsaredown37percent,andCOemissionsaredown86percent.Since

1990, PM2.5 emissions are down 48 percent; however transportation carbondioxide

(CO2) emissionsincreased 16percent. The reductions are due primarily tomore

stringentlight-dutyengineandfuelstandards.Theseoccurredatthesametimeas

a 56 percent increase in population, 241 percent growth in gross domestic product,

and166percentgrowthinVMT.

Percent Change in Motor Vehicle Emissions, Demographics, and Travel

(1970–2013)

Notes: Figures for licensed †

drivers, †registered vehicles and †VMT are from 2011. Data for ‡PM2.5 and ‡CO2

are from 1990.

Sources: Bureau of Transportation Statistics, National Transportation Statistics, Table 1-35, March 2014, 

http://www.rita.dot.gov/bts/sites/rita.dot.gov.bts/files/publications/national_transportation_statistics/

html/table_01_35.html

U.S. Census Bureau, Population Division, Table 1, Annual Estimates of the Resident Population for the 

United States, Regions, States, and Puerto Rico: April 1, 2010 to July 1, 2014 (NST-EST2014-01), 

http://www.census.gov/popest/data/national/totals/2014/NST-EST2014-alldata.html

U.S. Department of Commerce, Bureau of Economic Analysis, National Economic Accounts, May 2015, 

http://www.bea.gov/National/index.htm

U.S. Department of Labor, Bureau of Labor Statistics, The Employment Situation—February 2015, 

http://www.bls.gov/news.release/archives/empsit_03062015.pdf

Federal Highway Administration, Office of Highway Policy Information, Highway Statistics 2011, Chart DV1C, December 2012, http://www.fhwa.dot.gov/policyinformation/statistics/2011/dv1c.cfm

Federal Highway Administration Highway Statistics 2013, Public Road Mileage - VMT - Lane Miles 1920–

2013, Chart VMT-421C, http://www.fhwa.dot.gov/policyinformation/statistics/2013/vmt421c.cfm

U.S. Environmental Protection Agency, Clearinghouse for Inventories and Emissions Factors (CHIEF), 

Current Emission Trends Summaries, http://www3.epa.gov/ttn/chief/trends/index.html

U.S. Environmental Protection Agency, Inventory of U.S. Greenhouse Gas Emissions and Sinks, 

http://www3.epa.gov/climatechange/ghgemissions/inventoryexplorer/#allsectors/allgas/econsect/all

PROGRESS IN REDUCING MOTOR VEHICLE EMISSIONS
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Trends in Travel

What Are the Nation’s Trends in Travel?

Travel, Economic Growth, and Population (1970–2013)

Sources: Bureau of Transportation Statistics, National Transportation Statistics, Table 1-35, March 2014, 

http://www.rita.dot.gov/bts/sites/rita.dot.gov.bts/files/publications/national_transportation_statistics/

html/table_01_35.html

U.S. Census Bureau, Population Division, Table 1, Annual Estimates of the Resident Population for the 

United States, Regions, States, and Puerto Rico: April 1, 2010 to July 1, 2014 (NST-EST2014-01), 

http://www.census.gov/popest/data/national/totals/2014/NST-EST2014-alldata.html

U.S. Department of Commerce, Bureau of Economic Analysis, National Economic Accounts, May 2015, 

http://www.bea.gov/National/index.htm

U.S. Department of Labor, Bureau of Labor Statistics, The Employment Situation—February 2015, 

http://www.bls.gov/news.release/archives/empsit_03062015.pdf

Since1970,growthinVMThasfaroutpacedpopulationgrowth.VMTgrowth

tracked closely with economic growth until the mid-2000s, but has since leveled off

even as gross domestic product has increased.

TRENDS IN TRAVEL
15

How Many Miles Do We Travel in Vehicles?

Surface Passenger Miles Traveled (1990–2012)

Source: Bureau of Transportation Statistics, National Transportation Statistics, Table 1-40, January 2015, 

http://www.rita.dot.gov/bts/sites/rita.dot.gov.bts/files/publications/national_transportation_statistics/

html/table_01_40.html

Between1990and2012,thenumberofsurfacepassengermilestraveled

increasedby418billion.Americansusecarsmorethananyotherformofground

transportation—98 percent of all passengermileswere traveled in personal

vehicles (automobiles, motorcycles, and light-duty trucks) in 2012. From 1990 to

2012,transit and intercity bus and rail use increased by 18.8 billion passenger

miles, or 26percent.

FHWAanticipatesgrowthintotalVMTbyallvehicletypestoaverage1.04percent

annually from2013-2033.GrowthintotalVMTisexpectedtoslowtoonlyabout

0.2 percent annually during the ensuing decade (2033-2043), reducing the average

growth rate overthe entire 30-yearforecast period (2013-2043)to 0.76percent

annually. Thisrepresents a significantslowdown fromthe growth in total VMT

experienced over the past 30 years, which averaged 2.08 percentannually.

Source: FHWA Forecasts of Vehicle Miles Traveled (VMT): May 2015, Office of Highway Policy Information 

Federal Highway Administration, June 5, 2015, http://www.fhwa.dot.gov/policyinformation/tables/vmt/

vmt_forecast_sum.pdf
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How Do We Get to Work?

2000 2009

Source: U.S. Census Bureau, American Community Survey, 2009, Supplemental Table A, 

https://www.census.gov/newsroom/releases/pdf/supplemental_table_a[1].pdf

Americans commutetowork insingle-occupant vehiclesmorethanby anyother

method.In2000,76percentoftheworkforcedrovetoworkalone.Thatpercentage

increased to 79 percentin 2009. The share of people commuting by transit and

carpooling declined slightly in the same period.

Why Do We Take a Vehicle Trip?

2001 2009

Source: FHWA 2009 National Household Travel Survey, Summary of Travel Trends, Table 5, January 2011, 

http://nhts.ornl.gov/2009/pub/stt.pdf

Commutinghasdeclinedsignificantlyasashareofall vehicletrips.In2001,

22percentofall vehicletripsweremadetraveling toorfromwork.By2009,only

16percentofalltripsweremadefortravelingtoorfromwork.
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What is the Growth in Freight Movement?

The abilityof ournation’stransportationsystemto providefor andmaintainthe

efficient movement of freight isimportant to the continuing economic health of the

UnitedStates.Between1997and2012,theamountoffreightmovedby truck and

railgrewby5percenteach.

Growth in Truck and Rail Freight Movement (1997-2012)

Source: U.S. Department of Transportation, Bureau of Transportation Statistics, Transportation 

Commodity Flow Survey, June 2015, http://www.rita.dot.gov/bts/sites/rita.dot.gov.bts/files/publications/

commodity_flow_survey/2012/united_states/table1b
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How Does This Freight Move Around?

In 2012, trucks and rail carried the largest percentage of domestic commercialtonmiles, followed by pipeline, water, and multimodal.

Percentage of U.S. Commercial Freight Movement(2012)

Source: U.S. Department of Transportation, Bureau of Transportation Statistics, Transportation 

Commodity Flow Survey, Table 1a, June 2015, http://www.rita.dot.gov/bts/sites/rita.dot.gov.bts/files/

publications/commodity_flow_survey/2012/united_states/table1a

What About Freight and Air Quality?

Dieselexhaustfromfreight vehiclesisaprimary sourceofPM2.5, airtoxic

contaminants, and NOX emissions. Freight emissions comprise close to one-third

of U.S. transportation greenhouse gas emissions. This growth in freight movement

andaccompanyingincreaseinemissionshasledtoagrowingneedtofindnew

ways to address air quality concerns and greenhouse gas emissions associated

with freight movement.

Marine cargo vessels and port complexes are a large source of diesel freight

emissions. In addition to cargo ships, ports use cranes, hostlers, and other

equipmentpoweredbydieselfuel.Cargovesselstypicallyburnbunkerfuel.They

areamajor contributortoairquality issuesincoastalregions.Cargoships usually

switch to their auxiliary engines to provide power for ship operations while they are

in port.Although some auxiliary engines use cleanerfuelthan themain engines,

they still contribute to localized air pollution around port complexessince the ships

maybeidlingfordaysatatime.Tocombatthisproblem,manyportsare

constructing shore power (also known as cold ironing) systems that provide cleaner

electrical power to cargo vessels while they are in port.

Source: Federal Highway Administration, Freight and Air Quality Handbook, May 2010, 

http://www.ops.fhwa.dot.gov/publications/fhwahop10024/index.htm
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How Do Vehicle Miles Traveled and Lane Mileage Compare?

Growth of VMT and Lane Mileage (1990–2013)

Source: U.S. Department of Transportation, Federal Highway Administration, Highway Statistics 2013 

(Washington, DC: Annual Issues), Table VM-422C, http://www.fhwa.dot.gov/policyinformation/statistics.cfm

Despite significant growthinVMT, lanemiles have increased only slightly since

1990.Overmore than 20 years,VMT hasincreased 37percent,while lanemiles

have increased only 7 percent. This is mitigated somewhat by targeted traffic flow 

improvementsin some communitiesthat enhance capacity without additional lane

mileage.
20

Roads and Spending

How Much Are We Spending on Roads?

In2013,$19billioninfederalfundswere obligated, and65,500milesoffederal-aid

roadwayprojectswereimplemented.

Obligation of Federal Funds for Roads (2013)

Sixty-seven percent of the obligated funds went towardssystem preservation, new 

construction comprised nine percent, and additional capacity projectsreceived 24

percent.

Miles of Federal-Aid Roadway Projects (2013)

Ninety-one percent ofthe project milesinvolved system preservation,five percent of

the milesinvolved new construction, and four percentinvolved additional capacity.

Source: Federal Highway Administration, Highway Statistics 2011, Table FA-10, Obligation of Federal-Aid 

Highway Funds for Highway Improvements, http://www.fhwa.dot.gov/policyinformation/statistics/2011/

fa10.cfm

Note: This chart excludes funds and miles related to safety and environmentally related projects, and new 

bridges and bridge replacement/rehabilitation.

ROADS AND SPENDING
21

TRAFFIC CONGESTION

Traffic Congestion

Is Traffic Congestion in Our Cities Getting Better or Worse?

Congestion occurswhenthe free flowoftrafficon aroadway isimpeded dueto

excess vehicle demand, construction, maintenance, traffic incidents, weather, or

other road conditions and events.

Beforethe2008economic recession, congestionwasincreasing atbetween2 and

4 percent every year—which meant that extra travel time for the average commuter

increased slightly less than 1 hour every year. The economic recession set back that

trendafewyears,butthetrendinthelastfewyearsindicates congestionis rising

again.

Thismapshowsthe percent changefrom2011to2014inthe amount oftime per

tripittooktotravelinthepeakperiodforselectedareas.

Changes in Traffic Congestion (2011–2014)

Source: Texas A&M Transportation Institute, 2015 Urban Mobility Report, August 2015, Table 1, 

http://tti.tamu.edu/documents/mobility-scorecard-2015.pdf
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What Are the Major Sources of Congestion?

FHWA studies show that congestion is the result of seven root causes, often

interacting with one another.

• Physical Bottlenecks (“Capacity”)—capacity is the maximum amount of traffic

capableofbeinghandledbyagivenhighway section.Whendemandisgreater

than capacity, physical limitations can cause congestion.

• Traffic Incidents—events that disrupt the normal flow of traffic such as 

vehicular crashes, breakdowns, and debrisin travel lanes.

• Work Zones—construction activities on the roadway thatresultin physical

changes to the highway environment. These changes may include a reduction in

thenumberorwidthoftravellanes,lane“shifts,”lanediversions,reductionor

elimination ofshoulders, and even temporary roadway closures.

• Weather—environmental conditions can lead to changes in driver behavior that

affect traffic flow.

• Traffic Control Devices—Intermittent disruption of traffic flow by control

devicessuchasrailroadgradecrossingsandpoorlytimedsignals.

• SpecialEvents—demandfluctuationswhereby traffic flowinthevicinityof

an event (sports, concerts) will be radically different from “typical”patterns.

Special events occasionally cause “surges” in traffic demand that overwhelm

the system.

• Fluctuations in Normal Traffic—Day-to-day variability in demand leads to some

days with higher traffic volumes thanothers.

Source: Federal Highway Administration, Traffic Congestion and Reliability: Linking Solutions to Problems 

Report, July 2004, http://ops.fhwa.dot.gov/congestion_report_04/index.htm#toc
EMISSIONS SOURCES

23

Emissions Sources

What are Sources of Emissions?

Emissions that affect air quality come from many sources. EPA groups sources into

8 major categories. The table below provides more detail about which sources are

included in eachsector.

Type of source Emissions

Agriculture Crops & Livestock Dust; Fertilizer Application; Livestock Waste

Dust Construction Dust, Paved Road Dust, Unpaved Road Dust

Fire Agricultural Field Burning, Prescribed Fires, Wildfires

Fuel Combustion Commercial/Institutional (Biomass, Coal, Natural Gas, Oil, Other);

Electric Generation (Biomass, Coal, Natural Gas,Oil, Other);

IndustrialBoilersandInternalCombustionEngines(Biomass,Coal,

Natural Gas, Oil, Other); Residential (Natural Gas, Oil, Wood,

Other)

Industrial Processes Cement Manufacturing, Chemical Manufacturing, Ferrous Metals,

Mining,National Electric Code (NEC), Non-ferrous Metals,Oil&

GasProduction,PetroleumRefineries,Pulp&Paper,Storageand

Transfer

Miscellaneous BulkGasolineTerminals;CommercialCookingGasStations;

Miscellaneous Non-Industrial NEC; Waste Disposal

Mobile Aircraft; Commercial Marine Vessels; Locomotives;Non-Road

Equipment –Diesel; Non-RoadEquipment –Gasoline;Non-Road

Equipment – Other; On-Road Diesel Heavy Duty Vehicles; On-Road

DieselLightDutyVehicles;On-RoadGasolineHeavyDutyVehicles;

On-Road Gasoline Light Duty Vehicles

Solvent Consumer&CommercialSolventUse;DegreasingDryCleaning;

Graphic Arts; Industrial Surface Coating&SolventUse; NonIndustrial Surface Coating

Source: U.S .Environmental Protection Agency, http://www3.epa.gov/air/emissions/basic.htm
24

Emissions Trends

How Have VOC Emissions Changed since 1990?

VOC Emissions (2013)

VOCs are precursors of ground-level ozone and PM. In 2013, on-road vehicles

produced14percentofallVOCemissions,downfrom39percentin1990.On-road

vehicle VOC emissions have decreased 77 percent in the last two decades.

VOC Emissions Trends since 1990

Source: U.S. Environmental Protection Agency, Clearinghouse for Inventories and Emissions Factors 

(CHIEF), Current Emission Trends Summaries, http://www3.epa.gov/ttn/chief/trends/index.html

EMISSIONS TRENDS
25

How Have NOX Emissions Changed since 1990?

NOX Emissions (2013)

In combinationwithVOCs,NOX contributestothe formation of ozone. Itis also

aprecursortoPM.In2013,on-roadvehiclesproduced38percentofallNOX

emissions,thesameasin1990.However,totalon-roadvehicleemissionsofNOX

have decreased 48 percent since 1990.

NOX Emissions Trends since 1990

Source: U.S. Environmental Protection Agency, Clearinghouse for Inventories and Emissions Factors 

(CHIEF), Current Emission Trends Summaries, http://www3.epa.gov/ttn/chief/trends/index.html
26

How Have CO Emissions Changed since 1990?

CO Emissions (2013)

On-roadvehiclesareoneofthelargestsourcesofCOemissions.However,by2013,

on-roadvehiclesproduced34 percentof all COemissions, down from72percent

in 1990.

CO Emissions Trends since 1990

Source: U.S. Environmental Protection Agency, Clearinghouse for Inventories and Emissions Factors 

(CHIEF), Current Emission Trends Summaries, http://www3.epa.gov/ttn/chief/trends/index.html
27

How Have PM10 Emissions Changed since 1990?

PM10 consistsofdust,directsmoke,andliquiddroplets.ThemajorityofPM10

emissions come from sourcessuch as prescribed fires and wildfires, dust from paved

and unpaved roads,road construction, and agriculture operations(designated

Miscellaneousinthepiechartbelow).Sourcessuchasfuel combustion,industrial

sources,andtransportation,togethermakeuponly13percentoftotalPM10

emissions.

PM10Emissions(2013)

PM10 fromallsourcesdeclinedover 23percent between1990 and 2013,and onroad vehicle PM10 emissions decreased over 31 percent.

PM10 Emissions Trends since 1990

Source: U.S. Environmental Protection Agency, Clearinghouse for Inventories and Emissions Factors 

(CHIEF), Current Emission Trends Summaries, http://www3.epa.gov/ttn/chief/trends/index.html
28

How Have PM2.5 Emissions Changed since 1990?

PM2.5 resultsfrommotor vehicle fuel combustion and othersources.Most PM2.5

emissions come from sourcessuch as prescribed fires and wildfires, dust from paved

and unpaved roads,road construction, and agriculture operations(designated

Miscellaneousinthepiechartbelow).Sourcessuchasfuel combustion,industrial

sources, and transportation, together make up 29 percent of total PM2.5 emissions.

PM2.5 Emissions(2013)

Since 1990, PM2.5 fromallsources decreased by almost 17percent. Although onroadexhaustandemissionsfrombrakeandtirewearaccountedforonly3percent

of the direct PM2.5 emissions in 2013, on-road vehicle PM2.5 emissions declined more

than 43 percent since 1990.

PM2.5 Emissions Trends since 1990

Source: U.S. Environmental Protection Agency, Clearinghouse for Inventories and Emissions Factors 

(CHIEF), Current Emission Trends Summaries, http://www3.epa.gov/ttn/chief/trends/index.html

Note: 1990–1998 emissions do not include condensables. Condensables are the majority of PM2.5 that is

formed in the atmosphere from precursor gases such as SO2 and NOX.
29

How Have Emissions Changed in the Last 30 Years?

America made great progress cleaning the air. For nearly 30 years, national emissions 

trendsforpoint andarea sources and on-roadsources have declined.However,

non-road source emissions increased with the exception of PM10.

Source: U.S. EPA Office of Air and Radiation, http://www3.epa.gov/airtrends/index.html

Note: Data for PM10 begins in 1990.

What has Caused the Improvement?

Agreatdealofthecreditfortheimprovementsinon-roadsourcesgoestocleaner

carsandtrucks,andfuels.
30

Air Quality Trends

How Much Progress Is Being Made?

Ason-roadairpollutantemissionsdeclineovertime,airqualityimproves.

Reductionsin air pollutant concentrations are impressive, with concentrations of CO

decreasingby60percentsince2000.All criteriapollutantshaveshownsignificant

decreases over the last decade,showing a steady trend of improvement.

On-Road Air Pollution Concentration Reductions (2000–2013)

Sources: U.S. Environmental Protection Agency, Clearinghouse for Inventories and Emissions Factors 

(CHIEF), Current Emission Trends Summaries, http://www3.epa.gov/ttn/chief/trends/index.html

AIR QUALITY TRENDS
31

Mobile Source Air Toxics

What Are Mobile Source Air Toxics?

Mobile source air toxics are compounds emitted from highway vehicles and

non-road equipment which are known or suspected to cause cancer or other

serious health and environmental effects. Mobile sources are responsible for

direct emissions of air toxics and contribute to precursor emissions which

react to form secondary pollutants. Examples of mobile source air toxics

include benzene, 1,3-butadiene, formaldehyde, acrolein, polycyclic organic

matter, naphthalene, and dieselparticulatematter.

What Efforts Are BeingMade?

InFebruary 2007,EPAfinalizedtheControlofHazardousAirPollutantsfromMobile

Sourcesruletoreducehazardousairpollutantsfrommobilesources.Therulelimits

the benzene content of gasoline and reduces toxic emissions from passenger vehicles

andgascans.EPAestimatesthatby2030thisrulewillreducetotalemissionsof

mobilesourceairtoxicsby330,000tonsandVOCemissions(precursorstoozoneand

PM2.5)byover1milliontons.EPAadoptedmobilesourceemissionscontrolprograms

that,inadditiontocontrollingpollutantssuchashydrocarbons,particulatematter,

andnitrogenoxides,alsoresultinlargeairtoxicreductions.

The chart below shows the source contributions for air toxics. On-road vehicles 

account for about one-quarter of all air toxic emissions.

National Contribution of Each Emissions Source Type (2011)

Source: U.S. Environmental Protection Agency, http://www.epa.gov/orcdizux/toxics.htm

MOBILE SOURCE AIR TOXICS
32

What Are the Important Mobile Source Air Toxics in

Transportation?

The EPA 1999 National Air Toxics Assessment (NATA) identified seven compounds 

with significant contributions from mobile sources that are among the national and

regional-scale cancer riskdrivers:

• Acrolein

• Benzene

• 1,3-butadiene

• Formaldehyde

• Diesel particulate matter and diesel exhaust organic gases

• Naphthalene

• Polycyclic organic matter

Some ofthese chemicals(such as benzene) are presentin gasoline and diesel

fuel and are emitted through evaporation or when fuel passes through an engine

without being burned. Others (such as formaldehyde and diesel particulate

matter) are not present in the fuel itself; rather, they are byproducts of

incomplete combustion. These compounds have a variety of potential human

health effects. Benzene, for instance, is a known carcinogen.

The chart below shows the estimated proportion of emissions from the seven

notable MSATs from on-road vehicle activity in 2015.

Distribution ofMSATs Emissions from On-Road Vehicles (2015)

Source: Federal Highway Administration,

http://www.fhwa.dot.gov/environment/air_quality/air_toxics/policy_and_guidance/aqintguidmem.cfm
33

How Much of the Transportation MSATs Can Be Reduced?

The following chart shows the estimated on-road reductionsin seven MSATsthrough

2050despiteanexpected102percentincreaseinnationalVMT.Thedecreasesare

derived from improved vehicle technology and emissions modelingtools.

Estimated Percent Change in MSAT Emissions (2010–2050)

Trendsforspecificlocationsmaybedifferent,dependingonlocallyderived

information representing vehicle-milestravelled, vehicle speeds, vehiclemix,fuels,

emissions control programs,meteorology, and otherfactors.

Sources: Federal Highway Administration memorandum, Interim Guidance Update on Mobile Source Air 

Toxic Analysis in NEPA, December 2012, http://www.fhwa.dot.gov/environment/air_quality/air_toxics/

policy_and_guidance/aqintguidmem.cfm#fig1
34

Greenhouse Gas Emissions

What about Greenhouse Gas Emissions?

Greenhousegases(GHGs)trapheatwithinthe earth’satmosphere.Although

mostGHGsoccurnaturallyandhelpkeeptheearthhospitabletolife,theyarealso

generated by human activities. CO2 emissions account for more than 80 percent of

theU.S.GHGemissions.Theseemissionsarecontributingtochangesinthe

planet’s climatethatcouldleadtoharmfuleffects,suchassea-levelriseand

changesinglobal hydrological patterns. Although the United States makes up 4.4 

percent of the world’s population, it emits about 16 percent of carbon emissions

from fossil fuel combustion.

Sources of Greenhouse Gases (2013)

Source: U.S. Environmental Protection Agency, U.S. Greenhouse Gas Inventory Report: 1990-2013, 

http://www3.epa.gov/climatechange/ghgemissions/usinventoryreport.html

Greenhouse gas emissions from transportation primarily come from burning

fossil fuel for our cars, trucks, ships, trains, and planes. Overall,transportation

contributes 27 percent of national GHG emissions.

Source: Boden, T.A., Marland, G., and Andres, R.J. (2015). National CO2 Emissions from Fossil-Fuel 

Burning, Cement Manufacture, and Gas Flaring: 1751-2011, Carbon Dioxide Information Analysis Center, 

Oak Ridge National Laboratory, U.S. Department of Energy, doi 10.3334/CDIAC/ 00001_V2015. 

http://cdiac.ornl.gov/trends/emis/tre_coun.html

GREENHOUSE GAS EMISSIONS
35

Incontrasttomostcriteriapollutants,emissionsofGHGshavebeenrisingfrom

most sectors. From 1990 to 2013, carbon emissions from transportation grew by

16 percent.

Change in Carbon Emissions (1990–2013)

Source: U.S. Environmental Protection Agency, U.S. Greenhouse Gas Inventory Report: 1990-2013, http:// 

www3.epa.gov/climatechange/Downloads/ghgemissions/US-GHG-Inventory-2015-Chapter-2-Trends.pdf

Highway vehicles are the largest users of transportation energy, accounting for 82percent

ofthetotal.

Transportation Energy Use by Mode (2012)

Source: U.S. Energy Information Administration, Annual Energy Outlook 2015, Table A7—Transportation

Sector Key Indicators and Delivered Energy Consumption, April 2015,

http://www.eia.gov/beta/aeo/#/?id=7-AEO2015
36

The amount of energy used and itsintensity is an indicatorforthe amount of GHGs

produced. Per passenger mile, transit and passenger rail commuter transportationmodes 

arelessenergyintensethanlight-dutypassengervehicles.However,light-dutytrucksare

more energy intense. Forfreight transport, heavy-duty trucks are considerably more energy

intensethanrailonaton-milebasis.

Transportation Energy Intensity (2012)

Source: Bureau of Transportation Statistics, National Transportation Statistics, Table 4-20, 4-25: Energy

Intensity of Passenger Modes, April 2015,

http://www.rita.dot.gov/bts/sites/rita.dot.gov.bts/files/publications/national_transportation_statistics/

html/table_04_20.html

http://www.rita.dot.gov/bts/sites/rita.dot.gov.bts/files/publications/national_transportation_statistics/

html/table_04_25.html

Transportation Energy Data Book: Edition 33, ORNL-6990, Oak Ridge National Laboratory, July 2014, 

http://cta.ornl.gov/data/tedb34/Spreadsheets/Table2_17.xls
37

Emissions Standards

What Are the Federal Emissions Standards?

TheCAA setfederal emissions-controlstandardsfor allnewcars andlighttrucks

soldintheUnitedStates.TheCAAAestablishedstringentTier1emissions

standards,whichbecameeffectivein1994.MorestringentTier2emissions

standardswereadoptedin2004.InApril2014,EPAfinalized newTier 3 emissionscontrolstandards.

What Do the New Tier 3 Emissions Standards Do?

Startingin2017,Tier3standardswillsetnewvehicleemissionsstandardsandlower

the sulfur contentof gasoline, considering the vehicleanditsfuel as anintegrated

system. Thenewstandardsreducebothtailpipeandevaporativeemissionsfrom

passenger cars, light-duty trucks, medium-duty passenger vehicles, and some heavyduty vehicles.

Estimated Emissions Reductions from the Final Tier 3 Standards

Source: U.S. Environmental Protection Agency, Office of Transportation and Air Quality, EPA Sets Tier 3

Motor Vehicle Emission and Fuel Standards, EPA- 420-F-14-009, March 2014,

http://www3.epa.gov/otaq/documents/tier3/420f14009.pdf

What Are theBenefits?

The Tier 3 gasoline sulfurstandardwillmakeemissions-controlsystems more

effectiveforbothexistingandnewvehicles,andwillenablemorestringentvehicle

emissionsstandards.Removingsulfurallowsthe vehicle’scatalysttoworkmore

efficiently. Lower-sulfur gasoline also facilitates the development of some lower-cost

technologiestoimprovefueleconomyandreduceGHGemissions,whichreduces

gasoline consumption.

EMISSIONS STANDARDS
38

What Are the Heavy-Duty Diesel Emissions Standards?

InDecember2000,EPAissuedthefinalruleforatwo-partstrategytoreduce

diesel emissions from heavy-duty trucks and buses. This included new diesel-engine

standardsinmodelyear2004foralldieselvehiclesover8,500pounds.These

standards are based on the use of high-efficiency advanced emissions controls.

Estimated Vehicle Emissions Under Heavy-Duty Engine/Fuel Rule (2007–2030)

Source: U.S. Environmental Protection Agency, Heavy-Duty Engine and Vehicle Standards and Highway

Diesel Fuel Sulfur Control Requirements, December 2000,

http://www3.epa.gov/otaq/regs/hd-hwy/2000frm/420r00026.pdf
39

What Are the Fuel Efficiency and GHG Emissions Standards for 

Heavy-Duty Trucks?

The nation’s fleet of medium- and heavy-duty trucks are required to meet fuel

efficiency and GHG emissions standards for three categories of medium-and

heavy-duty trucks:

• Certaincombinationtractors,commonlyknownasbigrigsorsemi-trucks,are

required to achieve an approximately 20 percent reduction in fuel consumption

andGHGemissionsbymodel year2018,savingupto4 gallonsoffuelfor every

100 milestraveled.

• For heavy-duty pickuptrucks and vans,separate standardsarerequiredfor

gasoline-powered and diesel trucks. These vehicles are required to achieve up to

abouta15percentreductioninfuelconsumptionandGHGemissionsbymodel

year2018.Underthe finalizedstandards, a typical gasolineor dieselpowered

heavy-duty pickuptruck or van couldsave 1 gallonoffuelfor every 100miles

traveled.

• Vocational vehicles, including delivery trucks, buses, and garbage trucks, are

requiredtoreducefuelconsumptionandGHGemissionsbyapproximately

10percentbymodelyear2018.Thesetrucks couldsaveanaverageof1 gallonof

fuel for every 100 milestraveled.

In2015,EPAandtheNationalHighwayTrafficSafetyAdministration(NHTSA)issued

a notice of proposed rulemaking (NPRM) to establish a comprehensive Phase 2Heavy

DutyNationalProgramthatwillreduceGHGemissions andfuel consumptionfor

newon-roadheavydutyvehicles.Thisprogramwouldphaseinoverthelongterm, beginning in the 2018model year and culminating in standardsformodel

year2027.

Source: U.S. Environmental Protection Agency, Office of Transportation and Air Quality, Paving the Way 

toward Cleaner, More Efficient Trucks, EPA-420-F-11-032, August 2011, http://www3.epa.gov/otaq/

climate/documents/420f11032.pdf

U.S. Environmental Protection Agency, Cutting Carbon Pollution, Improving Fuel Efficiency, Saving

Money, and Supporting Innovation for Trucks, EPA-420-F-15-900, June 2015,

http://www3.epa.gov/otaq/climate/documents/420f15900.pdf
40

What Are the Non-Road Vehicle Emissions Standards?

Non-road diesel enginesinclude excavators and other construction equipment, farm

tractorsandotheragriculturalequipment,heavy forklifts,airportgroundservice

equipment, and utility equipmentsuch as generators, pumps, and compressors. Nonroad diesel engines are subjectto a set oftiered emissionsstandards. Tier 1 through 4

standards are currently in effect, having been implemented in steps since 1998. The

regulations have been phased in for newly manufactured enginesin different years

depending on the power output of the engine. Each tier represents a progression to

more stringentregulations.

Tier4standardswereimplementedfrom2008to2015andcallforstricterlimitson

NOX and PM (about a 90 percentreduction in emissions). EPAalso hasmandated

the use of lowersulfur diesel fuel to enable more advanced pollution control on

these engines. Since 2010,ultra-low sulfur diesel fuel isrequired, containing only

15ppm ofsulfur. Engine manufacturers must produce new engines with

advancedemissions controltechnologiessimilartothoseexpectedforhighway

trucks and buses. Exhaust emissions from these engines will decrease by more than 

90 percent.

In 2008, EPAadopted more stringent emissionsstandards and mandated the

application of idle-emission controls on newly manufactured and remanufactured

locomotives. The idle reduction technology automatically shutslocomotives down

if they are left idling unnecessarily. While such devices cannot eliminate all idling,

they can reduce most unnecessary idling.

What Effects Do the Standards Have on Pollutants?

Thesestandardsremoved99percentofthesulfurindieselfuelby2010,resultingin

dramatic reductionsin soot from all diesel engines.

The Non-road Diesel Rule complements the Clean Diesel Truck and Bus Rule.The

newstandardswereestimatedtoresultinreductionsofpollutionequivalentto

havingsome2millionfewertrucksontheroad.

Sources: U.S. Environmental Protection Agency, http://www3.epa.gov/otaq/nonroad-diesel.htm

U.S. Environmental Protection Agency, Clean Air Nonroad Diesel Rule-Regulatory Announcement, 

Publication No. EPA420-F-04-032, May 2004, http://www3.epa.gov/otaq/documents/nonroaddiesel/420f04032.pdf

U.S. Environmental Protection Agency, Clean Diesel Programs-Facts and Figures, Publication No. 

EPA420-F-04-040, May 2004, http://www3.epa.gov/diesel/documents/420f04040.pdf

U.S.Environmental Protection Agency, Control of Emissions from Idling Locomotives, December 2013, 

http://www3.epa.gov/otaq/regs/nonroad/locomotv/420f13050.pdf
41

Vehicle Emissions

What Are the Sources of Vehicle Emissions?

Thepowertomoveamotor vehiclecomesfromburningfuelinanengine.Emissions

from vehicles are the by-products of this combustion process. In addition, VOCs escape

throughfuelevaporation.As vehicleexhaustsystemshaveimproved,evaporative

emissions have become a larger component of total-vehicle VOC emissions.

What Emissions Come from Vehicle Exhaust?

ThecombustionprocessresultsinemissionsofVOC,NOX,PM,andCO,whichare

releasedfromthetailpipewhile avehicleisoperating. Exhaustemissionsoccur

during two modes:

• Start emissions—Starting a vehicle and the first few minutes of driving generate

higher emissions because the emissions-control equipment has not yet reached

its optimal operating temperature.

• Running exhaust emissions—Pollutants are emitted from the vehicle’stailpipe

during driving and idling afterthe vehicle is warmed up.

What VOC Emissions Come from Fuel Evaporation?

VOCs also escape into the airthrough fuel evaporation. Despite evaporative

emissions controls, evaporative losses can still account, on hot days, for a majority

of the total VOC pollution from current-model cars. Evaporative emissions occur in

several ways:

• Running losses—The hot engine and exhaust system can vaporize gasoline

while the vehicle is running.

• Hotsoak (cooling down)—The engine remains hotfor a period oftime after

thevehicleisturnedoff,andgasolineevaporateswhenthecarisparkedwhile

cooling down.

• Diurnal(whileparkedandtheengineiscool)—Evenwhenthevehicleisparked

forlongperiodsoftime,gasolineevaporationoccursasthetemperaturerises

during the day.

• Refueling—Gasolinevaporsescapefromthevehicle’sfueltankwhilethetank

is being filled.

Source: U.S. Environmental Protection Agency, Automobile Emissions: An Overview, Fact Sheet OMS-5, 

August 1994, http://www3.epa.gov/otaq/consumer/ 05-autos.pdf

VEHICLE EMISSIONS
42

What Are the Emissions from a Typical Drive in My Car?

Average Emissions of a Typical Car on the Road in 2015

Source: U.S. Environmental Protection Agency, MOVES2014 model run, assumed summer weekday for

VOC and NOX, winter weekday for CO, June 2015

Starting a car cold increases trip emissions compared to starting an engine that

isalreadywarm.Onagivenweekday,coldstartsofatypicalvehicleproduce

2.4 grams of VOCs(25 percent of the typical daily emissions), 24.2 grams of CO

(28 percent ofthe typical daily emissions), and 2.4 grams ofNOX (24 percent of

the typical daily emissions) per vehicle start. Running exhaust accounts for another

2.2 grams of VOCs, 61.9 grams of CO, and 7.6 grams ofNOX per mile traveled.

VOCs are also emitted through fuel evaporation. For example, parking your car all

dayproducesabout3.8gramsofVOCs.

Source: U.S. Environmental Protection Agency, MOVES2014 model run, assumed summer weekday for 

VOC and NOX, winter weekday for CO, June 2015, http://www3.epa.gov/otaq/models/moves/index.htm
43

Do Vehicle Emissions Rates Change with Vehicle Speed?

Emissionsratesvarybasedonthespeedavehicleistraveling.EPA’smodelfor

highway vehicle emissions, Motor Vehicle Emission Simulator (MOVES), shows how 

speedaffectsemissionsrates.VOC,NOX,CO,andPM2.5 emissionsratesaretypically

higheratlowerspeeds(0-20mph).

Thesecurvesdonotrepresentthefullrange ofeffects associatedwithtravel at

different speeds. Emissions rates are higher during stop-and-go, congested traffic

conditions than free-flow conditions operating at the same average speed.

Emissions Rates at Various Speeds

Sources: U.S. Environmental Protection Agency, MOVES2014 model run, assumed summer weekday for 

VOC and NOX, winter weekday for CO, June 2015, http://www3.epa.gov/otaq/models/moves/index.htm
44

How Have Car and Truck Emissions Changed since the Clean Air

Act?

The comparisons below show estimated in-use emissions rates, in grams per mile,

for cars and heavy-duty dieseltrucks with 2015controltechnology versus 1967

vehicles(beforesignificantemissions controls).The1967ratesarebasedonEPA’s

previous emissions model, MOBILE6.2, while the 2015 rates are derived from EPA’s

newer MOVESmodel.

Car and TruckEmissions

Car emissionsrates have declined bymore than 90 percent depending on

the pollutant, while heavy-duty diesel truck emissions rates have declined by

84 percentfor VOCs, 70percentforNOX, and 76percentfor CO.Newer cars

and trucks burn fuel much more efficiently now.

Sources: U.S. Environmental Protection Agency, MOVES2014 model run, assumed summer weekday for 

VOC and NOX, winter weekday for CO, June 2015, http://www3.epa.gov/otaq/models/moves/index.htm

U.S. Environmental Protection Agency, MOBILE6.2 model run, March 2005
45

Policy Responses

Federal Policy Helping to Achieve Our Clean Air Standards

Despitecontinuedimprovements,theairqualityissuesfacingstatesandregions

requirethatpolicymakersconsiderstrategiestoreduceemissionsfromall

sources—point and area, on-road mobile sources, and non-road engines. The

effortstoachieveNAAQSandGHGreductionswillrequiresubstantialemissions

reductions overthe nextfew decades and nearzero emissions of CO2 and other

greenhouse gases by the end of the century. Implementing such reductions poses 

substantial technological, economic, social and institutional challenges. Multiple

strategies are available to transportation and air quality officialsranging from

regulatory to voluntary, and from technology- and fuel-based strategies to marketbased measures aimed at changing driver behavior.

Transportation Conformity

Transportation conformity is a process to ensure that federal funding and approval

are given to those transportation activities that are consistent with air quality goals.

Theconformity regulationrequiresthatalltransportationplansandprogramsin

nonattainment or maintenance areas conform to the state’s air quality plan, the SIP.

Itensuresthattransportationactivitiesdonotworsenairqualityorinterferewith

thepurposeoftheSIP,whichisto attaintheNAAQS for a criteriapollutant.

The conformityprocessrequiresstateand localtransportation officialstomake

tough decisionsin ordertomeet both air quality andmobility goals. Where CAA

goalswerenotbeingmet,somestateandlocaltransportationofficialshavebeen

challengedtofindwaystoreducevehicleemissionsbydevelopingtransportation

plans, TIPs, and projects that will alter travel patterns, reduce the number of singleoccupant vehicles,andmakealternativemodesoftransportation(suchastransit

and bicycles) an increasingly important part of the transportation network.

Inspection and Maintenance Programs

Vehicleinspectionandmaintenance programs(I/M) helpimproveairqualityby

identifying high-emitting vehiclesin need ofrepair(through visual inspection,

emissions testing, and/or the downloading of information from a vehicle’s onboard

computer) and causing themto be fixed as a prerequisite to vehicle registration

withinagivennonattainmentarea.Ahigh-emittingvehicleisonewhichisemitting

atsome increment above the standard to which it was certified, either 2-3times

the standard for vehicles being tailpipe tested, or 1.5 times the standard for vehicles 

equipped with onboard diagnostic (OBD) systems and subject to the OBD test.

The CAAA made I/M mandatory for several areas across the country, based upon

variouscriteria,suchasairqualityclassification,population,and/orgeographic

location.

POLICY RESPONSES
46

Asmallpercentageofvehiclesemitalargepercentageofthepollutionfrom

on-road vehicles. These gross emittersinclude not only older-model cars, but

also some new cars with poorly maintained or malfunctioning emissionscontrol equipment.

Previousstudies estimated thatlessthan 10percent of a vehiclefleet emits

approximately 50 percent of the VOC emissions in a region. The same vehicles,

however, are not always gross emitters for all criteria pollutants; a different 10

percentofvehiclesmaybegrossemittersforCO,NOX,andothers.Additionally,

10to27percentofthevehiclesfailinginspectionsubsequentlydonot

ultimately passstateinspection andmaintenancetests. Severalstateshave

implemented vehicle scrappage or vehicle exchange programs to remove these

types of vehicle fromtheirfleetin nonattainment areas.

Improvements in Vehicle Technology and Fuels

Numerous alternatives and improvementsto vehicle technology and fuels have

been directed and/or promoted by Federal policy in the last two decades.

Diesel EngineRetrofits

Diesel engine retrofit programs have been funded and implemented over the last

decade. Several retrofit emissions-control technologies are available with varying

levelsof demonstrated effectiveness atreducing PM,VOCs, CO, and airtoxics. 

The diesel particulate filter hasthe potential to reduce particulatesto nearzero.

Advanceddieseloxidationcatalystsresultinlessdramatic reductionsbutare

less expensive and can run on diesel with higher sulfur levels. Various fuel

additives can also help reduce emissions ofsome pollutants compared to regular

diesel fuel.

Emissionscanbereducedbyusingfuelswithcertainpropertiesorbyusing

alternativefuels.Ultra-lowsulfurdiesel(ULSD)fuelaswellasbiodieselblendswill

reduce emissions. Engines certified to operate on alternative fuelssuch asliquefied

petroleumgas(LPG), compressed natural gas(CNG), and liquefied natural gas

(LNG) can also reduce emissions.
47

Heavy-Duty Diesel Truck Idling Reduction

Projects that reduce continuous idling of heavy-duty diesel truck engines at truck

stopsprovideelectricpowerfor cabspace cooling andheating, aswell asother

amenities,suchasInternetaccessandtelevisionviewing.Severaltechnologies

are available to reduce idling including direct-fired heaters, auxiliary power units,

automatic engine idle management systems, and truck stop electrification. Idlereduction measures can help meet multiple goals, including energy conservation,

energy security,andenvironmentalstewardship.

VehicleAerodynamics

Improving vehicle aerodynamics is one way to cut fuel consumption and emissions.

Aftermarket fairings attached to the front and/or belly of truck trailers can improve

fuelefficiencybyuptosixpercent.Therearealsomodificationstothetractorthat

can improve fuel economy,such asupgraded front bumpers, air dams, and side

mirrors. Rooffairings, cab extenders, and side fairingsinstalled on a tractor can

achieve fuelsavings of up to 600 gallons per year and emissionsreductions of

overfivemetric tonsofGHGs. Similarly, covering empty rail cars,modifyinghow

intermodal cars are loaded, and minimizing open areas between cars can help

improvetrainaerodynamics.

SmartWay Transport

EPA’sSmartWayProgramisapublic-privateinitiativetoreducegreenhousegas

emissions and air pollution created by freighttransportation. SmartWay Transport

is comprised of partnerships, policy and technicalsolutions, and research and

evaluationprojectsthatfindnewwaystooptimizethetransportationnetworks

inacompany’ssupply chain.Participatingcompaniesuseperformance-based

quantificationandreportingtoolsthatbenchmarkandinformindustryandthe

marketplace on freight operations, energy, and environmental efficiency.

Sources: The Intergovernmental Panel on Climate Change, http://www.ipcc.ch/pdf/assessment-report/ar5/syr/

AR5_SYR_FINAL_SPM.pdf

U.S. Environmental Protection Agency, http://www3.epa.gov/otaq/diesel/technologies/fuels.htm

FHWA, Freight and Air Quality Handbook, May 2010, http://www.ops.fhwa.dot.gov/publications/

fhwahop10024/index.htm

U.S. Department of Energy, Energy Information Administration, Alternative Fuel Vehicle Data, November 2015, 

http://www.afdc.energy.gov/fuels/

U.S. Energy Information Administration, March 2015 Monthly Energy Review, Table 10.2b Renewable Energy 

Consumption: Industrial and Transportation Sectors, April 2015, http://www.eia.gov/ totalenergy/data/

monthly/pdf/sec10_5.pdf

U.S. Department of Energy, Alternative Fuels Data Center, http://www.afdc.energy.gov/fuels/biodiesel.html

U.S. Environmental Protection Agency, SmartWay Transport Overview, EPA-420-F-15-033, July 2015, 

http://www3.epa.gov/ smartway/about/documents/basics/420f15033.pdf
48

Alternative-FuelforVehicles

A variety of alternative fueled vehicles are effective in reducing emissions:

• Liquefied petroleum gas (LPG)—a fossil-fuel derivative composed of 95percent

propaneand5percentbutane.ItproduceslowerCOemissions,butNOX

emissions may be higher.

• Natural gas—a fuelthat can becompressed(CNG)orliquefied. TheCNGform,

more common in the transportation sector, isstored in high-pressure cylinders.

CNGgenerateslowerCOandVOCemissionsthan conventional gasoline, and

lower NOX and PM than diesel fuels.

• Ethanol—grain alcohol made from corn, sugarcane, or woody biomass. Ethanol

blendsmayreduceCOemissions,buttheireffectonO3 isnegligibledueto

increases in NOX.

• Electricity—Electricvehiclesmaybepoweredbybatterieseitherchargedat

home or at charging stations with electricity from power plants. They have no

tailpipe emissions; overall emissions depend on power plant energy sources.

• Hydrogen—Two types of engines burn hydrogen. One is an internal combustion

engine;theotherisafuelcell.Hydrogenisaclean-burningfuelthatcanbe

produced fromcoal, natural gas, petroleum,solar,or wind energy.Avehicle

operatingonafuel cell,whichgenerateselectricitybyharnessingthereaction

of hydrogen and oxygen tomakewater,produces no COor VOC emissions and

extremely low NOX emissions.

Alternative-Fuel Vehicles in Use in 2003–2011

Source: U.S. Energy information Administration, http://www.afdc.energy.gov/data/
49

Use of alternative fuelsfor motor vehicles hasincreased in recent years. Nearly 1.2

millionalternative-fuel vehicles(AFVs)wereontheroadin2011,a124percent

increasesince2003.TheAmericanRecoveryandReinvestmentActof2009

supported a variety of alternative fuel and advanced vehicle technologies through

grant programs, tax credits, research and development, fleet funding, and other

measures.

Share of Alternative-Fuel Vehicle by Fuel Type in 2011

Sources: U.S. Energy Information Administration, Archive—Alternative Transportation Fuels (ATF) and 

Alternative Fueled Vehicles (AFV), March 2015, http://www.afdc.energy.gov/data/

AFVsdesignedtooperateonethanolwerethemostpopularin2011,followedby

LPG, CNG, and electricity.

Automated Vehicles/ConnectedVehicles

Automatedvehicles/connectedvehicles(AVCVs)arethoseinwhichatleastsome

aspect of a safety-critical control function (e.g.,steering,throttle, or braking)

occurs without direct driver input. AVCVs may be autonomous (i.e., use only vehicle

sensors) or may be connected (i.e., use communications systems such as connected

vehicle technology, in which cars and roadside infrastructure communicate

wirelessly).

AVCVshavethepotentialtobringabouttransformativesafety,mobility,energy,

andenvironmentalbenefitstoournation’ssurfacetransportationsystem.These

benefits could include reduced energy consumption and vehicle emissions,reduced

traveltimes,improvedtraveltimereliabilityandmulti-modal connectivity,and

improvedtransportation systemefficiency and accessibility.

Source: U.S. Department of Transportation, Intelligent Transportation Systems Joint Program Office, 

October 2015, http://its.dot.gov/automated_vehicle/index.htm
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Transportation Control Measures

States and localities can help reduce motor vehicle emissions by implementing

measuresto manage travel demand or improve traffic flow.

Underthe Transportation Conformity Rule, Transportation ControlMeasures

(TCMs) are strategiesthat are specifically identified and committed to in State

ImplementationPlans(SIPs); andareeitherlistedinSection 108ofthe CAA or will

reduce transportation-related emissions by reducing vehicle use orimproving traffic

flow. Measures that reduce emissions by improving vehicle technologies, fuels, or

maintenance practices are not TCMs.

Alternatives to Driving Alone

TCMs that focus on providing alternatives to single-occupant vehicle travel, such as 

carpooling, transit, and bicycling, include:

• Bicycle/pedestrian facilities—provision of paths, special lanes, lockers,showers,

or other facilities.

• Area-wide ridesharing—a program that provides carpool matching and

information services.

• Park-and-ridefacilities—parkinglotsorfacilitieslocatedtoprovideaccessto

transit stations, high-occupancy vehicle lanes, or bus services, or to encourage

carpooling.

• Improvedpublic transit—infrastructureimprovements,including system

expansion, provision of new expanded services, and financial incentives to use

existingtransitservices,suchasspecialfareprogramstoenticeriders.

Improving TrafficFlow

Traffic flow improvements focus on reducing stop-and-go traffic conditions 

through:

• Intelligent transportation system (ITS)—a system of information technologies 

andelectronicsthatareappliedtothetransportationnetwork.These

technologiesinclude the latestin computers, electronics, communications, and

safety systems. Someofthemore common applicationsinclude:

– Freeway management.

– Transit management.

– Incident management and emergency response.

– Electronic toll collection and electronic fare payment.

– Railroad crossings.

– Regional multimodal traveler information.
51

• High-occupancy vehicle lanes—highway lanes reserved for high-occupancy

vehicles such as buses, vanpools, and carpools.

• Signal timing improvements—intersection signal light changes to enhance the

flow of vehicles on arterials.

Freight Improvements

Policymakers employ a variety of operational and system managementstrategies 

toreducefreightvehicleemissions.Thesetaketheformoflocalregulationsand

ordinances,congestionmitigationeffortsgearedtowardsspeedingtheflowof

freight, and operational changes to reduce emissions. Some of the most common

approachesinclude:

• Idle reduction technologies and strategies that reduce emissions by cutting

downonthetimefreightvehiclesspendidling,includingtruckstop

electrification.

• Arterial signal coordination on routes with high truck traffic—adjusting signal

timing to optimize traffic flow on routes with a high percentage oftrucks.

• Port access improvements—projects and technologies that enhance main

accessroutesto seaports(eitherroad orrail) can reduce emissions by

minimizing wait times and queuing at the gates.

• Truck-only lanes—some states have experimented with freeway lanes wholly

or partially devoted to trucks. By separating trucks from other traffic, truck-only

lanes improve traffic flow and enhance safety.

Market-Based Measures

Measures that rely on pricing as an incentive to reduce travel congestion include:

• Parking pricing—increasesin parking fees orreduced feesfor carpools.

• Parking cash-out/transit subsidies—a program in which employees are given

theoptionoftakingthecashvalueofaparkingspaceoratransitsubsidy

instead of free parking at their jobsite.

• Managed lanes—designated lanes in roadways that assess road charges with

increased price during hours of peak demand.

• Emissions/VMT taxes—vehicle registration fees based on emissionsrates 

and/or miles driven.

• Fuel taxes—taxes paid at the pump on motor vehicle fuels.
52

Help from Businesses

Employers help implement measuresto reduce single-occupant vehicle travel,

including:

• Compressed workweeks—extension of the typical workday in order to reduce

the number of days worked,thereby reducing the number of work trips.

• Telecommuting—arrangements allowing employees to work at home orat

satellite offices close to home.

• Commuter choice—benefitsthat employers can offer employeesto commute to

workbymethodsotherthan driving alone. Thesemay include“qualified

transportation fringes” under Internal Revenue Service rules, such as transit and 

vanpool vouchers, biking, walking, teleworking, and others.

Other Measures

Other measuresto reduce congestion and emissionsinclude:

• Episodic—measuresthat are putin place during days when air quality is

expected to be poor to reduce exceedances of air quality standards, such as 

Ozone Action Days.

• Land use planning—incentives and planning to encourage development

patternsthatplacejobs,housing,andservicesclosertogetherandthat

encouragepedestrianandtransit-friendlyenvironments.Transit-oriented

development is an example of thistechnique.

• Parking restrictions—parking policies that discourage vehicle use, such as time

restrictions and/or elimination of on-street parking.

• Carsharing—programsfor peopletouse ashared vehiclefor ashort period

oftime,allowingusersaccesswithoutthefixedcostsandresponsibilitiesof

ownership.

• Bikesharing—programsthatprovidebicyclesforshareduseandrented

onanas-neededbasis,allowingusersaccesswithoutthefixedcostsand

responsibilities of ownership.

Program Funding

Numerous sources are available to fund measures that reduce transportationrelated emissions, including traditional funding sources,state and local sources,

userfees, and private-sectorresources.

One major source of funds, the Congestion Mitigation and Air Quality Improvement

Program(CMAQ),wasestablishedundertheIntermodal Surface Transportation

EfficiencyAct of 1991(ISTEA) and reauthorized under each subsequent

transportation bill since, including the Fixing America’s Surface TransportationAct

(FASTAct).
53

The CMAQ program allocates funds to states to implement transportation projects,

including TCMs, and other strategies to help nonattainment and maintenance areas 

meettheNAAQS forO3, CO, and PM. The actual amount offunding is based on the

severity ofthe air quality problemand the population ofthe area. State and local

governmentsselect the projectsto fund and coordinate them through metropolitan 

planning organizations. Eligible strategiesinclude:

• Transit improvements.

• Alternative-fuels and vehicles.

• Shared-ride services.

• Traffic flow improvements.

• Demand management strategies.

• Pedestrian and bicycle programs.

• Inspection and maintenance programs.

• Diesel retrofits and idle reductions.

EPA offers funding for projects that reduce diesel emissions from existing engines 

throughtheNationalCleanDieselCampaign.Authority forCleanDieselfunding

comes from the Diesel Emissions Reduction Act (DERA), part of the Energy Policy

Act of 2005. Seventy percent ofthe DERA appropriation isto be used for national

competitive grants and rebates to fund projects that use EPA or California Air

ResourcesBoard(CARB)verifiedorcertifieddieselemissionsreduction

technologies.

Thirty percent of the DERA appropriation is allocated to the states and territories to

fundprogramsfor cleandieselprojects.Inthepasttenyears,$1.8billioninfunding

has been allocated to theseprograms.
54

Where Can I Get More Information?

U.S. Department of Transportation (DOT)

https://www.transportation.gov/

U.S. DOT, Federal Highway Administration (FHWA), Planning, Environment and Realty

http://www.fhwa.dot.gov/hep/

U.S. DOT, Federal Highway Administration (FHWA), Office of Highway Policy Information

http://www.fhwa.dot.gov/policyinformation/index.cfm

U.S. DOT, Federal Transit Administration (FTA)

http://www.fta.dot.gov/

U.S. DOT, Bureau of Transportation Statistics (BTS)

http://www.rita.dot.gov/bts/home

U.S.DOT,TransportationandClimateChangeClearinghouse(TCCC)

http://www.climate.dot.gov/index.html

U.S. Environmental Protection Agency (EPA)

U.S. EPA,Office ofAir and Radiation

http://www3.epa.gov/air/

U.S.EPA,OfficeofAirQualityPlanningandStandards

http://www3.epa.gov/airquality/

U.S. EPA, Office of Transportation and AirQuality

http://www3.epa.gov/otaq

U.S. EPA, Climate Change Site

http://www3.epa.gov/climatechange/

Other Government Agencies

U.S. Department of Energy (DOE), Energy Information Administration (EIA)

http://www.eia.gov/

U.S. CensusBureau

http://www.census.gov/

Oak Ridge National Laboratory, Center for Transportation Analysis

http://cta.ornl.gov/cta/

WHERE CAN I GET MORE INFORMATION?
55
Office ofNatural Environment

1200NewJersey Avenue SE

Washington, D.C., 20590

http://www.fhwa.dot.gov/environment/air_quality/

Publication No. FHWA-HEP-16-019