USA Banner

Official US Government Icon

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure Site Icon

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

U.S. Department of Transportation U.S. Department of Transportation Icon United States Department of Transportation United States Department of Transportation

Chapter 7 Transportation Energy Use and Environmental Impacts

Monday, February 27, 2017


  • Despite transportation’s continued dependence on petroleum, recent trends show decreasing import dependence, small reductions in greenhouse gas emissions, and sharply reduced emissions of other air pollutants. U.S. dependence on imported oil decreased from a high of 60.3 percent in 2005 to 24.0 percent in 2015, largely as a result of increased domestic oil production.

  • Transportation continues to rely almost entirely on petroleum to move people and goods. However, the sector’s dependence on petroleum decreased from a peak of 97.3 percent of transportation energy use in 1978 to 91.7 percent in 2015. This reduction is due in part to increases in domestically produced ethanol in gasoline and improved fuel economy.

  • The highway mode continues to dominate transportation energy use. Highway vehicles used 84.1 percent of total transportation energy in 2014, with personal vehicles accounting for 73.2 percent of highway energy use and 61.6 percent of total transportation energy use.

  • The energy required to move one person one mile or one ton of freight one mile has generally declined over time. In 1975 the average miles per gallon (mpg) for all highway vehicles was 13.3 mpg. In 2014 the average was around 21.4 mpg and has continued to improve.

  • Transportation is the second largest producer of greenhouse gas emissions (GHG), accounting for 26.0 percent of total U.S. emissions in 2014. Aside from greenhouse gases, the six most widespread or common air pollutant emissions from transportation are below their 2000 levels and continued to decline from 2009 to 2015 due to many factors, including motor vehicle emissions controls that have contributed to considerable reductions.

  • Across 161 monitored urban areas, the total number of very unhealthy air quality days that could trigger health emergency warnings decreased from 290 in 2000 to 15 in 2015.

  • Since 1990, hybrid and electric vehicles have grown to nearly 3.0 percent of the U.S. market for new vehicles. However, with petroleum prices low, the sale of these alternative fuel vehicles has decreased and sales of pickup trucks and SUVs have increased.

This chapter reviews the patterns and trends in transportation energy use, other aspects of energy associated with our Nation’s transportation system, and transportation’s impact on the environment. Energy use is closely tied to the transportation sector as most vehicles in the United States rely on petroleum as a fuel. Therefore, developments in domestic oil production, alternative fuels, and improvements in vehicle energy efficiencies play a critical role in the vitality of the transportation system. Environmental impacts under consideration include greenhouse gas (GHG) emissions caused by the transportation sector and petroleum spills. These energy and environmental aspects of the transportation system are also important measures of performance, along with such primary measures as system reliability, efficiency, and safety.

Recent trends show reduced U.S. dependence on imported oil as a result of increased domestic production, improved fuel economy for vehicles, and the growth in advancements for alternative energy sources. U.S. dependence on imported oil peaked at 60.3 percent in 2005, but has since decreased by more than half, from 49.2 percent in 2010 to 24.0 percent in 2015 [USDOE EIA 2016c].

In 2015 the U.S. transportation sector used 27.6 quadrillion Btu (British thermal unit) of energy, second only to electricity generation but down from the peak of 28.8 quadrillion Btu in 2007 (figures 7-1 and 7-2). Transportation activities relied on petroleum for 91.7 percent of the transportation-related energy used in 2015, down from a record of 97.3 percent in 1978 (figure 7-2). The United States consumed more than 19.7 million barrels of oil per day, of which 13.8 million barrels (70.1 percent) were consumed by the U.S. transportation system in 2015 [USDOE EIA 2016c]. Despite transportation’s continued dependence on petroleum, recent trends show decreasing import dependence, small reductions in greenhouse gas emissions, and sharply reduced emissions of other air pollutants.

Greenhouse gas (GHG) emissions (carbon dioxide, hydrofluorocarbons, methane, and nitrous oxide) have historically closely paralleled transportation energy use and, as a result, were 3.2 percent lower in 2014 than in 2000, while transportation sector GHG emissions decreased by 4.0 percent [USEPA 2016a]. Transportation sector GHG emissions peaked in 2005, but saw an overall downward trend with a low point in 2012 due to increased use of alternative fuels and improved fuel economy tied to increased fuel prices. Since then GHG emissions have begun to increase due to lower fuel prices resulting in increases in both miles traveled and use of SUVs and light trucks [USEPA 2016a].

Energy Use Patterns and Trends

Transportation’s petroleum dependence decreased from 96.3 percent in 2005 to about 91.7 percent in 2015, chiefly due to increased blending of domestically produced ethanol from biomass in gasoline [USDOE EIA 2016c]. Today almost all gasoline sold in the United States contains 10.0 percent ethanol (E10). Nearly all transportation-related natural gas consumption, shown in figure 7-2, is used to fuel pipeline compressors. Natural gas use by motor vehicles remains a small fraction of total transportation energy use (figure 7-2).

Transportation’s petroleum use is expected to remain at about 13.5 million barrels per day through 2040 and beyond, despite decreases in personal vehicle gasoline use as a consequence of more stringent fuel economy standards [USDOE EIA 2016a]. This leveling off of petroleum consumption is expected because declining personal vehicle petroleum use is projected to be offset by growth in petroleum demand by other modes, particularly medium- and heavy-duty trucks. According to the Freight Analysis Framework (FAF), freight tonnage is forecast to grow 1.3 percent annually during this period (table 3-1 in chapter 3).

Alternative fuel use (excluding gasohol) by motor vehicles increased by 12.7 percent from 2010 to 2011 (the latest year for which data are available) [USDOE EIA 2016b]. Total alternative fuel use exceeded 500 million gasoline-equivalent gallons in 2011. In comparison, gasoline consumption1  in the United States grew from about 134 billion gallons in 2011 to more than 140 billion gallons in 2015—approximately 385 million gallons per day [USDOE EIA 2016c]. In terms of overall energy consumption, compressed and liquefied natural gas accounted for almost one half of the total alternative energy used by transportation activities, followed by E85, propane, electricity, and hydrogen. E85 is a blend of between 51 and 85 percent denatured ethanol and gasoline and can be used safely by approximately 10 million flex-fuel vehicles operating on U.S. roads. However, E85 is predominantly available in the Midwest corn- belt states as indicated in figure 7-3.

The highway mode dominates transportation energy use (figure 7-4). Highway vehicles accounted for 84.1 percent of the total, and used five times more energy than all other modes combined in 2014.2 Light-duty vehicles (passenger cars, sport utility vehicles, minivans, and pick-up trucks) accounted for 73.2 percent of highway energy use and 61.6 percent of total transportation energy use. Air transport came in a distant second with 6.2 percent of transport energy use, but this number excludes energy for international flights. Jet fuels supplied to international flights originating in the United States amounted to 931.6 trillion Btu [USEPA 2015a], which is nearly five times the amount of fuel used by domestic flights. Water transportation is third with 3.8 percent, but once again most of the energy used in international shipments is not included in this figure. An estimated 455.2 trillion Btu were supplied to international ships at U.S. ports [USEPA 2015a], an amount more than double that used by domestic waterborne shipping. Rail freight accounted for 2.0 percent of transportation energy use, although it carries roughly 30 percent of U.S. freight ton-miles. Pipelines used 3.4 percent of transportation energy, much of which is natural gas to fuel pipeline compressors. Transit operations accounted for 0.6 percent of transportation energy use.

Greenhouse Gas Emissions

The transportation sector is the second largest producer of greenhouse gas (GHG) emissions, accounting for approximately 26.0 percent of total U.S. emissions in 2014 [USEPA 2016a]. Electricity generation is the highest GHG producer. In recent years, transportation-related GHG emissions have been trending upward, but are below their 2005 peak (figure 7-5). Carbon dioxide (CO2) produced by the combustion of fossil fuels in internal combustion engines is the predominant GHG emitted by the transportation sector. In  2014 passenger cars were the largest source  of CO2 from transportation, accounting for 42.2 percent, followed by freight trucks (23.0 percent) and light-duty trucks (17.7 percent). Domestic operation of commercial aircraft  produced 6.6 percent of transportation CO emissions; however, as mentioned in chapter 2, there are now more air passenger miles in international flights originating and ending in the United States than there are domestic passenger miles, leaving much of the air travel emissions unaccounted for. Pipelines were responsible for 2.7 percent of emissions, followed by rail (2.6 percent) and ships and boats (1.6 percent) [USEPA 2016a].

Hydrofluorocarbons (HFC), methane (CH4),  and nitrous oxides (N2O) are the other principle GHGs emitted by the transportation sector. Each GHG has a different global warming potential, but all are reported using a common  metric of equivalent grams of CO2 for each emission (figure 7-6). Hydrofluorocarbons, such as those once used in automotive air conditioners,3 are second in abundance behind CO2. HFCs are the most detrimental GHGs known. GHG emission regulations for personal vehicles give manufacturers credits for reducing these HFC emissions, and it is likely that these emissions will decrease in the future. Nitrous oxides are chiefly produced in the catalytic converters of motor vehicles, and a very small quantity of methane emissions is produced by incomplete combustion of fossil fuels or by leakage.

Because 96.5 percent of transportation GHG emissions are CO2 produced by fossil fuel combustion and because petroleum comprises 91.7 percent of transportation energy use, modal GHG emissions closely track modal energy use. Transportation GHG emissions increased from 2000 to 2007 (figure 7-5), fell by 5.0 percent during the economic recession in 2008, and then stabilized at slightly under 1,800 teragrams (million metric tons) in the 2009 to 2014 period [USEPA 2016a]. The short-term decrease in economic activity and the related decline in transportation demand contributed, in part, to the decrease in CO2 emissions during the recession. Total transportation GHG emissions were 4.0 percent lower in 2014 than in 2000. Both the recession and also the improvements in availability of energy efficient vehicles likely contributed to this reduction [USEPA 2016a].

Evident in figure 7-7 are the results of the U.S. Environmental Protection Agency’s (EPA’s) decision to change the definitions of passenger cars and light trucks in 2007. Many vehicles formerly classified as light trucks, but designed predominantly for passenger transportation, were reclassified as passenger cars, causing an apparent jump in passenger car emissions that were offset by a compensating drop in light- truck emissions.

Energy Efficiency

Historically, improvements in the efficiency with which energy was used have enabled reduced energy consumption in the transportation sector. Fuel economies of passenger cars and light trucks have closely tracked the Corporate Average Fuel Economy (CAFE) standards since they took effect in 1978 (figure 7-7). The miles per gallon (mpg) values shown in figure 7-7 are the unadjusted test values on which compliance with the standards is based. The actual mpg values seen on window stickers and in public advertising are adjusted downward to better represent the fuel economy drivers will likely experience on the road.

The apparent decrease in on-road fuel economy estimates after 2005 more likely reflects a change in the definitions of passenger cars and   light trucks and the methods used to estimate their travel and fuel use than an actual decrease in mpg.

Another change in reporting (noticeable in figure 7-7) occurred when the U.S. Department of Transportation (USDOT), Federal Highway Administration (FHWA), started using the classifications of short- and long-wheelbase light-duty vehicles in 2007 rather than the previous categories of passenger cars and two- axle, four-tire trucks.4 As a result, the post 2006 on-road fuel economy data are not consistent with the data from 2006 and earlier years, unless the categories are combined.

Personal vehicle travel and fuel use before 1975 typically moved in parallel tracks (figure 7-8). Fuel economy improvements after 1975 broke the close connection as the amount of fuel used per vehicle-mile of travel steadily decreased. The gap widened as newer, higher mpg vehicles came to dominate the on-road fleet, eventually raising average mpg from 13.3 in 1975 to 21.5 in 2014, which is a slight increase from recent years due to a recent increase in consumer demand for light trucks and SUVs associated with low fuel costs [USEPA 2016f]. However, drops in fuel use are tempered somewhat by increases in travel stimulated by improvements in fuel economy, a phenomenon known as the “rebound effect.” The average price of regular gasoline in the United States in 2015 was $2.33 per gallon, or $2.00 before motor fuel taxes, compared to an average total cost of $3.55 per gallon in 2012, the highest annual average since 2000 [USDOE EIA 2016d]. As gas prices hover at these low values, auto manufacturers that had focused on small, fuel efficient vehicles are now seeing drivers once again demanding the large trucks and SUVs of earlier years, but due to CAFE standards, they are now more fuel efficient [WOODYARD 2015].

On August 28, 2012, the USDOT and the EPA set fuel economy and GHG emissions standards for passenger cars and light trucks through 2025. Nominally, the standards require a total fleet average of 54.5 mpg (163 grams  of CO equivalent) for new personal vehicles by 2025 [USEPA 2012]. However, this is based on laboratory test cycles rather than real world driving and does not consider the many ways manufacturers can earn fuel economy credits. Credits may be earned for solar panels on hybrids, engine shut off at idle, and other features that improve real world fuel economy but which are not reflected in the test cycle.

Furthermore, the new standards vary with the size of the vehicles a manufacturer produces. Medium- and heavy-duty highway vehicles (e.g., combination trucks and buses) are the second largest energy users among modes, accounting for 22.7 percent of transportation energy use in 2013 [ORNL 2015]. In 2011 the USDOT and the EPA announced the first fuel economy and emission standards for this vehicle class for model years 2014 –2018 [USEPA 2011]. By 2018 the requirements for combination tractor trailers specify fuel economy improvements ranging from 9 to 23 percent, depending on the truck type. Similar improvements are required for the diverse class of single unit commercial trucks and buses—vehicles as various as delivery trucks, dump trucks, cement mixers, and school buses. If a manufacturer produces mostly large vehicles, then its actual fuel economy requirement will be lower than if it produces mostly small vehicles.5 Taking all these factors into account, USDOT and EPA estimated that manufacturers would achieve fuel economy levels of 46.2 to 47.4 mpg on the laboratory test cycles [FEDERAL REGISTER 2012]. Fuel economies achieved in actual driving would likely be 15 to 20 percent lower.

The energy intensities6 of passenger modes have generally declined over time, with five out of six passenger modes now averaging less than 4,000 Btu per person-mile, or about 30 person-miles per gallon of gasoline equivalent (figure 7-9). These declines are largely the result of more aerodynamic vehicles and efficient engines as well as improved operating efficiencies (e.g., higher air carrier load factors). From 2000 to 2014, the energy intensity of short- and long-wheel base light- duty vehicles and bus transit rose while the energy intensity of other passenger modes—air and Amtrak—declined.

The energy intensity of rail freight transport decreased at an average annual rate of 1.6 percent per year since 1990. Moving one ton of freight one mile in 2013 required 88.4 percent as much energy as it did in 2000. This reduction was accomplished mostly through reducing energy use per freight car-mile by about 2.1 percent [USDOT BTS NTS 2015].

Alternative Fuels and Vehicles

A large part of the growing use of biofuels in transportation, shown in figure 7-2, can be attributed to the requirements of the Federal Renewable Fuels Standard (RFS). Enacted as part of the Energy Policy Act of 2005 (Pub. L. 109-58) and extended by the Energy Independence and Security Act of 2007 (Pub. L. 110-140), the RFS requires the introduction of increasing amounts of renewable energy into gasoline and diesel fuels each year, ultimately reaching 36 billion gallons by 2022 [USLOC CRS 2013b and 2015]. At least 16 billion gallons are required to be cellulosic ethanol,7 and no more than 15 billion gallons can be ethanol produced from corn starch. In 2014 the United States consumed nearly 13.5 billion gallons of fuel ethanol and 1.4 billion gallons of biodiesel [USDOE EIA 2016c]. More than 37 billion gallons of diesel fuel were consumed by vehicles in 2013 [USDOE EIA 2016e]. Diesel vehicles offerings, including new, clean diesel technologies are hitting the market, and these vehicles are providing more fuel efficiencies than similar-sized gasoline engines. Diesel fuel can provide up to 15 percent more energy than the equivalent amount of gasoline [USDOE and US EPA 2016]. These vehicles are a small percentage of the Nation’s fleet of motor vehicles, mostly medium and heavy trucks. As of April 2016, Ultra Low Sulfur Diesel (ULSD) prices were averaging $2.11 compared to $2.15 for regular grade gasoline at the pump8  [USDOE EIA 2016d]. In 2015 there were an estimated 1.74 million turbocharged direct injection (TDI) light-duty diesel vehicles in the United States out of 210.2 million conventional cars and light-duty trucks [USDOE ORNL 2015].


At present there is limited capacity to produce cellulosic ethanol, which has led the EPA to reduce cellulosic ethanol requirements each year, and the blending of ethanol from all sources with gasoline is very close to market saturation at the current 10 percent level. Additional ethanol production can only be absorbed by expanding the current distribution network of high ethanol blend fueling stations and increasing the numbers of vehicles capable of using these higher blends—up to and including E85 (85 percent ethanol, 15 percent gasoline). In 2013 the EPA decreased the requirement for cellulosic ethanol from 14 billion to 6 million gallons per year, less than one one-thousandth of the statutory amount, reflecting the absence of adequate production capacity for cellulosic ethanol [USEPA 2014b]. The EPA also has expanded the types of biofuels that can qualify under the RFS program to include such fuels as gasoline produced from biomass. At present, however, the capacity does not exist to produce these fuels in volumes that could make a meaningful contribution to achieving the RFS goals.

Nearly all U.S. gasoline now contains up to 10 percent ethanol. All automobile manufacturers’ warranties allow 10 percent ethanol/90 percent gasoline blends (E10). In 2015 motor vehicles used more than 140 billion gallons of gasoline, including almost 13.9 billion gallons of ethanol [USDOE EIA 2016a]. Higher levels of ethanol of up to 15 percent (E15) may pose difficulties and is not recommended for motorcycles, older vehicles, and off-highway engines [USEPA 2016e]. The 10-percent limit has been termed the “blend wall,” in that it appears to constrain the amount of ethanol that can be safely mixed with gasoline as a strategy for meeting the RFS. In 2011 after extensive study, the EPA issued a rule permitting E15 use in model year 2001 and newer motor vehicles. However, concerns exist about the potential for misfueling of older vehicles not capable of using E15 and risking mechanical problems. E15 is not widely available. It is currently sold at more than 100 stations largely concentrated in the Midwest. As a result of the United States Department of Agriculture’s Biofuels Infrastructure Partnership, $210 million has been set aside for the installation of new ethanol infrastructure in 2016, which will increase the availability of E15 [USDOE AFDC 2016b].

Flexible-fuel vehicles (FFVs) can safely use mixtures of up to 85 percent ethanol (E85) with gasoline.9  FFVs accounted for 75.7 percent of the nearly 19.7 million alternative fuel vehicles operating on U.S. roads in 2015 [USDOE EIA 2016a]. However, most on-road FFVs are fueled with gasoline or gasoline/ E10 blends only. Until 2016 automobile manufactures can earn extra credits toward meeting CAFE standards by making and selling FFVs. Future FFV sales are uncertain because the credits will be largely phased out unless actual use of E85 increases substantially. Together, liquid petroleum gas/ propane and compressed/liquefied natural gas- powered vehicles accounted for approximately 4.5 percent of alternative fuel vehicles in use in 2015 [USDOE EIA 2016a]. Electrically driven motor vehicles are gaining popularity in the consumer market; however, their limited range of travel on a single battery charge and the limited availability of charging infrastructure are hindering their acceptance in the mass market.

Hybrid and Electric Vehicles

Until recently gasoline or diesel fuel alone powered nearly all motor vehicles. The first mass-produced hybrid electric vehicle (HEV), powered by an internal combustion engine and an electric motor, was introduced in 1999. The internal combustion engine continues to provide energy for this kind of hybrid vehicle, but kinetic energy normally wasted during braking is instead used to generate electricity that is stored in an onboard battery for later use by the electric motor. Hybrid vehicles have become popular since the early 2000s as a replacement for traditional gasoline- or diesel-fueled vehicles. All hybrid electric vehicle10 sales have grown from 17 vehicles sold in 1999 to a high of 592,000 vehicles in 2013, before starting to decline in 2014. In 2015 about 499,000 HEVs were sold in the United States (figure 7-10). This decline is attributed mostly to the decline in gasoline prices at the same time, which in turn made HEVs less attractive for their fuel savings. In 2015, 54 makes and models of HEVs were offered for sale in the United States [USDOE and USEPA 2015b]. According to the U.S. Department of Energy (USDOE), Energy Information Administration (EIA), there were approximately 3.9 million hybrid electric vehicles (passenger cars and light trucks) or 1.6 percent of the approximately 230 million cars on the road in 2015 [USDOE EIA 2015a].

The first mass-produced “plug-in” hybrid electric vehicles (PHEV), able to draw electric power from the utility grid and store it on- board, were 2011 model year vehicles sold in 2010. In 2010 just 19 electric-only and 326 plug-in hybrid vehicles were sold. By 2015 combined sales of grid-connected vehicles totaled more than 115,000 units, a decrease of 9,000 units from 2014 [HYBRIDCARS 2016]. Over the same period, the number of new to the market makes and models of battery electric-only vehicles increased from 3 to 12, while plug-in hybrid offerings increased from 1 to 15 [USDOE and USEPA 2016b].

Hybrids and grid-connected vehicles comprised about 2.87 percent of the 17.4 million vehicle sales in 2015—a reduction of 75 thousand sales from 2014 [HYBRIDCARS 2016]. Both types of vehicles face several challenges: reducing costs, overcoming the market’s unfamiliarity with the new technology, decreasing the length of time required for recharging batteries, and developing a recharging infrastructure.

Refueling and Recharging Stations

The geographical distribution of refueling stations for alternative fuels partly reflects the numbers of vehicles in each state but also reflects the interests of residents and public policies (table 7-1). Considerable progress has been made in creating a nationwide recharging infrastructure. As of October 2016, there were more than 17,000 recharging stations including privately-owned stations with more than 37,000 nonresidential charging outlets across the United States, up from almost 3,400 outlets in 2011 [USDOE AFDC 2016a].11

E85 stations are disproportionately concentrated in states that grow corn and produce ethanol (figure 7-3). The distribution of electric vehicle recharging stations tends (figure 7-11) to favor states that have opted into California’s Zero Emission Vehicles (ZEV) standards.12 Manufacturers selling electric vehicles in these states earn credits towards meeting the ZEV requirements. The distribution of compressed and liquefied natural gas refueling stations, on the other hand, more closely reflects the number of CNG/LNG vehicles registered in a state (figure 7-12).

Transportation’s Energy Outlook

The EIA has projected the likely effects of current trends and existing policies on transportation’s future energy use and GHG emissions. The 2016 projections anticipate transportation energy use remaining at or near the current level of 27 quadrillion Btu through 2040 [USDOE EIA 2016a]. Existing fuel economy and GHG emissions standards are expected to decrease light-duty vehicle energy use by 19.7 percent by 2040, resulting in approximately 12.6 quadrillion Btu of energy use (figure 7-13). Most of this reduction is expected to be offset by growth in energy use by medium- and heavy-duty trucks, although that could change if fuel economy and emissions standards for those vehicles are further tightened.

For all other modes, activity growth is approximately balanced by improvements in energy efficiency. These projections are based on existing policies and increasing oil prices.13 Natural gas use by motor vehicles in compressed and liquefied form is projected to increase from just 0.06 quads in 2015 to 0.66 quads by 2040 [USDOE EIA 2016a]. EIA attributed all of the projected increase in natural gas use by motor vehicles to medium- and heavy-duty trucks and buses.

According to the EIA, the 2011–2025 fuel economy standards, together with the market’s response to higher gasoline prices, are projected to save personal vehicle owners about 40 billion gallons of motor fuel in 2025, compared to what consumption would have been at the same level of vehicle travel without any increase in fuel economy (figure 7-14).

By fuel type, EIA projects gasoline use to decline from 17.0 quads in 2015 to 12.6 in 2040, in line with light-duty vehicle energy use. Diesel fuel use will increase from 6.7 to 8.0 quads, which is consistent with the growth of truck freight energy use. E85 and electricity use will increase but will still amount to only 0.28 quads and 0.06 quads of energy in 2040, respectively [USDOE EIA 2015a].

Air and Water Quality, Noise, and Habitat Impacts

Beyond the greenhouse gases addressed earlier in the chapter, vehicle emissions controls and other policies have reduced transportation’s other six most common criteria air pollutant emissions to below their 2000 levels, a trend that continued through 201414 (figure 7-15). Smog-forming emissions of volatile organic compounds (VOC) and nitrogen oxides (NOx) were 49.8 and 43.0 percent lower, respectively, in 2014 than they were in 2000. In recent years, NOx emissions have decreased more rapidly, partly due to more advanced diesel emission controls and the use of cleaner, ultra- low sulfur diesel fuel.

Transportation’s share of total U.S. PM-2.5 emissions decreased by 27.1 percent from 2000 to 2014, while the share of PM-10 emissions decreased by 11.7 percent over the same period.

Emissions of sulfur dioxide (SO2) were 85.7 percent lower in 2014 than in 2000, due in large part to reductions in the sulfur contents of gasoline and diesel fuel. The Clean Air Act of 1970 led to the reduction in lead emissions, once a major air pollutant from transportation; lead is not shown in the figure because it has been virtually eliminated from transportation with the phase-out of leaded gasoline.

Emissions of ammonia (NH3), another air pollutant, also shows a significant decline from 2000 levels with a reduction of 60.0 percent in 2014. Transportation comprised 2.6 percent of total U.S. emissions of ammonia in 2014 [USEPA 2016b].

Reductions in transportation’s air emissions have contributed to improved air quality in the Nation’s urban areas. Figure 7-16 compares air quality days for 161 continuously monitored urban areas in 2000 and in 2015. The average number of days from the 161 urban areas in which air quality was reported to be unhealthy for sensitive groups (e.g., people with lung disease, young children, and older adults) dropped from 53.6 in 2000 to 7.1 in 2015; the average number of days with unhealthy air quality for the population as a whole declined from 12.2 in 2000 to 0.9 in 2015, and the total number of very unhealthy days (which could trigger health emergency warnings for the public) decreased from an average of 1.8 in 2000 to 0.1 in 2015. The great majority of days had good or moderate air quality in both 2000 and 2015, but 2015 had many more days of good or moderate air quality in these cities [USEPA 2016b].

Pipelines, ships, railroad cars, and tank trucks are among the sources of spills of crude oil and petroleum products into surface waters and navigable waterways.15 The annual volume spilled varies greatly from year to year and is strongly affected by infrequent, large events (figure 7-17). For example, in 2005 Hurricane Katrina caused numerous spills into navigable waterways from a variety of sources in Louisiana and Mississippi as the volume of petroleum spilled jumped to 9.9 million, more than three times the amount of petroleum spilled in any other year from 1995 through 2015.16 While the number fluctuates from year-to-year, the 1,375 spill incidents from vessels in 2015 were slightly less than the number of incidents in 2010 of 1,508 and much less than the 5,560 in 2000. The 681 spill incidents from pipelines and other non- vessel sources into navigable waters in 2015 show a similar declining trend from the 1,008 incidents in 2010 and 1,645 in 2000, indicating improvements in safety measures for all petroleum transport modes.

Pollution of waterways from spills, however, is not the only environmental challenge posed by marine transportation. Port and vessel operations can negatively impact air quality and have other detrimental impacts on the environment (see box A).

Additionally, the USDOT Pipeline and Hazardous Materials Safety Administration (PHMSA) reports that the number of serious pipeline incidents from 2000 to 2014 is down from 62 to 28. There is also a general declining trend in the number of fatalities associated with pipeline incidents, from 38 to 9, for that period [PHMSA 2016].

In 1985, in response to a congressional requirement, EPA began an effort to regulate underground storage tanks that can contaminate ground water, to clean up leaks, and prevent them in the future [USEPA 2016c].

Since then, the number of new leaks from storage tanks has been reduced by nearly an order of magnitude and over 85 percent of all leaks have been cleaned up (figure 7-18).

As rainwater or snowmelt runs off transportation infrastructure, like roads, parking lots, and bridges, it picks up de-icing salts, rubber and metal particles from tire wear, antifreeze and lubricants, and other wastes that may have been deposited on infrastructure surfaces. The runoff carries these contaminants into streams, lakes, estuaries, and oceans. An in-depth study of road-salt impacts on water quality examined U.S. Geological Survey historical data collected between 1969 and 2008 from 13 northern and 4 southern metropolitan areas. During the November to April period, when road salt application is most common, the concentration of chloride (an ingredient of salt) chronically surpassed EPA’s water-quality criteria at 55 percent of the monitoring locations in northern metropolitan areas; chloride levels acutely surpassed the criteria at 25 percent of these northern stations. From May to October, only 16 percent of the northern stations chronically exceeded the criteria, and just 1 percent showed acute exceedances. At southern sites, where road salt is less frequently applied, there were few samples in any season that exceeded the chronic water-quality criteria, and none exceeded the acute criteria [CORSI, ET AL. 2010].

Highways and other transportation infrastructure also affect wildlife via road kills, habitat loss, and habitat fragmentation. Numerous projects have been undertaken across the United States to mitigate these impacts, from salamander and badger tunnels to mountain goat underpasses on highways to fish passages through culverts.17  There are no systematic estimates of the numbers of wildlife killed by transportation vehicles in the United States. In certain circumstances, the population effects of road kill have been shown to be substantial, even threatening the survival of endangered species. In general, the number of bird kills exceeds the number of mammals killed. Insurance industry records indicate that there are between one and two million reported collisions between animals and vehicles each year. These numbers only include reported incidents; collisions with small animals resulting in no vehicle or human damage are not generally reported [GASKILL 2013].

Transportation noise is pervasive and difficult to avoid in the United States [USDOT FHWA HEP 2006]. It is generated by engines, exhaust, drive trains, tires, and aerodynamic drag. At freeway speeds tire-pavement noise dominates for highway vehicles, while exhaust and aerodynamic noise dominate for aircraft. However, a national noise exposure inventory does not exist. The United Kingdom has developed a noise inventory for 23 large urban areas by estimating noise levels using computer models that are based on transportation activity data [UKDEFRA 2016]. BTS in conjunction with the John A. Volpe National Transportation Systems Center is currently developing a national, multimodal transportation noise inventories.

Unwanted noise can have a variety of impacts including annoyance, sleep disruption, interference with communication, adverse impacts on health and academic performance, and consequent reductions in property values. There is almost no part of the United States in which transportation noise is not noticeable [WAITZ 2007]. When transportation noise levels are below 45 decibels (dB), the level of annoyance in the population is negligible, but when noise levels exceed 65 dB, impacts can be severe.18 Although highways are the most widespread source of transportation noise, exposure to transportation noise is systematically measured only for aircraft. In 2014, 321,000 individuals lived in high noise (>65 dB) areas around U.S. airports. The number of people residing in high noise areas around U.S. airports was down from nearly 7 million over 30 years ago and 847,000 in 2000. The number was reduced through a combination of changes in engine and airframe design and operational strategies [USDOT BTS NTS 2015]. Take-off and landing operations are the primary source of annoying aircraft noise, which per dB is generally more annoying to the public than highway or rail noise.

Under certain circumstances, unwanted and unnecessary light is considered “light pollution” [MRSCW 2016]. Transportation vehicles and facilities can be sources of light pollution. While light pollution is a special concern for facilities like astronomical observatories, it is also known to adversely affect biodiversity in urban areas and to have harmful effects on human metabolism [COE 2010]. No systematic data on light pollution due to transportation in the United States exists.

In addition to the primary performance measures of how efficiently, reliably, and safely people and goods move on the system, transportation’s energy usage and its environmental impacts are also important measures of how well the transportation system performs its societal function. In recognition of this, there have been efforts to mitigate transportation’s dependence on petroleum and environmental impacts. As detailed in this chapter, transportation has become more efficient over the past few decades in its use of energy and has reduced many of its environmental impacts even though activity levels have increased. It continues, however, to be the second leading emitter of greenhouse gases in the United States and has had other major impacts on the environment, such as oil pollution, habitat loss, and noise. Going forward, appropriate and accurate data will be needed to monitor progress and determine whether societal efforts to improve the system’s performance are having the desired effect.


Corsi, Steven R., et al. 2010. “A Fresh Look at Road Salt: Aquatic Toxicity and Water- Quality Impacts on Local, Regional, and National Scales,” Environmental Science and Technology, 44 (19), pp. 7376–7382.

September 1. Available at as of October 2015.

Council of Europe (COE). 2010. Resolution 1776: Noise and Light Pollution. Available at as of May 2015.

Diesel Technology Forum. 2016. Clean Diesel Vehicles Available in the U.S. Available at as of April 2016.

Federal Register. 2012. 2017 and Later Model Year Light-Duty Vehicle Greenhouse Gas Emissions and Corporate Average Fuel Economy Standards, vol. 77, no. 199, Part II, pp. 62623-63200. Available at http://www. as of May 2015.

Gaskill, Melissa. 2013. “Rise in Roadkill Requires New Solutions: Vehicle–wildlife collisions kill millions of animals—and harm thousands of people—each year. Scientists are working on solutions.” Scientific American. May 16. Available at http://www. as of October 2015.

Hybrid Cars. 2016. June 2016 Dashboard. Available at as of August 2016.

Municipal Research and Services Center of Washington (MRSCW). 2016. “Light Nuisances – Ambient Light, Light Pollution, Glare.” Available at as of August 2016.

Pipeline and Hazardous Materials Safety Administration (PHMSA). 2016. Pipeline Serious Incident 20 Year Trend. US DOT. Available at as of May 2016.

Problem and Response. FHWA-HEP-06-020.

Available at as of May 2015.

Renewable Fuels Association (RFA). Pocket Guide to Ethanol 2016. Available at uploads/2016/02/10823-RFA.pdf as of April 2016.

United Kingdom Department of Environment, Food and Rural Affairs (UK DEFRA), 2016. Noise Mapping England. Available at http:// as of August 2016.

U.S. Department of Commerce (USDOC), Bureau of Economic Analysis (BEA), Motor Vehicle Unit Retail Sales (April 2016). Available at as of April 2016.

U.S. Department of Energy (USDOE), Alternative Fuels Data Center (AFDC). 2016.

—2016a. Alternative Fueling Station Counts by State. May 2016. Available at www.afdc. as of May 2016.

—2016b. E15. Available at http://www.afdc. as of November 2016.

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

—2015b. Diesel Fuel Explained (October 8, 2015). Available at http:// as of August 2016.

—2016a. Annual Energy Outlook 2016. Available at as of October 2016.

—2016b. Alternative Fuel Vehicle Data. Available at www.eia.renewable/afv/ as of May 2016.

—2016c. Monthly Energy Review. April. Available at as of May 2016.

—2016d. Weekly Retail Gasoline and Diesel Prices (2015 annual). May 13. Available at as of May 2016.

—2016e. Distillate Fuel Oil and Kerosene Sales by End Use. January 15. Available at as of May 2016.

—2016f. Energy Efficiency and Renewable Energy. April 2016. Available at http://www. as of April 2016.

U.S. Department of Energy (USDOE), Oak Ridge National Laboratory (ORNL). 2015. Transportation Energy Data Book. Edition 34. September 30. Available at as of August 2016.

U.S. Department of Energy (USDOE), Oak Ridge National Laboratory (ORNL), 2015 Vehicle Technologies Market Report (2015). Available at as of September 2015.

U.S. Department of Energy (USDOE) and Environmental Protection Agency (USEPA). 2016. Diesel Vehicles. Available at as of August 2016.

—2016a. Diesel Fuels. Available at www. as of August 2016.

—2016b. Hybrids, Diesels, and Alternative Fuel Cars. Available at as of August 2016.U.S. Department of Transportation (USDOT), Bureau of Transportation Statistics (BTS). National Transportation Statistics (NTS). 2015. Available at as of May 2016.

U.S. Department of Transportation (USDOT), Federal Highway Administration (FHWA), Office of Planning, Environment and Realty (HEP). 2006. Highway Traffic Noise in the United States. Available at http://www.fhwa. guidance/usprbrsp.pdf as of May 2016.

U.S. Environmental Protection Agency (USEPA):

—2016a. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2014, EPA 430- R-15-004 (Washington, DC: Apr. 15, 2016). Available at as of May 2016.

—2016b. Air Quality Index Basics. Available at as of May 2016.

—2016c. Semiannual Report of UST Performance Measures Mid Fiscal Year 2016 (Oct. 1, 2014 –Mar. 31, 2016). Available at as of May 2016.

—2016d. Air Pollutant Emissions Trends Data. Available at as of May 2016.

—2016e. Fuels Registration, Reporting, and Compliance Help: E15 Fuel Registration. Available at as of October 2016.

—2016f. Light-Duty Automotive Technology, Carbon Dioxide Emissions, and Fuel Economy Trends: 1975–2015. Available at https://www3. as of October 2016.

—2014a. Light-Duty Automotive Technology, Carbon Dioxide Emissions, and Fuel Economy Trends: 1975–2015 (December 2015). Available at as of September 2015.

 —2014b. EPA Issues Direct Final Rule for 2013 Cellulosic Standard, EPA-420-F-14-018. April. Available at as of May 2015.

—2012. EPA and NHTSA Set Standards to Reduce Greenhouse Gases and Improve Fuel Economy for Model Years 2017-2025 Cars and Light Trucks, EPA-420-F-12-051, August. Available at as of May 2015.

—2011. EPA and NHTSA Adopt First-Ever Program to Reduce Greenhouse Gas Emissions and Improve Fuel Efficiency of Medium- and Heavy- Duty Vehicles, EPA-420-F-11-031, August. Available at as of May 2015.

—1995. Controlling Nonpoint Source Runoff Pollution from Roads, Highways and Bridges, EPA-841-F-95-008a. Available at http://www. as of May 2015.

U.S. Library of Congress (USLOC), Congressional Research Service (CRS):

—2015. Renewable Fuel Standard (RFS): In Brief. Kelsi Bracmort, Congressional Research Service, R43325, January 16. Available at as of May 2015.

—2013a. Deepwater Horizon Oil Spill: Recent Activities and Ongoing Developments, 7-5700, R42942, Jan. 31, 2013, Washington, DC.

—2013b. Renewable Fuels Standard (RFS): Overview and Issues, Washington, D.C., March.

Waitz, I.A., R.J. Bernhard and C.E. Hanson, 2007. “Challenges and Promises in Mitigating Transportation Noise,” The Bridge, vol. 37, no. 3, pp. 25-32. Available at as of July 2016.

Woodyard, Chris, 2015. “Automakers Challenged by Low Gas Prices”. USA Today (Jan. 4, 2015). Available at http://www. as of May 2016.

1 The U.S. Energy Information Administration (EIA) uses product supplies to approximately represent con- sumption of petroleum products. It measures the disappearance of these products from primary sources, such as refineries, natural gas processing plants, blending plants, pipelines, and bulk terminals.

2 Data on energy use by mode from 2013 and beyond are projected data.

3 The original coolants were chlorofluorocarbons (CFCs), which when released into the atmosphere were found to create holes in the stratospheric ozone layer that helps to protect the Earth’s surface from harmful radiation.

4 A vehicle’s wheelbase is the distance from the center of its rear axle to the center of its front axle. “Short- wheelbase” light-duty vehicles include passenger cars, pick-up trucks, vans, minivans, and sport-utility vehicles with wheelbases less than or equal to 121 inches. The same types of vehicles with wheelbases longer than 121 inches are classified as “long-wheelbase” light-duty ve- hicles. Typically, light-duty vehicles have gross vehicles weights of less than 10,000 pounds.

5 The size of a vehicle is defined as the rectangular “footprint” formed by its four tires. A vehicle’s footprint is its track (width) multiplied by its wheelbase (length).

6 Energy intensity is the amount of energy used to pro- duce a given level of output or activity (e.g., energy use per passenger-mile of travel).

7 Cellulosic ethanol is produced from non-food based feedstock, such as wood and crop residues (corn husks, cobs and stalks), and switch grass.

8 Gas prices reported are from a weekly survey of ap- proximately 800 fuel retailers across the United States of prices paid at the pump including taxes.

9 E85 may contain anywhere from 51 percent ethanol to 85 percent ethanol. Because fuel ethanol is denatured with approximately 2 percent to 3 percent gasoline, E85 is typically no more than 83 percent ethanol.

10 The total includes hybrid electric vehicles, “plug-in” hybrid electric vehicles, and battery electric-only vehicles.

11 A single electric vehicle recharging station may include multiple recharging outlets. Residential recharging locations are included in the station count. Transportation Statistics Annual Report 2015 presented the number of recharging outlets rather than both stations and outlets as noted here.

12 Connecticut, Maine, Maryland, Massachusetts, New Jersey, New Mexico, New York, Oregon, Pennsylvania, Rhode Island, Vermont, Washington, Delaware, Georgia, and North Carolina have adopted the California Air Re- sources Board (CARB) regulations for a vehicle class or classes in accordance with the Section 177 of the Clean Air Act.

13  EIA’s Annual Energy Outlook include multiple sce- narios, one of which includes oil price decreases over the 2014-2040 period. However, for this TSAR report, the Reference Case was used, which is based upon exist- ing policies and increasing oil prices.

14 Often called “criteria pollutants” because the U.S. Environmental Protection Agency sets permissible levels for these air pollutants using criteria based on scientific guidelines on human health or welfare under the Clean Air Act.

15 Safety issues associated with spills of hazardous mate- rials are covered in Chapter 6.

16 The much larger Deepwater Horizon oil platform fire and spillage in the Gulf of Mexico of 207 million gal- lons is not considered to be a spill into navigable waters of the United States or a spill from a transporting vessel (USLOC CRS 2013a).

17 While there are no comprehensive statistics on mitiga- tion efforts, numerous case studies of highways mitiga- tion efforts can be found at

18 Noise (sound) is measured in decibels (dB) on a logarithmic scale. Each increase of 10 dB represents a doubling of the noise level.