Transportation and Energy Issues


The transportation sector is a major segment of the US economy: about 1 in 5 dollars in the US is spent annually on transportation products and services, and 1 in 16 Americans is employed directly by this sector (and up to 1 in 7 jobs, including indirect employment). (2,3,6) The Department of Transportation Bureau of Transportation Statistics data (2) for the highway mode show that in 1994 there were in the US about 134 million cars, 3.7 million motorcycles, 670,000 buses and 63.5 million trucks. The latest figure for the nation's motor vehicle fleet is 204.1 million vehicles on the roads, which represents a third of the number of motor vehicles worldwide. Passenger cars and trucks account for the bulk of vehicle-miles traveled in the US: since 1992 the per capita passenger miles has hovered at about 17,000. The U.S. transportation sector is a major consumer of fossil fuels, both directly, by motor vehicles burning gasoline and diesel fuel in internal combustion engines (ICE), and aircraft burning jet fuel; and indirectly, by consuming electric power for urban mass transit.

There are over half a billion fleet and commercial cars in the world today, a number anticipated to grow at 3% annually and double within the next 20 years. Passenger car ownership in the US is now more than one car per 2 people (561 per 1,000 residents in 1993). This number for other OECD countries (366/1,000) is also high and projected to rise worldwide(8) . Motor vehicles account for 1/3rd of world oil consumption, but 2/3 of US oil consumption: in 1994, cars consumed 73,825 million gallons (1.8 Bbl) , and trucks 65,125 M gallons (about 1.3 Bbl) (2).

The latest Department of Energy's Energy Information Administration Energy Outlook (1,4) projects that growth in global oil demand for all modes of transportation (personal and freight vehicles, aircraft, rail and marine) over the next two decades will exceed demand for the previous two, largely because of projected transportation sector growth in developing countries.(4) US energy use is concentrated primarily in the highway mode (ref. 4a, Table 2-9), which took 76% of the total consumption in 1993 (of which 40.7% was for automobiles and 34.4% trucks, the latter being dominated by light trucks). The energy distribution by transportation mode is illustrated in Fig. III.2-1, and is currently divided as follows: 47% for cars, 28% trucks, 0.7% bus, 8% air, 3% rail, 9% water and 5% pipeline. (2)

This fuel consumption growth in the transportation sector reflects the steady growth in vehicle-miles traveled: light vehicle highway travel grew by 4.4% annually in the eighties, but has slowed to 2.8% since 1993. Light duty vehicles (cars, vans, light trucks) currently account for more than half of all transportation energy consumption (Figure III.2-1), and their dominance will persist in the next decade as their fuel share will steadily grow by about 1% annually, while fuel consumption by freight trucks and air transport is forecast to grow at 2% per year (highway travel rose at 2.5% in 1994, but a t 2% in 1995). Air travel growth has also slowed, from 4.1% per year in the eighties to half of that in this decade. Both rail and waterborne freight are projected to increase at modest annual rates (1.1-1.3%). The more energy-efficient electric transit and commuter rail, which use electrical power primarily generated at fossil-fuel burning plants, represent a very small fraction of the US transportation mileage(1, 2): about 40 billion passenger miles in 1993 for transit and and about 7.5 billion for passenger rail, compared to six times the combined total for cars and trucks.

The Energy Information Administration Annual Energy Outlook(1) projects that the gasoline share of transportation energy use will decline through 2010, as the overall fuel efficiency of conventional light-duty vehicles continues to improve and the sales of alternative fuel vehicles increase (displacing about 465,000 bl/day by 2010). Other projections (8) forecast that the global demand for petroleum will increase and perhaps double by 2020 from the present level of about 75 million barrels per day, primarily because of growth in automobile usage in S. Korea, China, India, and other Asian nations. This growth rate will exacerbate urban congestion, air pollution and associated adverse impacts on economic productivity, human health and quality of life. (3. 4, 7)


Figure III.2-1: U.S.Transportation Energy Consumption, 1970-1992.

The fuel efficiency for a vehicle depends on many factors: type and maintenance of internal combustion engine, fuel quality, engine burn rate and burn temperature, curb weight of the vehicle and passenger occupancy factor (loading) or ton-miles load. The annual fleet energy efficiency average also depends on motor vehicle fleet make-up and age; the units in either energy per vehicle-mile, or per passenger mile show the advantage of high occupancy for transit bus and rail, vs. low and decreasing occupancy for personal vehicles. The Oak Ridge annual Transportation Energy Data Book provides the energy efficiency characteristics by transportation mode and by vehicle type (4) (TT. 2-15, 16 ibid.) Between 1975 and 1992 the fuel efficiency for new mid-size sedans doubled, and the PNGV program discussed below intends to triple the current level in the next decade.

However, annual fleet average data show that between 1970 and 1993 the automobile fleet became less efficient (-2.1% per vehicle-mile), while transit buses became only marginally more efficient (+3.7% per passenger mile, but only .5% per vehicle-mile), largely due to higher utilization rates. Average transportation efficiency per vehicle-mile also decreased for all other modes, except for rail transit rates per passenger-mile (+ 1.8%), again due to increasing usage of commuter rail. For comparison, an average automobile in 1993 consumed 5,748 Btu/passenger-mile, vs. 4,374 for transit bus and 3,687 for rail transit. Fuel efficiencies for freight vehicles also decreased slightly over the past decade, ranging from 8,780 Btu/vehicle-mile for light trucks, to 22,332 for heavy trucks, and to 14,195 for railcar in 1993. Real gains in fuel efficiency were made for commercial air travel , due to both more seat-miles per gallon and to more passengers per seat, as a result of airline deregulation and of improved jet engines.

Unfortunately, in recent years and on a fleet-wide average basis, consumer preference for less energy efficient sports-utility vehicles, such as heavier vans and light trucks, has eroded gains made by improvements in the average fuel efficiency of automobiles. Since the average vehicle life expectancy is about 12 years, energy savings as a result of turnover in the motor vehicle fleet and replacement of older less fuel-efficient cars with newer models will be slow. In addition, the Congress prohibited the US Department of Transportation from increasing fuel efficiency standards FY96. The federal speed limit was also repealed by Congress in 1995 as part of the National Highway System Act, allowing the states to set higher speed limits than the 55 mph, thus ensuring that the existing fleet will burn fuel less efficiently.

Nevertheless, the average fuel efficiency of automobiles is expected to gain modestly over the next decade at 1% per year, or about one third of the rate of improvement in the 80's. Furthermore, in spite of fuel intensity gains through improved engines and lower curb weight, it is unclear if the known fossil fuel reserves can accommodate projected growth in transportation sector oil consumption.


All modes of transportation that consume petroleum based fuels contribute to air and water pollution of the environment, both through burning and through the refueling cycle, regardless of specific fuel intensity (economy per mile and normalized to occupancy). Even today's relatively clean car will generate about 1 ton of pollution over its lifetime. Electric transit and commuter rail contribute pollution at the electricity source, through fuel burned by electric power plants. In addition, the vast infrastructure network needed to produce and supply motor vehicles fuels - from trucks transporting pressurized Liquefied Petroleum Gas (LPG) and oil, to fugitive emissions of unburned fuel at the gas pump - further contributes to air pollution.

The increase in the number of vehicles and in the aggregate overwhelm the gains made in pollution control technology for tailpipe emissions and increases in fuel efficiency for motor vehicles. Vehicle-miles traveled figures and trends correlate well with the toxic emissions of atmospheric pollutants regulated by the EPA, and with non-toxic greenhouse gases (GHG) like Carbon dioxide (Figure III.2-2) which may have adverse effects on the global climate.

In the US, over 60% of air pollutants on EPA's list of "criteria pollutants" that must be controlled to meet national ambient air quality standards (NAAQS) come from mobile sources. Tailpipe emissions are the major source of air pollution, and of precursors of ozone and acid rain: CO, NOx, SOx, Hydrocarbons, volatile organic compounds and particulates. Transportation accounts for about 60% of carbon monoxide, 40% of nitrogen oxides, and about 40% of volatile organic compounds including hydrocarbons, which lead to atmospheric ozone when photoreacting with NOx. Sulphur dioxide vehicle emissions produce acid aerosols, which are eye and respiratory irritants; and, as acid rain, pollute the groundwater. In addition, soot and aerosol particles from unburned fuel cause respiratory problems. Carbon monoxide is poisonous and causes respiratory problems, while volatile organics are both toxic and carcinogenic.


Figure III.2-2: 1992 U.S. CO2 Emissions by Transportation Mode

Motor vehicle CO2 emissions amounted to 446.3 Millions metric tons (mmt) of carbon in 1994. Although carbon dioxide is non-toxic and is a byproduct of efficient and complete fuel burning, there are international and US efforts to limit its production rate and accumulation in the atmosphere. It is a major greenhouse gas , whose increasing concentration is expected to lead to global climate change. Other greenhouse gases of concern include chlorofluorocarbons, including freon used for airconditioning in cars, methane, ozone and nitrous oxide. The Intergovernmental Panel on Climate Change has made recommendations for curbing anthropogenic greenhouse gases, but adherence in the US and worldwide is largely voluntary. The scientific basis for specific adverse climate and health impacts of global warming, such as 3-D climate models, is still evolving; therefore, transportation control measures are not constraining energy consumption or utilization patterns.

Within industrialized nations, the fractional atmospheric carbon loading (carbon dioxide and monoxide) due to transportation rose from 21% in 1970 to 30% in 1990, largely due to more cars, albeit cleaner burning. In today's megacities (with population >10M people), about 90-95% of lead and CO, 40-50% of NOx and 40-50% of Hydrocarbons are due to cars, trucks and buses.

Higher fuel efficiency requirements introduced as Corporate Average Fuel Economy (CAFE's) Standards, combined with improved catalytic converters and environmental mandates in the 70's limiting tailpipe emissions, have drastically reduced emissions per car. Real progress was made over the last decade, when mobile source contributions of CO decreased by 21%, volatile organic compounds by 33%, NOx by 7% and fine particulates by 14%, with gains made in overall air quality as well.

However, these gains were more than offset by the growth in number of cars and vehicle miles traveled: vehicular CO2 has increase by 63% (at 3% annually) from 1970 to 1987, with the US continuing to be the largest contributor of this greenhouse gas. Over this period, US CO2 emissions from motor vehicles increased by 30%, OECD countries share of automotive CO2 increased by 70% and the Less Developed Countries contribution grew by 120%. At this rate atmospheric deposition of CO2 from motor vehicles will increase by 50% by 2010, exacerbating the threat of climate change.

It is becoming apparent that in addition to the safety-related toll (in deaths, injuries and property loss) due to transportation accidents, there is a comparable health cost from adverse impacts of air pollution. A Harvard study (Dockery et al, 1993) showed a clear association of urban air pollution and mortality, indicating that smog reduces the lifespan on the average by 2 years. This prospective cohort study of about 8000 people in 6 representative urban centers, showed a 26% increase in the adjusted mortality rate in more air-polluted cities, correlating with particulates (soot, sulfate and nitrates particles and acid condensates), while correcting for personal risk factors (smoking). Mortality associations with air pollution included lung cancer and pulmonary disease, but no positive association with other causes of death. Both morbidity (disease incidence) data and mortality were associated with air pollution (ozone and particulates). Other studies of the health impacts of urban air pollution found that soot and other diesel fuel particulate emissions may cause about 60,000 premature deaths from cardiopulmonary diseases annually in the US (more than the roughly 40,000 annual road fatalities and 3 million injuries). These acute health effects compound the chronic health problems from air pollution, and the balooning health care costs engendered. (7)

The World Health Organization and the UN Environmental Programme have evaluated the potential health effects of climate change, based on a global environmental monitoring system. They contributed an assessment of global health impacts to the Intergovernmental Panel on Climate Change, with active US agencies and scientists' participation. The major human health effects of global warming and ozone depletion are very complex to model and predict, and would vary geographically, but include: heat stress and air pollution effects, including general immunosuppression increasing the susceptibility to disease, skin problems and cancers , eye (cataracts) and respiratory problems, as well as pandemics of vector-borne diseases.

The EPA has announced in late 1996 stricter health-based standards for ozone and particulate emissions, which would place many more cities across the US in non-compliance with air quality standards and trigger strict transportation control measures and loss of federal highway funds unless mitigation plans are adopted.


Oil consumption for transportation and its adverse environmental, health and economic impacts can be mitigated through the application of more advanced technologies: advanced automotive materials that are lighter, stronger and more durable; improved fuel efficiency or use of alternative and renewable fuels; new energy storage and delivery sources (advanced batteries, fuel cells, ultracapacitors and flywheels). In particular, new types of power plants for vehicle propulsion, such as clean fuel cells that obtain the energy of oxidation electrochemically, combined with ultralight capacitors and with flywheels to store and redistribute electrical and mechanical energy, could dramatically curtail noxious fumes and urban pollution. Focused research efforts are needed to achieve substantial technological improvements in all these enabling technologies in order to offer practical alternatives to the growing use of fossil fuels in transportation.

These anticipated research benefits could be integrated into the next generation of lighter and cleaner motor vehicles, that are expected to be comparable in cost, safety and efficiency to current cars that burn fuel in internal combustion engines. This objective is embodied in a major R&D federal-industry research partnership effort the Partnership for Next Generation Motor Vehicle (PNGV), aimimg to triple the current automobile corporate fuel efficiency of cars within ten years. Concepts under study by numerous industry, governement and university participants include new subsystem technologies (propulsion, braking) to be incorporated in various transitional alternatives burning cleaner fuels and hybrid (burning fuel as backup to electricpropulsion) cars, to more advanced electric-powered and hydrogen-burning or fuel-cell cars.

Active areas of research include vehicle control electronics, to precisely time the engine fuel injection and burn cycle, automatically adjust burn rate to external or motor temperature, perform thermal and mechanical energy management and electric loading more efficiently , monitor and display the status of vehicle subsystems and provide failure diagnostics, and enable automated alert, alarm and safety warning (collision avoidance) communications. Much current research is devoted to micromechanical sensors and transducers and to embedded software for intelligent intra-vehicle, vehicle to vehicle and vehicle to roadside communications within the multi-modal Intelligent Transportation Systems program. The goal is to use advanced traffic technology and centralized traffic management to reduce urban road congestion through widespread application of wireless or dedicated radio roadway sensors and vehicle transponders. Rapid progress has been made in in-vehicle navigation using the Global Positioning System (GPS) of satellites for accurate location, combined with digital maps to allow navigation in cities and along highways. In addition to economic productivity benefits, selecting the shortest way and sending less time idling will cut air pollution from vehicles gridlocked in traffic jams. Other intelligent technologies (like vehicle radars) focus on collision avoidance and on intelligent automatic cruise control to enhance safety, while development of uncooled infrared detectors combined with Heads-Up Displays to enhance night-time and fog visibility , will both assist the driver and improve road safety.

R&D is also needed to advance emission controls and catalyst technologies for conventional internal combustion engines , to develop higher quality and alternative fuels for vehicles, to design improved and more efficient lean-burn versions of internal combustion engines, such as direct-injection engines; and to develop viable Electric Vehicles and fuel-electric hybrids. Alternative fuels include Compressed Natural Gas (CNG), liquefied petroleum gas (LPG), cleaner gasoline-based or biomass-based renewable oxygenated fuels (methanol and ethanol with potentially toxic additives), and hydrogen fuel cells. However, at present these emerging vehicles and fuels require costly and extensive modification to both vehicles fuel tanks and to the existing refueling infrastructure. For instance, there are about 50,000 vehicles in the US today using CNG (though widespread in Italy and Canada), which have 85-90% lower emissions of CO and volatile organics, 30% less CO2 and no particulate emissions. However, there are no manufacturers of the pressurized fuel tanks (2 per car) needed, nor filling stations for CNG as yet. The most promising near term application is for bus fleets and for niche vehicle fleets (on military bases, postal and other delivery vehicles).

Many competing concepts for the future car are in development, or in protoyping stages, that include serial and parallel electric motors, both DC or AC versions thereof, advanced gas turbines, various types of flywheels for energy storage, numerous types of batteries and capacitors, and even hydrogen cars. For the transitional period, ultralight hybrid automobiles have been proposed, but safety concerns, combined with the lack of manufacturing base for the advanced materials in structural and non-structural applications, pose barriers to deployment. The electric utility industry in partnership with federal agencies is actively pursuing efficient and rapid charging or refueling technologies for electric or alternative fuel vehicles compatible with low-cost and widely distributed charging infrastructure at home, work and in public settings.

Advances in materials will produce ceramic engines for hotter and more complete burn and lighter cars with enhanced fuel efficiency and sufficient crash strength. Advanced materials are also needed for advanced batteries and for strong flywheels for the vehicle energy storage and management. Advanced zeolitic catalytic converters that can be recycled to cut tailpipe emissions appear promising, but require test, evaluation and demonstration efforts.

Major technological challenges for fielding environmentally benign and fully recyclable transportation vehicles remain, from component-level to subsystems and system integration, to fuel efficiency, safety, manufacturability and economic challenges. In addition, major investment in associated infrastructure is needed, and some economic dislocation in traditional automobile manufacturing is anticipated as a result of changing the transportation fleet makeup. To progress beyond the R&D phase to commercialization of new transportation energy technologies, government assistance might economically stimulate the private sector in making the transition to new transportation fuels (e.g., hydrogen), by developing and demonstrating the safety of the infrastructure for refueling.


Faced with the prospects of growing pollution, urban congestion and unsustainable oil consumption for transportation, a strategic R&D program is needed to achieve a sustainable energy production base and efficient energy storage, transmission and utilization. There is a pressing need for cleaner, more energy efficient vehicles, environmentally benign energy sources (including renewables, and biomass fuels or oxygenated alternative fuel mixes), and more efficient energy storage, conversion and propulsion technologies for transportation.

Technical improvements in transportation vehicles promise to alleviate air pollution transportation impacts, particularly if combined with market push effects, such as tax or other economic incentives (e.g., feebates at the pump) and with regulatory requirements involving stricter Corporate Average Fuel Economy (CAFE) standards, and improved, centralized, and standardized road and vehicle inspection and maintenance. Major fuel savings and pollution reduction could result from: Transportation Control Measures mandated by the Clean Air Act Amendments of 1990; replacing gasoline-fueled cars with expanded urban mass transit and advanced rail options; switching to cleaner fuels in congested non-attainment areas (e.g., from diesel to compressed natural gas, or alternative fuels); and reducing vehicle-miles travelled personal travel demand with both incentives and disincentives (restricting downtown parking and charging for parking at work, promoting high occupancy vehicles or car-pooling, offering transit subsidies); and instituting pollution controls at the source (stricter inspection and maintenance requirements, improved catalytic converters). However many of these options require social reengineering and are not politically palatable or economically acceptable to the American public. taxpayers objected to even marginal increases in gasoline tax or carbon taxes to pay for pollution prevention, green technologies and environmental clean-up.

Research efforts and technology deployment are necessary, but not sufficient, since transportation and related energy consumption are tightly coupled to the economy and to the quality of work and life in our society. Major changes in vehicle fuels would entail some social re-engineering, such as: devoting resources to farmland/woodland and production plants for ethanol and methanol from biomass; developing infrastructure for refueling and central charging stations for EVs, as well as workplace and home-based fast-charging options. Regulatory mandates and/or tax incentives are needed for their adoption on a large scale, coupled with vehicle materials and technologies optimization for more efficient burn and longer operating life.

To meet the projected growth in travel demand worldwide over the next two decades, without adverse impacts of fossil fuel consumption by the transportation sector, including economic instabilities from oil pricing shocks, security threats from potential Middle East and even global conflicts, and environmental, climate and health consequences, it is necessary to:

  • maintain a vigorous and diversified energy research program as a hedge for future energy shock due to overdependence on imported petroleum fuels;
  • continue to evaluate and integrate improvements due to science and technology findings into policy and regulatory decision making;
  • couple scientific and technological improvements in energy sources, energy storage and energy transmission and power trains to economic and regulatory incentives to speed up penetration into industry production and market share gains;
  • enhance the value of hard scientific research and technology with social and health research to achieve the benefits of improved transportation vehicles and infrastructure .


  1. DOE Energy Information Administration Transportation (DOE/EIA) internet postings of Annual Energy Outlook, 1995 and International Energy Overview, 1993, and also demand statistics through 1993,

  2. DOT Bureau of Transportation Statistics data , see Energy and Transportation statistics posted
    1. 1997 National Transportation Statistics, and 1996 National Transportation Statistics.
    2. Transportation Statistics Annual Report, 1996.

  3. J. McKenzie and M. Walsh, "Driving Forces: Motor Vehicle Trends and their Implications for Global Warming, Energy Strategies and Transportation Planning", World Resources Institute, Washington, DC, 1990.

    1. Stacy Davis and David McFarlin, Oak Ridge National Labs, Transportation Energy Data Book-16 (1996), ORNL-6898.
    2. D.L. Greene, D.W. Jones and P.N. Leiby, ORNL, "The Outlook for U.S. Oil Dependence", paper prepared for DOE Office of Transportation Technology, May, 1995.
    3. David Greene and Yehui Fan, "Transportation Energy Efficiency Data Trends, 1972-1992", ORNL- 6828, 1995

  4. "Energy R&D: Shaping Our Nation's Future in a Competitive World", Yergin Report, June 1995, 3 vols., DOE.
  5. AAMA-World Motor Vehicle Data, 1994 ,
    1. Dockery et al., "An Association Between Air Pollution and Mortality in Six U.S. Cities", The New England Journal of Medicine, vol 329, # 24, 1753-1759, Dec. 1993;
    2. "Diesel Exhaust: a Critical Analysis of Emissions, Exposure and Health Effects", The Health Effects Institute (HEI), April 1995;
    3. Natural Resources Defense Council (NRDC) , : see issues and fact sheets under Energy: Health: Air and Atmosphere; and report "Breath Taking: Premature Mortality Due to Particulate Air Pollution in 239 American Cities".

  6. World Resources 1996-1997, Ch.1, section on Urban Environmental problems; Ch. 4, Urban Transportation; Ch. 12- Energy and Materials; Ch. 14- Atmosphere and Climate.