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U.S. vehicles travel over three trillion miles per year. The vast majority (99.64%) of these miles are traveled on U.S. roads, with the greatest portion of these miles attributed to passenger and light-duty vehicles (US Dept of Transportation, 2014). Transportation accounts for 28% of the energy used in the U.S. (US Energy Information Administration, 2016) and 26% of U.S. greenhouse gas emissions equaling 1.786 billion tons of CO2 equivalent.(EPA 2016). U.S. vehicle travel increased from 724 billion miles in 1960 to the current level by 2006, at which point the total vehicle miles stabilized (US Dept of Transportation, 2014). 90% of the fuel used for transportation in the U.S. is petroleum based (US Energy Information Administration, 2016).
Transportation has become the leading and most-rapidly increasing contributor to greenhouse gas (GHG) emissions both globally and the U.S (Schipper, Saenger, and Sudardshan, 2011). Between 1991 and 2006, nearly half of the growth in U.S. carbon emissions was attributable to transportation. CO2 emission growth due to transportation has been driven by several factors, including increasing demand for passenger and freight transport, urban development and sprawl, lack of rail and bus transit and cycle infrastructure in many regions, fuel-inefficient vehicles, relatively low oil prices, and the limited availability of low-carbon fuels (Brown, Southworth, & Sarzynski 2008). Given the scope and growth of transportation and associated emissions, it is becoming increasingly important to understand and quantify impacts and trends in various transportation modes.
Light trucks (pickups, minivans, and SUVs) and passenger cars account for 34% and 24% of U.S. transportation fuel usage, respectively (US Energy Information Administration, 2016). Light trucks and passenger cars combined contribute 59% of U.S. transportation carbon emissions (U.S. Department of Energy, 2014). Fuel efficiency standards in the U.S. were initially established by Congress 1975. Corporate Average Fuel Economy (CAFE) standards set the average, sales-weighted, fleet fuel economy for new vehicles starting with the 1978 model year, with the intention of doubling average fuel economy to 27.5 mpg by 1985. The Department of Transportation also established CAFE standards for light trucks ( pickups, minivans, and SUVs) beginning with the 1978 model year. In 2007, CAFE standards for light trucks were increased to 22.2 mpg, with further increases scheduled. No increases were made beyond 1985 levels for passenger cars until until 2007, when the Energy Independence and Security Act raised the fuel economy standards of America's cars, light trucks, and SUVs to a combined average of at least 35 miles per gallon by 2020 (Union of Concerned Scientists, 2017). However, it seems likely that this standard will be scaled back by the current administration before it is implemented.
Minimum fuel efficiency standards for cars and light cars are set at different levels. A passenger car is any 4-wheel vehicle not designed for off-road use that is manufactured primarily for use in transporting 10 people or less. A light truck is any 4-wheel vehicle which is designed for off-road operation (has 4-wheel drive or is more than 6,000 lbs. gvwr and has typically truck-like physical features); or which is designed to transport more than 10 people, provide temporary housing, provide open bed transport, permit greater cargo-carrying capacity than passenger-carrying volume, or with the use of tools can be converted to an open bed vehicle by removal of rear seats to form a flat continuous floor (NHTSA, 2006). The ambiguity of this definition enables manufacturers to define vehicles as trucks or cars at their discretion. Many sport utility vehicles (SUVs) produced today which seem to meet the passenger car definition above are classified as light trucks, allowing their manufacturers far greater leeway to meet CAFE standards .
It is useful to analyze vehicle carbon intensity for cars and light trucks. This is defined as the amount of carbon dioxide emission per vehicle distance traveled. Carbon intensity is inversely proportional to fuel economy. From 1973 to 2008 carbon intensity decreased 33% per vehicle mile and 15% per passenger mile. After 1973, new cars became much lighter, less powerful, and gradually more efficient. By 2007 a new cars and light trucks used half as much energy per unit weight as ones sold in the 1970s. However, new car weight had increased to 80% of the 1975 values for cars, and light truck weight increased above 1975 values. As a result the decline in fuel usage per mile of new cars and light trucks sold in the 1990s was closer to 33% less than those sold in 1973 (Schipper, Saenger, and Sudardshan, 2011).
Low-density suburban development, or "urban sprawl", has dominated development in the U.S. since World War II. This also includes scattered and commercial strip development, as well as large expanses of single-use development . Suburban households drive 31 percent more than urban households, while western households drive 35 percent than northeastern households (Kahn, 2000). What's more, households in low density areas tend to own more cars, are more likely to own less fuel efficient vehicles such as SUV's and trucks, have lower vehicle occupancies, and use public transportation less than households in high density areas (Brownstone, 2008).
Domestic air carrier service accounts for 6% of the total U.S. transportation energy use and 11% of the U.S. transportation carbon emissions. U.S. planes traveled 6 billion vehicle miles (608 billion passenger miles) for such travel in 2014. Domestic airline mileage increased from 858 million vehicle miles (31 billion passenger miles) in 1960 to 6.7 billion vehicle miles (588 billion passenger miles) in 2006 before decreasing to current levels. (U.S. Department of Transportation, 2016). It is interesting to note that although total vehicle miles have decreased by over 10% since 2006, total passenger miles have increased 3.4% during the same period. This is likely due to efforts by airlines to increase cost efficiency by increasing plane occupancy.
While automobile fuel use was affected by efficiency standards, there were no similar policies for air travel. Instead, technological progress and efforts to support profitability have led to reduced carbon intensity in air transportation. Many unprofitable non-stop flights between smaller cities have been eliminated in favor of hub-and-spoke patterns developed by the major airlines, which increased plane loads. In addition, air travel intensity declined as plane occupancy increased to about 80% capacity in 2006 from around 50% in the early 1970s. This created more crowding on aircraft, but led to considerable reduction in fuel consumption. As a result, the carbon intensity of air travel declined by 60% between 1973 and 2006, greatest for any major mode of transportation (Schipper, Saenger, and Sudardshan, 2011).
Rail and bus shares of U.S. transportation decreased from just over 7% in 1960 to around 4% in 2008, in terms of passenger miles. This is disconcerting given that the carbon intensities of bus and rail travel are (potentially) significantly lower than both passenger cars and air travel. Rail intensity can vary considerably. Heavily used intercity passenger (Amtrak) or commuter rail lines (Metro North, LIRR) typically have very low energy intensities, well below that of auto or air travel. Unfortunately, only a few large urban transit systems provide energy intensities that are competitive with automobile travel. As a result, North American public transportation service is overall not very energy efficient (energy consumption per passenger-mile). Under current conditions, U.S. transit vehicles consume about the same energy per passenger-mile as cars, although less than vans, light trucks and SUVs (Litman, 2015) .
Bus travel, including intercity buses, school buses, and urban buses, has a mixed record. In fact, because buses carried so few passengers, city buses released more CO2 per passenger-mile on average than cars/light trucks during periods in the 1990s. But by 2000, newer, more efficient buses used progressively less fuel/mile, to the point where the intensit of a bus with an average of 9 passengers fell below that of automobiles again. (Steiner & Mauzerall, 2006). Efficiency of public transit vehicles is highly dependent on passenger occupancy. A bus with seven passengers is about twice as energy efficient as an average automobile, while a bus with 50 passengers is about ten times as energy efficient. Rail transit tends to be about three times as energy efficient as diesel bus transit. New hybrid buses are about twice as energy efficient as current diesel buses. Chester and Horvath (2008) and Chester, et al. (2013) calculate life cycle energy consumption and pollution emissions for various modes of transportation, including fuel used in their operation, and energy used in vehicle and facility construction and maintenance. While, public transit typically uses less than half the energy of a passenger car and a quarter of the energy of a light truck or SUV, these efficiencies vary significantly with on travel conditions. During peak periods, when occupancy is high, buses are the most energy efficient mode, but during off-peak, when occupancy ise low, buses are least efficient. (Litman, 2015)
Although public transit is on average only modestly more energy efficient than automobile travel, and less efficient than some commercially available cars, this reflects the relatively low occupancies of transit vehicles. Transit services with high passenger occupancy rates are relatively energy efficient. Public transit improvements can provide significant energy savings and emission reductions by increasing operation efficiency, reducing traffic congestion, and substituting for automobile travel. Residents of transit-oriented communities tend to drive significantly less than they would in conventional, automobile-oriented locations. Transit improvements support other energy conservation strategies, such as efficient road and parking pricing policies. Without high quality transit such strategies are less effective and less politically acceptable. Current demographic and economic trends are increasing demand for high quality public transit and transit-oriented development (Litman, 2015).
American railroad passenger traffic grew steadily from the late 1800s until the 1920s, when long distance travel shifted to private automobiles and rail travel began a long decline. This decline was interrupted briefly due to gasoline rationing and the suspension of auto production during World War II when railroads were put back into service to transport the great volume of soldiers and war workers. Intercity bus service, which had been very limited before 1940, expanded during this period, as well. After the war, however most rail companies discontinued passenger service entirely. Passenger stations were demolished or abandoned, and railroad cars were taken out of service. In an effort to preserve rail service, Congress created Amtrak in 1970 and provided federal funds to support the new rail system. Commuter lines provided the remaining service. This was followed within a few years with the federal government's deregulation of U.S. airlines. The great increase in air travel that began after mid century is projected to continue indefinitely, offering speedy and safe transportation that strain air transit facilities. Bus travel provided a low-cost alternative airplane or train travel and has retained a small but relatively stable niche (Caplow, Hicks and Wattenberg, 2000).
The MTA, which is the New York Metropolitan area's transit system, is a noteworthy case study of a large-scale US public transit system. The Metropolitan Transportation Authority is North America's largest transportation network, providing service for 15.3 million people in 5,000 square miles including New York City, Long Island, southeastern New York State, and Connecticut. MTA subways, buses, and railroads provide 2.73 billion trips each year to New Yorkers, including about one in every three users of mass transit and two-thirds of the rail riders in the U.S. While 15 percent of the nation's workers use public transit to get to their jobs, four of every five of New York City's central business district rush-hour commuters use transit service, most of it operated by the MTA (MTA, 2017). The MTA accounts for 65 percent of all New York City commutes while using just 5 percent of New York City's total energy consumption (MTA, 2008).
The MTA also boasts the largest bus fleet in the U.S. and more subway and rail cars than all the rest of the country's subways and commuter railroads combined. According to the MTA, ridership on it's mass results in a 15 million metric ton net reduction of pollutants, making New York the most carbon-efficient state in the nation (MTA, 2017). New Yorkers consume one quarter as much energy per capita as the average American, largely attributable to the MTA system (MTA, 2008). While the energy and carbon emission efficiencies of the MTA system is impressive, the economy of such an operation poses significant ongoing challenges. Fares and tolls provide 53% of the MTA's $14.6 billion dollar annual operating revenue, but the system relies on taxes and subsidies for the remaining operating funds (MTA, 2015). In addition the agency relies heavily on debt to fund capital projects, with debt payments consuming a growing share of the MTA's annual operating budget, increasing the likelihood of fare increases and, creating an estimated debt service cost of $3.5 billion a year by 2030 (Tri-State Transportation Campaign, 2017). It seems that the inspiring environmental and fuel efficiency gains attributed to a large-scale public transportation system comes with a burdensome cost.
Freight accounts for about 26% of all petroleum-based fuels consumed in the U.S. transportation sector. Freight transportation demand is typically measured in tons, ton-miles, and value (dollars) of goods moved by the freight sector. The Federal Highway Administration estimates that 18.5 billion tons of goods worth $16.7 trillion were moved in the United States in 2007, for a total of 5.4 trillion ton-miles of travel (U.S. DOT). Trucks moved about 72% of all freight tonnage, accounting for 42% of all ton-miles and 70% of freight commodity value. Rail accounted for only 11% of tons moved, but 28% of ton-miles and 3.5% of total value, reflecting rail's cost effectiveness in hauling heavier, but generally lower-value, commodities, such as coal and grain, over long distances. Excluding international maritime shipments, waterborne transportation accounted for a smaller percentage of tons and ton-miles. Air freight transportation constituted an even smaller share, except when measured by value (Grenzeback, Brown, Fischer, Hutson, Lamm, Pei, Vimmerstedt, Vyas, Winebrake, J.J., 2013).
Between 1960 and 2008, the share of trucks to almost 42% of ton-miles, while rail fell from 36% of freight in 1960 to 33% in 2008. The share of waterborne freight decreased significantly while air freight grew ten-fold over the entire 48 year period, despite accounting for less than 1% of total freight travel in 2008. Disconcertingly, the modes of travel and freight that consume the most energy per unit grew faster than those that use the least energy. Freight demand is estimated to grow to 27.5 billion tons in 2040 and to nearly 30.2 billion tons in 2050, requiring ever-increasing amounts of energy. In the coming decades, all modes of domestic freight transportation are expected to increase significantly, but trucking's share, when measured in both tons and ton-miles, is projected to continue to grow at the expense of rail and waterborne freight (Grenzeback, Brown, Fischer, Hutson, Lamm, Pei, Vimmerstedt, Vyas, Winebrake, J.J., 2013).
The cost and volatility of fuel prices in the past decades as well as increasing interest by shippers in decreasing fuel costs and carbon emissions from goods movement have led the motor carrier industry to search for better fuel efficiency. The U.S. Environmental Protection Agency's (EPA's) SmartWay Transport Partnership program works with the shipping and trucking community to reduce fuel use and emissions by promoting cleaner and more efficient engines and transmissions, more aerodynamically clean truck shapes (including nose cones, skirts and gap fairings), idle reduction technologies, low rolling resistant and single-wide tires, lower weight components and aluminum wheels, driver training, and more efficient routing and dispatching (EPA 2011).
Railroads spend relatively less than trucks on fuel, due to the economies of scale and fuel savings by hauling very large volumes of freight over long distances. In 2008, railroads consumed approximately 320 Btu per ton-mile, compared to trucking, which used approximately 1,390 Btu per ton-mile. The difference in fuel use is reflected in the generally higher price of trucking services and the generally lower price of rail services, but the services provided by truck and rail also differ substantially in load capacity, routes and destinations served, frequency of service, transit time and reliability of travel time (Grenzeback, Brown, Fischer, Hutson, Lamm, Pei, Vimmerstedt, Vyas, Winebrake, J.J., 2013).
Understanding trends in fuel consumption by mode of travel merits an analysis of public investment in transportation and transportation infrastructure. In the U.S., transportation infrastructure is funded primarily by user-related taxes and fees which support construction and maintenance. Congress created the Highway Trust Fund (HTF) in 1956 to provide money for construction and maintenance of the Interstate Highway System. In 1982, the Mass Transit Account (MTA) was created to invest in public transportation systems. Taxes paid by highway users are credited to the HTF and are used solely to pay for highway and mass transit improvements. Currently, a federal excise taxes on gasoline, gasohol, diesel fuel, compressed natural gas, and taxes on heavy trucks and truck tires provide revenue for this fund. Revenue from motor fuel taxes are divided between the Highway Account (HA) and the Mass Transit Account, while all revenues from heavy truck taxes are dedicated to the Highway Account. In recent years, revenues have totaled $38 billion to $42 billion per year, with about $5 billion for the Mass Transit Account and the rest for the Highway Account. In 2015, Congress passed the $305 billion "Fixing America's Surface Transportation (FAST) Act", a five year plan to increase highway investment from $40 billion per year to $46.4 billion per year and increased public transportation funding from $10.7 billion per year to $12.6 billion per year (The American Road & Transportation Builders Association, 2016).
There is also a federal Airport and Airways Trust Fund, financed by fees on air travelers and taxes on aviation fuels.which finances airport improvements and the air traffic control system. State governments finance highway construction and maintenance through a variety of primarily user-related taxes and fees including taxes on gasoline and diesel fuel, vehicle registration fees, driver license fees, sales taxes on motor vehicles and heavy trucks, and traffic violation fines (The American Road & Transportation Builders Association, 2017).
Given the considerable and increasing concern regarding greenhouse gas emissions and global warming, understanding and adapting energy use seems increasingly urgent. The transportation sector's share of energy usage and carbon emissions makes it ripe for such analysis. While improvements in fuel efficiencies in all modes of transport, conservation efforts, and expansion of non-carbon based fuels provide hope for long term sustainability of transportation in the U.S., fundamental underlying factors make significant and meaningful improvement difficult to achieve. A U.S. landscape and infrastructure which was initially designed with an emphasis on rail-based public transportation has shifted over the last century to an auto-based transportation system. The Northeast United States is littered with bike paths that used to carry an extensive rail network that has been largely abandoned. Remaining public transit systems such those run by the Metropolitan Transit Authority have provided extensive, well utilized bus and subway service, and salvaged right of way remnants to recover rail for commuter service that is also heavily utilized. Unfortunately, high operating and capital costs combined with a dependence upon public funds make them difficult to sustain, particularly during periods of economic difficulty. What's more, the cost of maintaining the nation's extensive highway, road, and bridge infrastructure is becoming increasingly burdensome, crowding out funding for public transit systems.
Similarly, shifts in freight transport modes to more carbon-intensive forms such as heavy trucking, and the rapid expansion of air travel over the last 60 years have led to an increase in fuel usage and carbon emissions in trends that are difficult to reverse. And while the introduction of CAFE requirements for autos and trucks have improved efficiency, increases in vehicle and passenger miles and vehicle weights have limited these benefits. The Energy Independence and Security Act of 2007 promises a future fleet of significantly more efficient cars and trucks by 2020, but this gain could be abandoned by the current Congress and President. Despite great advances in technology and awareness, sustainable transportation in the U.S. will require greater initiative on the part of the public and government. Until that occurs, transportation sustainability will remain elusive.
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