Renewable fuels are produced from plants, crops and other biomass, and can reduce greenhouse gas emissions when compared to burning the fossil fuels they replace. EPA is also now working through the process of potentially setting domestic regulations under the Clean Air Act that address GHG emissions from certain classes of engines used in aircraft.
The Energy Independence and Security Act requires federal agencies to only acquire cars, light trucks, or medium-duty passenger vehicles that are low greenhouse gas emitting. Each year, EPA evaluates the greenhouse gas emissions performance of the fleet to determine which vehicles in each class emit less harmful greenhouse gases.
The law requires federal agencies to purchase these high performing vehicles. Over time this will result in a greener federal fleet. SmartWay helps the freight transportation sector improve supply chain efficiency, reducing greenhouse gases and saving fuel costs for companies who participate.
Since the mids EPA has required automakers to display a label on new cars and light trucks with information on vehicles' fuel economy and fuel costs. Labels on today's cars also include ratings on greenhouse gas and smog-forming pollutants.
These freight modes account for the remaining 30 percent of transportation-related emissions Davis et al. Table In the United States between and , energy intensity—the amount of energy required to produce a unit of transport activity—declined for nearly all transportation modes for example, energy intensity declined by 0.
However, these increases in efficiency were more than offset by an increase in total transportation activity for example, the number of passenger-miles flown grew by 4. TABLE There are four possible strategies that could be employed to reduce GHG emissions from the transportation sector:. Shift transportation activity to modes that emit fewer GHGs per passenger-mile or ton-mile;.
Reduce the amount of energy required to produce a unit of transport activity that is, increase the energy efficiency of each mode ; or. Each of these strategies is briefly discussed below. The most basic—but perhaps most difficult—way to reduce transportation-related GHG emissions is to reduce the total amount of transportation activity.
While there has been some attention devoted to reducing total freight transport volumes—by, for example, promoting consumption of locally produced food and goods—most of the attention in this area has focused on reducing personal transportation activity, especially activity by light-duty vehicles.
Since , the number of light-duty vehicle passenger-miles has grown at an average rate of 2. This growth has been spurred by, among other factors, the suburbanization of America. As recently as the s, the majority of daily commutes were from downtown to downtown or from close-in suburbs to downtown.
Now, the majority of commutes are from suburb to suburb, with the attendant traffic and pollution issues NRC, a; see also Chapter Suburbanization has also stimulated the increased use of light-duty vehicles for trips other than commuting—for example, according to the National House-.
Both logic and empirical evidence suggest that developing at higher population and employment densities results in trip origins and destinations that are closer to one another, on average, leading to shorter trips on average and less vehicle travel. Shorter trips can also reduce vehicle travel by making walking and bicycling more viable as alternatives to driving, while higher densities make it easier to support public transit. Another trend that has led to increased travel activity has been the reduction over time in the average number of people traveling in each automobile and light truck.
In , the average vehicle carried 1. For travel to and from work, the average declined from 1. Increasing the average vehicle occupancy could lead to reductions in total vehicle miles traveled and thus GHG emissions, even considering small offsets due to the need to pick up and drop off the additional passengers. Many municipalities have instituted policies to encourage carpooling; however, few of these policies were developed based on research on patterns and determinants of human behavior or effective mechanisms for informing such behavior, and there is a need for more evaluation of effectiveness.
Because commuting only accounts for about a quarter of passenger trips, carpooling strategies have limited potential for reducing transportation-related GHG emissions. However, it may be possible to increase the prevalence of ridesharing through more effective conveyance of information and the provision of incentives, both in monetary and convenience terms.
New technologies could help in this regard; for instance, it is already possible to use personal telecommunications devices and computers to connect drivers with prospective riders to create casual forms of carpooling. Such op-. Indeed, it is conceivable that in some locations public transit services will evolve away from the large fixed-route systems into smaller van-type vehicles that employ dynamic routing technologies to offer transportation services similar to that of private cars but with higher average occupancy WBCSD, While such concepts are in limited use in Europe, they have not been explored in the United States.
Because there are significant differences in the energy expended per passenger-mile or ton-mile among the major modes of transportation, a second candidate strategy for reducing transportation-related GHG emissions is to shift people or freight to more energy efficient modes.
The two most widely discussed options are 1 inducing people to substitute some of their driving with public transportation service, bicycling, and walking; and 2 shifting more freight from truck to rail. The viability of public transportation as well as walking and biking as an alternative to driving hinges in part on there being favorable urban land use patterns, as discussed in the preceding subsection and in the recent report Driving and the Built Environment NRC, e.
For public transportation to be an energy efficient alternative to the private vehicle, however, requires that the services be heavily used. At present, except in a few very dense urban areas such as New York City, public transportation load factors are not high enough to make these services more energy- and GHG-efficient than driving. Because demand is especially low outside of rush hours, transit systems often operate with very low levels of occupancy for much of the day NRC, c.
As a consequence, buses—the most prevalent form of transit—used 24 percent more energy per passenger-mile than private cars in Davis et al. Subways and commuter rail systems, in contrast, used about 20 percent less energy per passenger-mile than private cars, but these systems accounted for a minority of total public transportation ridership.
There is also significant geographic variability in the availability of public transportation: 97 percent of all subway and transit rail trips occurred in metropolitan areas with a population of over 5 million, and the New York metropolitan area alone was responsible for 38 percent of all national transit use for travel to and from work NRC, a. Bicycling and walking do not emit any GHGs and are associated with health co-benefits, but they currently constitute a very small share of all miles traveled by people when compared with motorized modes.
Strategies designed to facilitate and promote these modalities could yield multiple benefits. There has also been interest in using passenger rail for medium-distance miles or less intercity travel in the United States, which is currently dominated by automobiles and, to a lesser extent, air travel. In Europe and Japan, high-speed rail is succeeding in winning substantial market share away from automobiles and air transport for city-to-city travel at distances of up to miles FRA, There are many challenges, however, to duplicating such a system in the United States.
While high gasoline and deisel fuel taxes and road tolls tend to discourage intercity travel by private car in Europe and Japan, the ease and low out-of-pocket cost for automobile travel in the United States favors their use. Automobiles also offer flexibility for local travel once at the final destination, which is particularly important for families and leisure travelers who make trips between suburbs rather than center cities. A large share of business travel also takes place in suburban areas, which are poor locations for high-speed rail terminals.
Another challenge is that there are relatively few large U. Because of the long distances between cities, aviation is the only practical alternative for timely intercity travel in the United States. Moreover, U. This ability to offer a dense schedule of flights—which is highly valued by time-sensitive business travelers—cannot be matched by high-speed rail.
The recent uptick in intercity bus travel in the United States, which has been attributed both to the recent economic downturn and to higher fuel prices, is another longer-distance travel option that could potentially be promoted to reduce overall energy use and GHG emissions, particularly among leisure travelers.
The practicality and benefits of shifting additional freight traffic from truck to rail has been studied and debated for years. In , 64 percent of freight ton-miles moved by rail, while trucks carried only 9 percent, with most of the remainder moved on water-ways Department of Commerce, Although moving freight by rail is generally more energy efficient than moving freight by truck, it is not clear that a significantly larger share of freight could be practically moved by rail.
For example, because many rail sidings have been abandoned, most freight traffic, and especially manufactured goods, are moved by truck for at least a portion of the journey. Observers who study freight movements. Increasing the efficiency of transportation—especially light-duty vehicles—has been a major strategy for reducing U.
The companion report Limiting the Magnitude of Future Climate Change NRC, c includes a summary of changes in fuel economy standards over the past 30 years, the effectiveness of these standards, and their implications for climate policy.
For example, the fuel economy potential of new passenger cars and light trucks measured in terms of ton-miles per gallon has improved at a rate of about between 1 and 2 percent per year since EPA, c , mainly through a series of technological advances in engines and aerodynamics. However, this potential has not been reflected in actual new vehicle fuel economy; since the mids, the fuel economy of new automobiles and light trucks as tested by the Environmental Protection Agency EPA has essentially been stable.
Instead, vehicles have become heavier by about pounds on average [Davis et al. The EPA estimates that if the potential improvements in fuel economy had been realized, model year cars would have averaged 33 to 34 mpg instead of the 30 mpg they did average, and new light trucks would have averaged 27 to 28 mpg instead of 22 mpg.
Congress has called for a fleetwide combined fuel economy for cars and light trucks that reaches 35 mpg by model year , representing a 30 percent increase over current levels Energy Independence and Security Act of , P. Tapping the reservoir of unrealized fuel economy potential with continued modest improvements in the efficiency of conventional gasoline and diesel engines would be the easiest way for motor vehicle manufacturers to meet these new efficiency standards.
Doing so, however, would require consumers to sacrifice certain desired performance attributes such as acceleration capabilities. In order to meet the new standards under these constraints, manufacturers will need to increase the use of hybrid-electric propulsion systems, make cars and trucks lighter typically through the use of materials such as fiberglass and carbon fibers , and develop next-generation propulsion systems—batteries and fuel cells being the two main candidates see next subsection.
It will be important with respect. To advance the technologies required to enable the production of more fuel-efficient light vehicles, the federal government has over the years funded cooperative research and development programs such as the Program for a New Generation of Vehicles. In addition to such federal actions, some states, led by California, have set their own fuel economy standards and taken other actions, such as requirements to sell a certain number or fraction of low-emissions vehicles.
If the United States passed a climate bill that priced transportation carbon and linked it to a transportation bill that would reinvest the revenues into a green transportation system, the United States would be on track to meet its stated obligation of a 17 to 20 percent absolute decrease in greenhouse gas emissions by That would give comfort to other countries—particularly China, India, and other emerging economies—that the United States is serious about reducing its transportation carbon and it would contribute to the likelihood of a global climate agreement.
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