Technology using electrical energy to power automobiles has been in existence for over a century. However, for a number of reasons, including the energy density of petroleum fuels, the internal combustion engine has been the power source of choice for automobiles and most other vehicles. However, with the oil shocks of the past few decades, as well as an increasing awareness of the emissions of air pollutants and greenhouse gases from cars and trucks, interest in the use of electrical power train systems has grown. While there are other potential replacements for the internal combustion engine, such as compressed air, these other technologies have not been the subject of much interest scientifically or politically. Much of the federal advanced vehicle research has come through the Partnership for a New Generation of Vehicles (PNGV) and the FreedomCAR program, consortia of the federal government and the "Big Three" American automobile manufacturers. PNGV focused on near-term goals and the development of hybrid electric vehicles, while FreedomCAR, which replaced PNGV in 2002, focuses on long-term research on fuel cells and hydrogen fuel. The United States is not alone in pursuing these new technologies. Japanese manufacturers were the first to introduce high-efficiency gasoline-electric hybrid vehicles in the U.S. market. The development of these vehicles is a response to global pressures to lower emissions and improve fuel economy. In that context, it is worth noting that in most developed countries, gasoline and diesel fuel prices are considerably higher than they are in the United States. Four advanced propulsion technologies of key interest are electric vehicles, hybrid vehicles, plug-in hybrids, and fuel cell vehicles. In an electric vehicle, the vehicle runs exclusively on electricity that is supplied from an electric utility provider, eliminating combustion on-board the vehicle. A hybrid vehicle integrates an electrical system with an internal combustion engine to utilize the benefits of each system. A plug-in hybrid system allows a vehicle to be charged in the same manner as a pure electric vehicle, allowing an all-electric range. The additional hybrid system allows for the vehicle to use a combustion engine when the batteries are depleted. In a fuel cell vehicle, instead of combustion, a chemical conversion process is used, leading to higher levels of efficiency. In addition to altering the propulsion system, many other efficiency-related technologies, such as improved aerodynamics and low-resistance tires can be incorporated into both new and conventional vehicles. While these various technologies are promising, they must overcome certain obstacles before they will be competitive in the marketplace. There are three main barriers to their widespread use: cost, infrastructure, and performance. Cost is a factor since without subsidies, consumers are unlikely to purchase new vehicles in large numbers if the new vehicles are not cost-competitive with conventional vehicles. Also, convenient infrastructure must exist for both fueling and maintenance of these vehicles. Finally, the performance of the new vehicles must be comparable to that of conventional vehicles. An electric vehicle (EV) is powered by an electric motor, as opposed to a gasoline or diesel engine. Power is supplied to the motor by batteries, which are charged through a central charging station (which can be installed in the owner's garage) or through a portable charger on board the vehicle, which is plugged into an electrical outlet. Because no fuel is consumed in EVs, and the vehicles therefore do not produce emissions, they are considered to be zero emission vehicles (ZEVs) in certain air quality control regions. Although there are emissions attributable to the production of electricity to charge the vehicles, the overall fuel-cycle of EVs tends to lead to lower levels of toxic and ozone-forming emissions—as well as greenhouse gases—than those of conventional vehicles. Also, since pollution attributable to electric vehicles occurs at power plants, it is generally emitted in areas with relatively low population density. Another potential public policy benefit of electric vehicles is that they can reduce U.S. dependence on foreign oil, since only about 3% of electricity in the United States is generated from petroleum. Furthermore, transportation dependence on all forms of fossil fuels can be reduced, since approximately 30 to 35% of electricity in the U.S. is generated from non-fossil fuels. However, high electricity costs in recent years have led to questions about the long-term viability of EVs. Commercially, these vehicles have not been well-received by consumers. By 1998, only about 4,200 EVs were on the road, mainly in California. By 2005, this number had increased to roughly 51,000. However, only a few car companies currently produce electric vehicles, and most of those are only available for lease by large fleets. EVs for personal use have declined as automakers have taken most personal EVs off the market. While the number of EVs has increased, the amount of fuel used per vehicle has decreased, as have, presumably, miles traveled per vehicle. Between 1998 and 2005, while the number of EVs increased more than 10-fold, electricity for vehicle fuel increased only about five-fold. One of the most significant barriers to wide acceptance of electric vehicles is their higher purchase cost. For example, the manufacturer's suggested retail price for a 1999 General Motors EV1 was approximately $33,995, which was considerably higher than a comparable 1999 Chevrolet Cavalier at $13,670. However, fuel costs tend to be much lower for EVs than for conventional vehicles. In 2002, a small conventional vehicle could achieve a fuel cost of approximately $690 per year. An electric vehicle, however, could achieve a considerably lower cost of $390 to $480 per year. This difference, while significant, fails to make up for the additional purchase or lease cost for an electric vehicle. With increased petroleum prices, the cost savings for EVs may make them more attractive. However, its is unlikely that even a very large increase in petroleum prices would be sufficient to make the current generation of electric vehicles cost-competitive. In terms of maintenance costs, electric vehicles have fewer moving parts, which reduces wear. However certain parts, such as replacement batteries, tend to be expensive. Through 2006, there was a federal tax credit for the purchase of an electric vehicle. The federal credit was worth 10% of the purchase price of the vehicle, up to $2,000. This credit was part of the Energy Policy Act of 1992. In some areas, EVs are also exempted from high occupancy vehicle (HOV) lane restrictions, parking restrictions, and/or vehicle registration fees, which may provide an additional incentive for their use. Another key obstacle to more widespread use of electric vehicles is the lack of fueling (charging) and maintenance infrastructure. For example, there were approximately 700 public charging stations in 2004, mostly in California and Arizona. Currently, that number is only 440. This represents less than 1% of the roughly 125,000 gasoline stations nationwide. The lack of recharging infrastructure is not only inconvenient, but also limits long-distance travel, since Arizona and California account for the vast majority of all recharging sites currently in operation. Adding to the problem of fueling infrastructure, is the lack of maintenance infrastructure. Few mechanics have experience servicing EVs, and most work must be done at a certified dealer. For this reason, most EV leases include free dealer maintenance over the period of the contract. On the other hand, one advantage of electric vehicles is that they have fewer moving parts and thus may be more durable, and require less frequent maintenance. Another major concern with electric vehicles is their performance. The batteries used to power the vehicles tend to be quite heavy, limiting the range of these vehicles. While a conventional passenger car can travel 300 to 400 miles before refueling, until recently, electric cars generally could only travel about 100 to 150 miles before needing to be recharged. With new developments in battery technology, EV range has increased. However, even new EVs are unlikely to have the range of a conventional vehicle. Furthermore, while refilling the tank of a conventional vehicle requires only a few minutes, a full residential recharge for an electric vehicle can take five to eight hours, although new chargers may shorten this time significantly. For fleet vehicles, or for short-distance commuting, these performance characteristics might not greatly affect their marketability, but the feasibility of EVs for long-distance, intercity travel is unlikely with current technology, even if the fueling infrastructure is greatly expanded. A lesser concern with electric vehicles is an unconventional driving style. To provide maximum efficiency and range, the driver must accelerate and brake very smoothly, or range is significantly diminished. Because of this, some drivers may not be comfortable or proficient operating an electric vehicle. The greatest performance benefit from an EV is that, as was stated above, there are no emissions from the vehicle itself. Furthermore, the overall toxic and ozone-forming emissions tend to be much lower than with conventional vehicles since it can be easier to control emissions at a power plant than it is to control combustion vehicle emissions. An added benefit is a reduction in noise pollution since EVs are significantly quieter than conventional vehicles. Greenhouse gas emissions caused by EVs tend to be lower than those from conventional vehicles, depending on the local fuel mix used in power generation and the efficiency of the power distribution grid. But if electricity transmission and distribution losses are high, total energy consumption attributable to electric vehicles may exceed conventional vehicles. A major issue for vehicle manufacturers, and a motivation for increased research and development on electric vehicles, is California's zero-emissions mandate. This mandate would require manufacturers to sell ZEVs and other super-low-emission vehicles. However, many technical and market barriers have hindered the implementation of the program. Most recently, the California Air Resources Board amended the program, allowing manufacturers two methods to certify compliance. First, manufacturers were able to generate credits for future use by introducing a limited number of fuel cell vehicles (see discussion below on fuel cells) by 2005.