transportation is dominated by automobiles, which operate on a network of four million miles (6.4 million km) of public roads, including one of the world's longest highway systems at 57,000 miles (91,700km). The world's second-largest automobile market, the United States has the highest rate of per-capita vehicle ownership in the world, with 765 vehicles per 1,000 Americans. About 40% of personal vehicles are vans, SUVs, or light trucks. Rail Mass transit accounts for 9% of total U.S. work trips. Transport of goods by rail is extensive, though relatively low numbers of passengers (approximately 31 million annually) use intercity rail to travel, partially due to the low population density throughout much of the nation. However, ridership on Amtrak, the national intercity passenger rail system, grew by almost 37% between 2000 and 2010. Also, light rail development has increased in recent years. The state of California is currently constructing the nation's first high-speed rail system. Airline The civil airline industry is entirely privately owned and has been largely deregulated since 1978, while most major airports are publicly owned. The three largest airlines in the world by passengers carried are U.S.-based; American Airlines is number one after its 2013 acquisition by U.S. Airways. Of the world's thirty busiest passenger airports, twelve are in the United States, including the busiest, Hartsfield–Jackson Atlanta International Airport. Energy The US is the second-largest energy consumer in total use. The U.S. ranks seventh in energy consumption per capita after Canada and a number of other countries. The majority of this energy is derived from fossil fuels: in 2005, it was estimated that 40% of the nation's energy came from petroleum, 23% from coal, and 23% from natural gas. Nuclear power supplied 8.4% and renewable energy supplied 6.8%, which was mainly from hydroelectric dams although other renewables are included. American dependence on oil imports grew from 24% in 1970 to 65% by the end of 2005. Transportation has the highest consumption rates, accounting for approximately 69% of the oil used in the United States in 2006, and 55% of oil use worldwide as documented in the Hirsch report. In 2013, the United States imported 2.808 billion barrels of crude oil, compared to 3.377 billion barrels in 2010. While the U.S. is the largest importer of fuel, The Wall Street Journal reported in 2011 that the country was about to become a net fuel exporter for the first time in