The development of megacities, which are cities over 10 million people, means that cities will need to develop efficient, low-emission mass transportation systems in the next decade. The population explosion of many of the world's largest cities has created traffic jams that are mammoth in size. For example, Sao Paulo can have traffic snarls that are 110 miles in length, and people in the U.S. now spend an average of 9 years in their cars (see the BBC link below). Increased car ownership in China now means that Chinese cities are clogged with traffic. Some new forms of transportation are also planning to use stilts to carry commuters on buses well above the gridlock on the ground. This level of congestion is not tenable, so people will have to use efficient mass-transit systems that produce low emissions. The high cost of gas and the laws in many cities that require new forms of transportation to produce less pollution will necessitate the use of new forms of technology to power vehicles, such as liquid natural gas, battery power, hydrogen, and compressed air.