In preparation for the 2008 Summer Olympics and to fulfill promises to clean up the city's air, nearly US$17 billion was spent. Beijing implemented a number of air improvement schemes for the duration of the Games, including halting work at all construction sites, closing many factories in Beijing permanently, temporarily shutting industry in neighboring regions, closing some gas stations, and cutting motor traffic by half by limiting drivers to odd or even days (based on their license plate numbers), reducing bus and subway fares, opening new subway lines, and banning high-emission vehicles. The city further assembled 3,800 natural gas-powered buses, one of the largest fleets in the world. Beijing became the first city in China to require the Chinese equivalent to the Euro 4 emission standard.