You have to keep your eyes peeled for the bus at the station in Shenzhen's Futian central business district these days. The diesel behemoths that once signalled their arrival with a piercing hiss, a rattle of engine and a plume of fumes are no more, replaced with the world's first and largest 100% electric bus fleet. Shenzhen now has 16,000 electric buses in total and is noticeably quieter for it. We find that the buses are so quiet that people might not hear them coming," says Joseph Ma, deputy general manager at Shenzhen Bus Group, the largest of the three main bus companies in the city. The benefits from the switch from diesel buses to electric are not confined to less noise pollution: this fast-growing megacity of 12 million... is also expected to achieve an estimated reduction in CO2 emissions of 48% and cuts in pollutants such as nitrogen oxides, non-methane hydrocarbons and particulate matter. Shenzhen Bus Group estimates it has been able to conserve 160,000 tonnes of coal per year and reduce annual CO2 emissions by 440,000 tonnes. Its fuel bill has halved. China's drive to reduce the choking smog that envelops many of its major cities has propelled a huge investment in electric transport. Although it remains expensive for cities to introduce electric buses - one bus costs around 1.8 million yuan (208,000) - Shenzhen was able to go all-electric thanks to generous subsidies from both central and local government. Typically, more than half of the cost of the bus is subsidised by government," says Ma.