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At the end of 2013, Beijing launched its first dual-mode trolleybus line, an executive action in the Beijing Clean Air Action Plan (2013-2017). Within the five-year time-frame, this Plan aims to fulfill the promise that there will only be zero-emission buses in the Beijing city center within the Beijing 5 Ring Road. The total number of electric vehicles is expected to reach 4700 ((You gai dian gongjiao che beijing shanglu, http://society.people.com.cn/n/2013/1220/c1008-23894059.html, accessed on 14 January 2014.))[caption id="attachment_8340" align="aligncenter" width="355"] Source of image: You gai dian gongjiao che beijing shanglu, http://society.people.com.cn/n/2013/1220/c1008-23894059.html, access on 14, January 2014.[/caption]In fact, since 2001, there have been 15 traditional trolleybus lines in Beijing; the traditional system depends only on electricity wire. If there is a malfunction in the grid, buses would have to stop in the middle of roads, causing traffic congestion. The new dual-mode trolleybus system is different from the old traditional one in that the buses are equipped with a battery; the electricity grid connected to the buses will continue to run them, and at the same time charge the battery. Therefore, if an error occurs in the electricity supply, the dual-mode trolley buses can be "off-line" and still be able to function ((Beijing jiang piliang zhihuan shuang yuan diandong gongjiao che bi chun diandong che pianyi 20 duo wan, http://big5.ce.cn/gate/big5/wap.ce.cn/cy/201312/13/t20131213_1915572.html, accessed on 14 January 2014.)). This will increase the mobility of the buses, which will no longer be shut down easily from grid problems.&#160;