The fact of the matter is that pollution caused by burning auto fuel has continued being a problem with no solution. Adding on concern about the energy crisis, experts forecast that at the end of the 21st century there will be need for another transportation revolution.
事實上，汽車燃燒石油所帶來的空氣汙染，一直無法有效消除；加上能源危機的顧慮，專家預估廿一世紀下半葉，交通工具就需要再一次「革命性」的改變。