in shanghai, there're mainly four means of transportation,the  underground,  taxis,  buses  and  electric  bikes.   the underground is much faster than taxis and buses. buses are the cheapest but the slowest of the four. taxis run more quickly than buses, but they are more expensive. now, more and more people choose to use electric bikes.  the electric bike is both clean and mobile. it also saves money. it is the most convenient means of transportation.