Imaginearoadcrossingwithoutthethree-waytrafficsignal.Ca...
问题详情：Imaginearoadcrossingwithoutthethree-waytrafficsignal.Cars,trucks,schoolbuses,bicyclistsandstudentswalkingtoschoolwouldtrytorushthroughthecrossing61_______thesametime.Theirsafetywouldbeindanger...