Blue flag, red flag and green flag can represent several different signals if they are hung only on two sides.

Blue and red flags, red and green flags, blue and green flags. Blue flag and red flag: it means no passage. This signal is used to inform vehicles and pedestrians not to pass during the maintenance or construction of roads, bridges, tunnels and other traffic facilities. Red and green flags: stop and wait. This signal is used in traffic control, vehicle scheduling and other occasions to tell vehicles to stop and wait or follow the instructions. Blue flag and green flag: open traffic. This signal is used to inform vehicles and pedestrians that they can pass after the maintenance or construction of traffic facilities such as roads, bridges and tunnels is completed.