Ⅰ. At a pedestrian crossing: a) When you have stepped off the pavement on to a zebra crossing, traffic must stop to let you cross. But don't be foolish: make sure that drivers have enough time to give way to you. b) If a police officer is directing the traffic, watch for his signals. Do not cross until he holds up the traffic. c) At a crossing with traffic lights, do not cross in front of traffic which has the signal to go.Ⅱ. At a bus or tram stop: If you want to get on a bus or a tram' give a clear signal for it to stop and do not step into the road until it has done so. When you get off a bus or a tram and wish to cross to the opposite side of the road, do not walk behind the vehicle. Wait until the vehicle has moved off and make sure there is no other traffic before you cross the road According to the passage, at a pedestrian crossing you should not ______.