In England, traffic keeps on the left.Cars, buses and bikes all move on the left side of the road.But the traffic keeps on the right in China.So when you are in England, you must be very careful in the street.Before you cross a street you must stop and look both ways.Look right and look left and look right again.If the traffic lights are red, the traffic must stop.Then the people on foot can cross the road.If the traffic lights are green, the traffic can go.People on foot mustn't cross. In the morning and in the evening, when people go to or come back from work, the streets are very busy.Traffic is very dangerous  . When you go by bus in England, you have to be careful, too.Always remember the traffic moves on the left.If you don't have a look first, you will go the wrong way. In many English cities, there are big buses with two floors.You can sit on the second floor.From there you can see the city very well.It's very interesting. ,. When the traffic lights are   _  , the traffic must stop.
red yellow green we don't know