Sarah drives to work each morning. On the way she passes through a busy junction with a set of traffic lights. One morning she stops behind a line of traffic just as the lights go red. She can see 14 cars between her and the red light. The red (stop) light will remain on for 42 seconds. The green (go) light will come on for just 10 seconds, and this will be enough time for 7 cars to get through the junction before the red light comes on again and the line of cars is brought to a stop.