Explanation

"At junctions controlled by traffic lights, you must stop behind the white line until the lights change to green. A red light, an amber light, and red and amber lights showing together all mean stop. You may proceed when the light is green unless your exit road is blocked or pedestrians are crossing in front of you. If you’re approaching traffic lights that are visible from a distance and the light has been green for some time, be ready to slow down and stop, because the lights are likely to change."