Road markings, signals and traffic-control instructions
Markings and signals
Road markings are part of the legal control of the road. A solid white line normally may not be crossed; a broken line may be crossed only when it is safe and permitted. Double solid lines forbid crossing. Stop lines, give-way lines, pedestrian crossings, lane arrows and hatched areas all tell you where to stop, wait or drive. Traffic lights are commands: red means stop, green means proceed only if the way is clear, and amber means stop unless you are too close to stop safely.
Traffic-control instructions
Police officers and authorised controllers override lights and signs. Their arm and body signals may direct traffic from any direction, so watch the officer, not only the light. Flashing amber at a crossing or junction usually means proceed with extra caution and yield as required.
Worked example
You are behind a slow tractor on a road with a broken centre line. The line is broken, so crossing it to overtake is legally possible, but only if you have clear sight, no oncoming traffic, and no solid line ahead. You check mirrors, signal, move out, and return only when the broken line and safety allow. If the line became solid, you would wait.
Traps that cost marks
Crossing a solid line because the road is empty; stopping on a pedestrian crossing; treating a green light as an unconditional right to go; ignoring a stop line because the light is green; misreading a hatched area as a normal lane.