Alternately flashing red lights

The image shows two circular red lights that flash alternately. In the driving-theory questions they require road users to stop. They are used at level crossings with or without half-barriers, at the entrance to a lifting bridge and at a ferry landing stage. While the lights are operating, vehicles and pedestrians must not pass and drivers must not cross the transverse stop line. At a level crossing they begin flashing shortly before a train arrives and may be accompanied by an audible warning. They do not indicate a police checkpoint, a mandatory motorway exit or that a green light is about to appear.