Traffic light failure
When they fail they flash red in all directions, it is hardwired into them. If the power is lost and the battery backup is exhausted they'll be off, that's really the only potentially dangerous time (i.e. if you don't see them / don't know they're there and think you have the right of way) Cars would be programmed with maps that know where the traffic lights are, so if they can't "see" the lights they would know something is up and could stop and "look both ways" before proceeding. They'd likely handle that particular situation better than some people.