What does the traffic signal ahead sign mean?

This sign is posted on a rural road where traffic moves quickly. What does it warn of?

Traffic lights ahead that may already be changing

Lights on a fast road stay hidden behind crests and trees until quite late, and a driver who arrives with no warning meets an amber with no good option. The sign gives you time to lift off and cover the brake before the lamps are even in sight.

This sign appears on a fast road you have not driven before. What is the point of it?

Traffic lights are ahead where you would not expect them, so be ready to stop

New lights, or lights just over a crest, catch drivers who are travelling at open road speed and reading the road as though it had no intersections on it. The sign asks you to cover the brake and hunt for the signal instead of trusting the road to stay clear.

This sign appears just before a crest on a road with a high limit. What is the point of it?

To warn that lights are ahead and may change before you reach them

You cannot see over a crest, so the sign gives you the reaction time the road itself takes away. Come over the top at a speed that lets you stop for a red you have not seen yet. Drivers get caught here because they set their speed by what is visible, and the crest hides the one thing that matters.

You are on a road with a high speed limit and this sign appears where the road bends away to the left. What is the point of it?

Lights are ahead that you cannot see yet, so ease off and be ready to stop

The sign goes where a bend, a crest or a line of trees hides the signal until you are almost on top of it. Coming off the accelerator early turns a hard stop into a normal one if the light changes as you arrive. It also gives the driver behind you your brake lights in time to do the same.

You pass this sign and then see that the light ahead has been green for as long as you have been able to see it. What should you expect?

The light may change as you arrive, so ease off and be ready to stop

A light that was already green long before you got there is close to changing, which is why an amber can feel like an ambush. Easing off early leaves you a real choice between stopping and going instead of a hard decision at the line.

You pass this sign and the light ahead has been green for as long as you have been able to see it. What is the sensible way to read that?

It is closer to changing than a light that has just turned, so ease off

A light that has been green a long time is a stale green, and every second you watch it makes a change more likely rather than less. Easing off on the approach lets you stop smoothly if it does change, instead of forcing the hard choice at the amber.