When thinking about how (real-world) traffic lights work, I wonder if there is a more intuitive way to display the yellow light. The meaning of Green and Red are states:

Green: Ok, you can go!
Red: Not ok, you need to wait!

However, yellow indicates progress:

Yellow: Attention, the light will become red in an instant!

(And depending on the country, it may mean as well:)

Red+Yellow: Just a second, Green (or Red, in other countries) will follow in an instant!

This can be judged as imperfect user experience, as "instant" is somewhat unspecific, and similar methaphors (lamps) are used for different concepts (state vs. progress). (The "user" here is the driver of a vehicle.)

Now, of course, traffic lights are heavily institutionalized, it would be very difficult to establish a different norm. But are there variations, design studies or research of traffic lamps that try to adress this problem?