a traffic light.
I have three colors,
Red, yellow, and green
each with a different meaning.
Red is a no-go, yellow is to prepare, and green is go.
When I'm old they become dull,
Excessive sunlight make the color-coating of the traffic lights fade, or alternatively the light bulbs can lose their strength.
but when I'm new they are gleaming.
Shiny new lights are nicely visible.
I help keep people safe,
Traffic lights keep you in order.
but sometimes they hate me.
You don't like to wait when it is red.
You won't see me in a forest,
but you will in a big city.
Indeed, at major traffic junctions.
Some of me have short lengths,
and some of me have long.
And in the time that the longest of me take,
you could sing a very long song.
The time of changing color depends from place-to-place, and also from color-to-color. Red takes forever!
When my colors change,
people get happy or sad,
Time to start your car!
and when you guess what I am,
you will be very glad!
Indeed. Although I would not be surprised, if this would be a duplicate. We should go and check.