n. A traffic light that has been green for a long time and so is likely to turn orange at any moment.
1998
"Stale green" is a term used in transportation circles. It means the traffic light has been green a long time.
1994
Check for stale greens. "What's a stale green?" Well — a fresh green is a green traffic light you know has just turned green — you were waiting for it or approaching it — you know it is going to stay green long enough for you to cross the intersection. Any other green light is a stale green, and may turn yellow as you cross.
1986 (earliest)
Green traffic lights — May change (stale green).