green light

From the standard red-green color scheme used in traffic lights and railway signals.

• Audio (General Australian):

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green light (plural green lights)

1. A traffic light in its green state (indicating that vehicles may proceed).

   The drive home was much faster than usual; somehow we managed to hit every green light.

2. (idiomatic) Approval, or permission to proceed.

   Synonym: go

   We have a green light on the project. Work starts today.

   • 2020 May 20, “Network News: Class 345s finally able to serve Heathrow Airport”, in Rail, page 15:

     And Crossrail's Class 345 Aventra fleet finally has the green light to carry passengers to Heathrow Airport, after the Office of Rail and Road granted approval for the European Train Control System (ETCS) Level 2 overlay system.

3. (slang) A go-ahead to commit a crime, specifically to carry out a hit on someone (i.e. to kill).

   Yo man, I heard someone had put a green light out on you, so watch your back.