Consider the following items:
- the warning track around a baseball outfield
- the red stripe near the end of the a roll of receipt paper
- the margin bell on a typewriter
- the rumble strips when a highway becomes a street
They all seem to be of a class: things that give you a warning that something is about to change, that you're almost done. I'm looking for an expression or word for this.
(I was thinking "canary in the coal mine", but that is for emergencies, disasters, that kind of thing -- I want something for the routine changes of life.)