An oddity in applied electronics

You'll notice that light-controlled intersections do in fact need a "go" light (the green light).   You can't have an intersection where the default action in the absence of any other lights is to go because then you'd have a lot more chaos than usual when the power goes out.

That's Hen3ry's point - the absence of a signal can indicate "power or equipment failure" rather than "battery charged".   He'd prefer a positive indication that shows everything is working the way its supposed to.