Purple Light

Conventional traffic lights typically consist of three colours. Proposed is an additional smaller light, in purple, just above the red one.

The purple light illuminates if, and only if, the time remaining until the red light changes is greater than 20 seconds. If you see the purple light, turn your engine off, save the planet, and so forth. If it's not there, you've not got long to wait.

I believe this is much simpler, and with far fewer drawbacks, than a countdown timer.