There are traffic lights used the concept LED Sand Glass, which can help you know exactly how much time is left.

LED lights trickle down in a virtual sand glass that countdowns how much more Red and Green there will be. When it approaches 3 seconds left, the LEDs switch to yellow and give you an exact count on what's left.

It’s an amazing idea. But how it turns yellow before it turns green and red could be a little confusing.