When we have space colonies, in the distant future, on planets with low gravitational acceleration, there will surely still be traffic to keep under control.
Today’s invention is a set of low-g traffic lights which operate without electronic controls.
Two pendula swing side-by-side. One has a green lens as a bob, the other red.
The red lens is stationary in front of a sealed lamp unit and behind an aperture, as shown on the left of the diagram.
The green lens swings (slowly) inwards and gradually creates the appearance of an amber light. A bar on the green lens impacts the red lens and, just as in a Newton’s cradle, exchanges momentum with it elastically, so the green lens stays put and the red lens begins to move away.
Eventually, the amber is replaced entirely by the green so that any waiting skycars can proceed.
The absence of air resistance on the space highway of the future, allows this cyclic process to proceed -effectively forever.