13. Red, amber, green and... blue? Indeed, some cities in Germany added a blue aspect to their traffic lights in the 1980s. The blue light also showed a white key, but what did it mean?
From Quiz Have You Got a Light, Boy?
Answer:
Long red - turn off engine
The "engine off" blue was one of the early attempts at helping drivers conserve fuel. It was typically used at intersections where the red duration was 60 seconds or more. The light was solidly lit for most of the red, asking the driver to turn off the engine. It went off about 20 seconds before green (keep engine off, but do not turn off if just arriving) and then, 5-7 seconds before green, came on flashing (restart engine).
While a good idea in principle, the light had several flaws. First, too few drivers heeded it, second, of those who did heed it, few restarted the engine at the correct time (either immediately upon going off, massively reducing the benefit or only when the red-amber came on, leading to delays) and finally, it broke the important rule that the only permitted usage of blue in a traffic-relevant lighting fixture is that of the blue emergency vehicle light.