Streetlights aren't really necessary when there's a full moon out, what with it bouncing all that sunlight down at us and everything. But your average streetlight isn't smart enough to know when it isn't needed, so it sits there, dumbly shining away for no real reason. That's not the case with the Lunar-Resonant Street Lights, conceptual LED-based lamps that dim down and turn off completely as the moon waxes. When it's a new moon, it'll be fully lit up, showing you your path. What results is an energy savings of 90-95%. Not a bad idea.
Street lights typically have sensors (usually done in banks of lights rather than in each individual light) to determine when to turn on and turn off but the sensors do not have the ability to modulate light output.
HPS lamps which are common in street lamps have an efficiency of 100-150 lumens per watt while a typical LED gets only 70 lumens per watt although there have been some advances where they are able to get 130 lumens per watt. So LEDs are not really inherently more efficient.
The advantage of LEDs is that they can produce a variable range of light. So I'm sure the sensors will detect how much light is being provided by the moon and adjust the output of the LEDs accordingly. That is the likely reason for the energy savings.
Of course, I haven't really studied HPS lamp design so I don't know if there isn't an easy way to make variable output lamps.