In case you weren't already convinced that most street lights are time-varying, here's proof. This is a shot I took of a small US city (I don't know which) from an airplane at night. I swept the camera while taking the shot, using a 1/10 second exposure time. As you can see, there are many lights and groups of lights which show strong periodic traces. Most of these lights are pressurized sodium street lights, which flash at 120 times per second (in the US).
I had to adjust the exposure and tone curve in Lightroom to compensate for the fact that a city at night isn't very bright as far as a camera's concerned; and when you sweep the camera, it's even dimmer.