The following is a discussion from the RFI reflector on finding RFI from street lights by Frank Haas KB4T, a utility RFI investigator in Florida. With these RFI sources being a common irritant to the active contester, Frank's explanations and suggestions are good information. The article was reprinted in the recent ARRL's Contest Update.
"As a Utility Interference Investigator, I run into bad street lights all the time. The most common failures are cycling and constant "invisible" RFI. Listening with a radio tuned to a quiet spot (or 1710) on the broadcast band, you can usually hear the repetitive street light symphony. Sometimes the RFI produced by these failed cycling lights can be heard as high as 325 MHz.