Very large triangular humming (pulsing) craft with several lights moving slowly

In 1978 myself and a friend (& his younger brother)went from Cary, Il. to Chicago to film a movie for a high school project.  Upon returning home on Hwy 90, I noticed several lights off to the North.  Several minutes later I noticed the lights were still there.  Just about at that moment we were exiting Hwy 90 onto Barrington Rd.
After traveling on Barrington Rd and continuing to watch these lights we turned onto Algonquin Rd & traveled several miles where Algonquin & Dundee Rds intersect.  I had lost track of the lights for about a minute or so, but suddenly to my right (I was in the passenger seat)the lights were up in the air about 100-150 feet or so above us at this intersection.  
Screaming at the driver, I had him pull off the road and  I jumped out of the car to look up at an extremely large triangular object moving to the south west very slowly and making a pulsing humming sound. Hum-hum-hum etc.
After it fell out of site we drove home.  The very best I can remember the lights were green & blue
Witness indicates that date of sighting is approximate.  PD