Two glowing disks seen over an Interstate highway in Western Pennsylvania.

I was on my way home from a job interview (1980s approx.).  It was late at night, about 10pm or so. I was on I-79 (I think that is the name of the Interstate).  
Suddenly, I saw two glowing disks floating over the mediam strip. I couldn't tell exactly how big they were, but they were huge and hovering just a few feet above some pine trees: approximzte altitude-40 to 50 feet.  I did not see any details, just their glow;they never moved. I was driving south at about 60mph and the sky was overcast.  In about 15 minutes I was past them and nothing else happened.  I checked my watch and did not notice any missing time.  It was a bitter cold night in January or February; I do not remember exactly.
Witness indicates that the date of the sighting is approximate.  PD