viewed 1-4 cigar/disc shaped dark spots emitting no light nor sound for approximately 6 minutes

I left Robstown Texas at about 7 PM heading north on US 77.  Then 77 fed onto I37 from Corpus Christi to San Antonio.  I was driving along and noticed a dark object moving about the same speed as I was.  It seemed to increase in number from 1 up to 4 from time to time.  I took notice of the next mile marker which was 42.  I then rolled my window down to see if perhaps there was something on my window creating the affect, plus to listen for noise.  The spot(s) were still there, but I heard no noise other than my car and general traffic noise.  The spot(s) were dark and the best I could describe the shape was from a cigar shape to a disc (flat)shape.  I flashed my lights on and off to signal I had seen them... and anytime a car passed me, I pointed my arm out the window towards the spots... hoping someone else would look in that direction.  I watched the spots for about 6 miles... then they dissappeared.  I made mental note of the time, 7:17PM and the next mile pole, which was 48.
The spots did not reappear.