Glowing ball of light

We were driving to Laughlin at night and this light the size of approx a basketball was following us right next the passenger side window where I was riding.  I ignored at first but after a while I noticed no cars either behind or infront nor any buildings or airplanes.
I then checked inside the car for any light source and rolled the window down to get a better look.  It was quiet no noise at all and easy to look at not too bright.  I asked my friend whom was driving to look at it and she said she had glanced a while back and not sure what she was seeing either. 
At this point we were not scared just trying to figure it out.  As I continued to be curious and stare at it, it took off straight ahead far enough to see it but moved really fast.  
Then it showed up on my friends side window and she started to freak out right away.  It was only there a minute and then gone.  We did not see where it went it was just gone. We did not see it again.  
It happened a long time ago but we still talk about it and   I just wanted to share it with someone.  Thanks for listening.