Ball of Light

Last night I was driving home around 11pm along the shores of the river Tay between Newport and Newburgh. It was just starting to rain in big spots. Shortly after it would turn into a torrential downpour.
Anyway, as the big spots were falling, I saw a white light drift swiftly down the hill on my left, cross the road about 100yds in front of me and continued down towards the river. It looked egg shaped and roughly the size of a car wheel. The area was all fields. I lost sight of it as I reached the spot it had crossed the road.
Was it ball lighting, a ghost, or did I fall asleep at the wheel and like all the exciting things in my life, just a dream?

http://en.wikipedia.org/wiki/Ball_lightning