Ampere was the Newton of Electricity. ~ James C. Maxwell
Every day on my ride home, I start the climb up the Altamont Pass, and if I look up through the open sunroof at the right moment, i see these little orangish balls on the high power lines. Of course, they are for aviators to see, and seem to be made by, among other companies, the Tana Company (award for ugliest web site).
These are cheaper, I suspect, than actually using conductor marking lights, those little strobes you see at night (picture right). Maybe those are only for the towers themselves, not the spans.