Time for a nonmigraine-related visual disturbance.
The background condition. I am extremely nearsighted, especially in my left eye. If I don't wear glasses, I can see colors and that's it for normal distances, though I can read if I hold the print within three inches of one eye and shut the other. Even with glasses, I see halos around light sources. I have only been driving a car for two years, and I don't do it very often. (Boring and stressful at the same time - does it get any worse?)
Last year, my husband and I drove down to the Gulf Coast for birdwatching. I did most of the driving so as to get experience. Driving out and back on the coastal plain, I kept seeing sheet lightning flicker to the right, but the sky was clear, I never heard any thunder, and it wasn't nearly hot enough for heat lightning. When I asked my husband about it, he could not see it. This only happened at highway speeds and only when I was driving, not when he drove and not driving from birding spot to birding spot. I decided it was an optical illusion.
Between last summer and last week I haven't seen the lightning, so when I went to the optometrist for my checkup I forgot to ask about this effect. Thursday (before my new glasses came; however, my right eye prescription did not change, so I presume it wouldn't have made a difference) I had to drive down to Houston, which is also on the Gulf Coastal Plain. It was overcast and we kept driving in and out of rain, so at first I thought the sheet lightning effect of to the right was real, but my companion couldn't see it and sure enough, as soon as we got into Houston I couldn't either.
So - any ideas? Nobody else I know gets it, which makes me think it must be related to my eyesight problems, but I can't convey to you how weird it is for my right eye to be the culprit and not my troublesome left; and I'm puzzled as to why it only starts up once I get down off the Balcones Escarpment onto the coastal plain - and I mean right down onto it; I've driven to my sister's on the highway through prairie that's plenty flat for most purposes and never gotten the sheet lightning. I tend to drive right at the speed limit or a little lower (65-70 mph most places) and stick to the right lane (US rule is slower traffic keeps to the right; remember that the driver's side is on the left here), so any physical objects contributing to the lightning effect would have to be stationary, not other vehicles. Tall objects you pass on Texas highways are trees, utility poles, oil derricks, and cell towers, none of which I am likely to notice much when driving.
I don't know whether it would happen at night because I haven't done any prolonged night driving yet.