I don’t have epilepsy, but I do get migraines, and what you described is a perfect trigger for me.

Best way to avoid it is don’t drive when the sun is low in the sky or on roads lined with very tall trees. If for some reason I was trapped in such a situation, which is not really likely since it’s very predictable if it’s something you’re interested in avoiding, I would probably slow down, find somewhere to pull over, and just wait it out. But yeah, it is bad. I don’t do it if I can at all help it, because I avoid getting into that situation like the plague.