I just give them enough room, and go on my way. If I don't know them, there's no good way to show support from a car. If I do, I'll honk after i'm a good distance away.
I've considered putting a SAG sticker on the back of my car, cause a couple times i've seen people with a flat or whatever along the road, and think about stopping, but then conclude that that'll just freak them out.