It won't always fly so low. I'd bet the wind direction was not favorable to gain height. But, it wanted to move so it began to travel lower. I've seen them follow the major roadways here. A very good book is "Chasing Monarchs: Migrating with the Butterflies of Passage" By Robert Michael Pyle He talks about them following rivers, too.http://books.google.com/books?id=gO9eXQ ... q=&f=false
Parts of it are on google books. Pyle is an incredibe writter.
I haven't seen them, yet. But, I am hoping soon they will be here following the roadways and crossing the intersections. I hold my breathe when I am stopped at a light and have to watch them try to nagivate the intersection--especially, when semi trucks roar through the intersection.