I missed my chance this year at the only place I have seen them over the years. French Prairie rest area, south bound I-5, Oregon.
I don't ever remember seeing any when I grew up in upstate New York.
I know that in NH, they plant native wildflowers along the middle and sides of the freeways. This may have been the case going across NY state too, as they were right along the freeway.
In the case of the ones I saw, they were randomly spaced in a little groove of trees. There are some photos of them in my gallery.
We call it Japanese Knotweed - same stuff - pretty flower and all, but what the hell were they thinking planting a non-native plant like that?? Oh wait! They weren't thinking! Now we can't get rid of it. The Crown Vetch is much better. And since the Vetch likes it open and sunny, it doesn't invade our forests! Like you said, they add nitrogen back into the soil to repair where it was damaged during road building.
In NY, we had a neighbor across the road wage a three year scorched earth campaign to get rid of a patch of it. No, really! Old motor oil and fire! My mother was not happy, she blamed the rats in our garage on him destroying there habitat.
Crowned Vetch is a better fit, for sure!
Yeah, there are a few towns here that have used the scorched earth method to eradicate the stuff. Still takes a few years, though, to work. LOL Rats in the basement! That's actually kinda funny.