Californians

[Donger]

During my last trip to California, I noticed that there is some kind of detector (I think) about every mile on the highways. I've searched for a picture of what I'm talking/asking about, but can't find one. Each location seems to be identical, and includes a solar package and a white box thing that is pointing down toward the road. And, no, the white box does not appear to be a camera.

Californian CP'ers, have you seen these and do you know what they are, please?

Thanks.