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Context would help. General location (state, country, etc)? What kind of road? Were you near any other car infrastructure? Are the directionals pointing at anything informative?
I suspect the box facing down is some kind of short range antenna. Like for reading toll passes kind of thing, though I doubt this is that. The directionals first reminded me of like IR sensors for like emergency vehicle signalling, but you'd normally see that on a traffic light.
Can't find an exact one, but here's an interesting example of a traffic detector:
And in the road there would be a buried paired receiver. And it would detect if a vehicle blocked the signal between the two.
Oooh this setup just got installed on a highway in my town, I've been wondering what it was since it was installed just past one of the vehicle counting strips they use
it was on a street light after an intersection, in California. from the comments it would make sense to be for traffic control. My original thought was a gunshot detector since I remember hearing they installed them in the city but doesnt seem that this is it