Streetlights are usually white, which means there’s a phosphor, which drastically limits the modulation frequency. Also, the lights themselves (at least where I live) are owned by the city or PG&E. Good luck getting them to cooperate that deeply with carriers. Also, the lights are off in the daytime.

That being said, in the absence of fog and heavy rain, optical communication between light poles isn’t that crazy.

Was thinking mostly of messaging for the lights themselves, telling lights to tun on or off, checking that they are on or off and also that they aren't misbehaving, that sort of thing, so relatively slow modulation would probably be fine.

I’m pretty sure OP was referencing streetlights for how dense you need your base stations for millimeter wave communication networks.