air quality or traffic statistics gathering…

Is it the only one?
Where is it located?

If it were intended for public use like a fire or police box it would be marked.

Could it be a control panel for traffic lights or walk/don’t walk signs?

Do you hear the sound of water in it? If so, It’s quite possibly a neighborhood station that taps into the water supply for testing. We have many of them around Seattle, and only city water employees have access to them via a lock. They do look a bit different though.

Is it attached to the sidewalk, or just sitting there?

Its a phone.