Pelotons - the race variety of a paceline - can only be safely formed with road closures because they typically swell to fill at least one lane of the road. They are not just 1 rider behind another, but often 2-4 riders wide.

Pacelines for group rides and the like can be formed whenever, and they do not form just because of a crosswind. When I used to train with a group, we would almost always form pacelines in single file, on every ride.