Single or double?

It's much faster for cars to pass a group of cyclists that is riding two abreast than in a long single file.... And it is safer for the cyclists, too, as doubling up really allows you to control the lane when it is too narrow to allow cars to squeeze past without changing lane (and you are more visible and predictable to drivers, unlike folks at the right edge of the lane who are liable to swerve into the lane/traffic to avoid stuff like broken glass, gravels, pedestrians, etc).