too much 'it depends' variability. road width, lane count; shoulder or bike lane present or not; number of riders, rural vs urban. Personally, in my experience: safety in numbers: any size group is often safer (from cars) than solo. I feel safer in urban (slower traffic) than I do suburban. Safest in lightly travel rural roads. Two abreast on quiet roads (talking, rotating, etc) but if it's a busy two-lane, it's time to single-up.