I’ve got a (well rehearsed ) drill now, when I’m leading a ride with lots of riders on it. If it’s a single track road, no more than 5 riders per ‘pack’ all in single file, at least 3 car lengths between ‘packs’. If it’s on a normal two way road, and traffic approaches from behind, the customary “car up” call is given ( for traffic from behind) or “car down” for anything approaching from in front, then the riders form into two abreast formation, to ‘shorten the bus’. No more than 10 riders per ‘pack’. This also makes the drivers minds up for them, as they have to treat us like a car, we’ve taken any doubt away ( from the sane, mentally competent ones ). It takes a bit of time for the penny to drop, about how this actually works in practice, but sometimes, motorists do actually ‘get it’.