Actually, remembering this common gripe I made a special note to observe group dynamics at the event I was at on Saturday. It was a huge group, probably pushing 100 cyclists in convoy.
From my position at the back (
) it looked like just a complete mess of bikes everywhere, taking up the whole road; 3, 4, 5 abreast, just mucking in anywhere.
But as I took the opportunity for closer look, moving forwards and backwards through the group, I realised that in fact there were very few people riding 3 abreast, if any. In most cases they were riding two abreast, the odd time a 3 abreast might occur when someone was falling back or overtaking.
However, the reason that they looked like 3 or 4 abreast was because the line wasn't neat. In some cases the guys were cycling right beside eachother, in other cases they were riding with 1.5 metres between them, effectively hogging the whole lane.
From behind, this looked like 3 or 4 abreast, when in basically all cases it was 2 abreast. In fact from what I could tell, people were actively avoiding riding 3 abreast - not because they thought they should, but because riding with 3 bikes in a 2.5 metre wide lane at 35km/h is quite uncomfortable with little room for error. Any time a rider turned a twosome into a threesome, someone would move forward or back to give themselves more room, and the three-abreast position was abandoned.