I am not just speaking out of my ass. I have researched this. The larger the group the greater the time advantage to passing side by side cyclists, but the advantage applies to passing 3 side by side as opposed to strung out. This obviously assumes the motorist has to enter the oncoming lane to safely pass the cyclist. Where there's a useable shoulder or a lane wide enough for a motorist to safely share with the cyclist, then single file is best for the motorist.
https://www.youtube.com/watch?v=8NGdQDEkWCESure, except that in the instance with the riders two up, the motorist was way too damned close while following them. If two had tangled and gone down, the car didn't have enough room to avoid running over them. So car should have started a couple seconds further away. So now it's 18 seconds vs 14.
Secondly, here in the US, I'm aware of no laws requiring the motor vehicle to move completely to the other lane. CA requires 3 feet between car and bike, other states require only 2. So there's more time made up not moving fully over into the other lane. Tons of places here where you don't even need to cross the centerline, and yet leave plenty of room to pass a group of single file cyclists.
Thirdly, what does the motorist do when something appears suddenly on the left. (or from the right in the English video)? Quite possibly swerves to the right. Where there is now less room for error because the riders are three up. Again, riders potentially getting mangled. Oh sure the motorist may, or may not face civil penalties. I hope the families of those lost take comfort in that. Best remember there's a jury in that civil action. And those people on the jury? They're made up of folks just like the ones found in this thread. So good luck with that massive lawsuit. Oh, and BTW, if you're not riding your bicycle "as close as practicable to the curb or right hand edge of the roadway", well here in CA, that's illegal. So, super dooper good luck with that lawsuit...
And last, there's the emotional/psychological effect that three riders abreast, blocking the entire lane has, vs those same three riding single file. The former incites the kind of anger found here in the thread. The latter, not nearly as much. Especially if this single file group is entirely within the shoulder.
Stay safe out there.
