Maybe I’m just being pedantic (likelihood is high), but I’m reasonably confident while it is true that when you’re moving faster, you move faster into the space someone else used to be. I have to question whether the space they used to be is in the same state as it was when they were there.
Taking an easy factor into consideration, say an angled crosswind, where does the exhalate go from the front rider? I would argue that there’s a non-zero possibility that most of it is not directly behind them. With the possibility directly proportional to the strength of the crosswind. If there’s a straight tailwind, or headwind, obviously the modeling gets more complicated. Clearly you do not want to be in their “pocket”, but as anyone who has drafted knows, when the wind gets squirrely, the pocket follows.
Making it even more complex, and well beyond my ability to understand, is the way air swirls around after it detaches from your blunt body. We aren’t teardrops, so there are significant eddy currents created when we break through the air. I don’t have the tools, or training, to model how spittle flows, drops, or attaches to these currents, but I can’t imagine that it just sits there like a cloud of gnats waiting patiently for me to pass through.
Finally, there’s gravity. As anyone who’s taken basic physics knows. All objects fall at the same speed; 9.8m/s. So unless you’re very, very tall, that snot rocket will hit the floor well under a second after you fire it. I don’t want to double check the author’s math here, so let’s just roll with their in-the-lead-rider’s-airspace in 1.3s @ 20kph number… Hmm…
Now, just to be 100% clear, I don’t think it’s safe, or smart, to ride in a group. I also don’t think it’s safe, or smart, to linger in anyone who isn’t in your quarantine group’s airspace, however we calculate that, but I also can’t help but think that this is unnecessarily alarmist, and the guidance suspect.
The issue is that if you’re out on the road and riding fairly close behind another cyclist, you’ll quickly arrive in the airspace that they’ve just vacated. Even if you’re more than two metres behind them, you could be in that exclusion zone just a fraction of a second later.