When riding the ACA USA Pacific Coast ride, I ended up rider with another rider whose average speed was the same as mine, but he was faster climbing, and I was much faster descending and faster on the flats. On long hills, greater than 10 Km, Jim would wait at the top for me, then I would descend fast, and back pedal on the flat until he caught up. On short, less than three km climbs, Jim would keep going and I would pass him at 15 to 20 km/hr faster than him on the descent giving him 1.5 m clearance minimum, and normal 2.5 m, as he could not hear my Ring/Ring bell, OR me yelling out "Passing on the left", as load as I could, until I was just passing him.
Anything less than 1.5 m, and Jim was surprised when I passed him.
It was good to be able to ask question of a Passee to find out what was the most effective warning method, and lots of side distance, OR slowly right down to his speed, warning him, then accelerating past him, were the only effective methods for me on a tour with 13 riders.
On large tours with more than 2,000 riders, I would only pass if I could safely pass on the other side of the road, as many riders within a pelaton would suddenly pull out, to pass, without checking behind them.
I think a prerecorded "Passing on the left" horn of variable volume would be the ultimate!!!