Back when people still gathered, I rode sweep for a 5K charity walk/run, which meant I had to ride slower than the slowest walkers. It got challenging at times, but the speeds were down around or below the speeds being discussed here and never had to unclip (nor did I fall over while clipped in.)

If someone can devise the rules and testing methods .... start a "Sub-3 mph quarter-mile challenge." it would be the only race I could ever win. Make the purse big please .... a good way would be offering all the people who say it is impossible, 2-to-1 returns on their donations if no one won. It's a safe bet.