I had a similar experience last summer trying to catch some guy in jeans on a mountain bike. He was a couple hundred yards up on a loud, busy road when I spotted him - I'm not really a cat 6 racer but still I thought that here was a guy that even I could take. The area where this happened is a nice flat where I can maintain 21-22 mph for a couple of miles, depending on wind conditions. Each time I looked up he was the same distance up ahead. I kept thinking, "I should be gaining on this guy!" Eventually he turned off. My turnoff was on the next street and sure enough not long after I see the guy buzzing (literally) on a side street, powered along by a little motor from a weed whacker or a chainsaw or something.