It looks like a typical segment local to me averages around 600 riders. We have a few 1,500-2,000 within 10 miles of my house. To get up into the ridiculous numbers, you have to get out near the coast-- there are some +15,000 rider segments out there.

The upside/downside is that we have an annual bicycle race here, the Redlands Bicycle Classic-- upside is we get to watch racing every year, downside is there are segments within a mile of my house where guys are doing 30mph up hills. Getting into the top 300 is a challenge.