It's the Monday Night Ride out of Mueller Park. Some Mondays (in addition to a number of smaller, slower groups) will attract a peloton of 50 or 60 riders. There aren't any rules and some riders have been known to cross the center line and do all manner of other things that I don't approve of. Fortunately, my wimpyness prohibits me from riding with that bunch so it's not a big problem for me. If you're not used to riding in close quarters, slower riders can become unnerved when they brush your elbows as they swoop past with no warning.