Organizer David Barstow Robinson exhaustively explained the Ladd’s 500 in the Facebook event:

It’s not a race, it’s a relay

500 laps of Ladd Circle is a century. Form teams of at least 2, switch riders at least 10 times during your laps, and turn left for a while. If you want to run it solo, bring two bikes so you can make your switches. Riders are responsible for accurately counting their own laps. Any bike is acceptable, but weirder is funnier. If you’re not riding, let’s party in the middle.