|Radical City did not originate from fanon, and more information can be found on their respective wiki(s). You may read more about it on the following wiki(s):
Radical City is a location in the Sonic series. It made its first appearance in Sonic R.
Radical City makes another appearance in Sonic Heroes 2. It is a racecourse which spreads across several zones, and of the five racecourses, is considered to be the hardest due to its sheer length; it goes through Metropolis, Oil Ocean Zone, north-east Aquatic Ruins, Casinopolis, south-west Marble Garden Zone, an otherwise impassable part of Mystic Ruins South and back into Metropolis.
In Sonic Heroes 2, Radical City is the only racecourse that is at any point played without Team Babylon; Team Rose chases Team Archaic right the way through it.