Nah - 520 starts on maybe the bridge, or on the west end of it on the causeway. Streets on the island are under the control of the JIA. This is a sign error (or at least a cludge meant to say "To West 520"
(I grew up on Jekyll in the 80s and 90s and my parents are still there. You can find lots of old maps that show 520's predecessor GA 50 assigned to the causeway and then in a loop around the island itself on Beachview and Riverview. No evidence that it was ever signed as such, though.)
