About  east of the western terminus, SR 312 passes over the Florida East Coast Railway main line via an overpass. from the western terminus, SR 312 intersects U.S. Route 1 (US 1), with SR 312 continuing a few blocks east through St. Augustine until the Mickler-O'Connell Bridge over the Intracoastal Waterway.
SR 312 traverses eastward from its western terminus, passing over the Florida East Coast Railway main line via an overpass. It then connects with US 1 before continuing east through St. Augustine to the Mickler-O'Connell Bridge over the Intracoastal Waterway.