On further topo map research, it appears that this was likely a rail bridge that was later converted to a road bridge before being abandoned completely; when the forest area was owned by the military, a short distance railroad was plotted across this point.
So, apparently, it was both at different times.
This bridge does have a reinforced bar in front of it. That's what actually does the "can opener" effect as locals call it; the truck tops get sheared off by the bar, not the bridge.
The protection bar has a clearance of 11 feet, 8 inches; the bridge itself if unprotected would have clearance of approximately 11 feet, 10 inches, but it would be *extremely* vulnerable to damage. The road is one-way under the bridge, and there is not a protection bar in the other direction.
Of note is that Ponce de Leon Ave / GA 8 / US 78 / US 23 / US 29 (this road carries a lot of routes) is a four lane arterial road, but the outer two lanes have clearance of less than 10 feet under the bridge. The bridge still gets hit regularly by trucks, and the concrete reinforcement has to be patched up periodically.
Part of the problem with these bridges is that it's not always easy to fix the issue. Railroad bridges can't change height in a short distance, so without a couple miles of rail height change, the only other option is lowering the road. Given drainage and other infrastructure, this is not always an option either.
This was definitely a road bridge. The road involved isn't named, but shows up at this location on the USGS 250k:1 topo map for Rome, GA in 1955, 1958, and 1961, and even still in the 24k:1 quadrangle for Matt, GA for 1997 (historicalmaps.arcgis.com). This is the same road that connects to North Gate Rd/Sweetwater Rd on the north side of the river.
It's likely that this was originally planned to be part of, or planned to be replaced for, Georgia Highway 318 (now defunct).
GA 318 ran along what is now Dawson Forest Road in the southern area of the forest up until just short of that bridge, visible on the Matt, GA quadrangle. The original road going over this bridge took a little more meandering path than the one currently called Clark Rd. It never officially extended northward onto North Gate Road on the Georgia highway system maps, and that portion of it was decommissioned by 1972. (The number itself was dropped in the mid-1980s.)
Other evidence of association: Part of North Gate Rd in the middle of the forest is paved, with asphalt that looks to be about 1960s in age. Strangely, this segment is not connected to any other paved road. It's likely the Highway 318 project was abandoned at some point, as the current dirt part of North Gate Road meanders a lot more than would be expected for a state highway of the time.
Background: I tried hard to find out when this closed to vehicular traffic, but cannot nail down the actual year. I drove across this many times in 1998-1999 (the first two years I lived inside the city of Atlanta). I can find evidence that it was closed by 2003 and derelict with weeds growing out of it by mid-2004.
Its status is still unclear as of this writing, as it is sometimes closed even to pedestrian traffic, and then quietly reopened again.
Orange temp sign said "bridge out" a few weeks ago as I was going past the west end of CR 290. I'll visit up close soon for status and update appropriately.