This is the Big Green Road Sign of its era — a humble highway milestone.
When the National Road was new in the early 1830s, milestones were posted in several states. You’ll still find lots of old milestones along the road in Maryland and Ohio. I’m not aware of any in Pennsylvania and West Virginia, but that doesn’t mean there aren’t any. Illinois has none.
I don’t know how many milestones were posted in Indiana, but two remain. This is one of them. It’s west of Richmond on the north side of the road, in someone’s front yard. It reads: SL 9M, R 4½, C 1 — that is, State Line 9 miles, Richmond 4½ miles, and Centerville 1 mile. You have to just know that the first two are to the east, and the last is to the west.
The other milestone is three miles west of Centerville, also on the north side of the road in someone’s yard. I looked for it but didn’t find it as I bicycled by on my Ride Across Indiana. I found it on a 2009 road trip, however; see both milestones here.