I noticed something today that seemed odd. This is Peachtree Street, looking north toward 16th.

What I'm looking at here in particular is the mile marker on that lamppost over on the right. To the best of my knowledge, this part of Peachtree has not been a state highway in at least 75 years (US 19/SR 9 join Peachtree about 2 1/2 blocks downstream). So what is the mile marker referring to? Some other observations:
- One mile north of here would be somewhere between 25th and 26th Streets. This is, as I mentioned, part of SR 9, which would presumably fall under normal GDOT mileposting rules (around milepost 2.2 or so). I see no marker there, though neither do I see one at any of the locations where I would expect SR 9's first three mileposts.
- One mile south of here is roughly the intersection with 5th Street. No mile marker there either.
- There is, however, one thing that does line up:
the course for the Peachtree Road Race, held annually since 1970. Mile marker 5 matches up just about exactly. But it's only posted northbound, and the race course runs southbound, so I'm skeptical even of that. (And I couldn't find a mile marker 6 in the appropriate place on 10th Street, either.)
The mile marker and the lamppost it's mounted on have both been present since at least 2010 per Street View, but don't seem to have been there in 2007 (though as blurry as 2007-era imagery is, it's hard to say).