Hadrian's arch was built to honor the Emperor Hadrian and likely to celebrate the consecration of the Temple of Olympian Zeus abutting.
This is well worth the the visit.
Its unique as still defining, a thousand odd years later, an ancient stretch of road allowed this tiny bit of ancient Athens' remaining open land.
The busy 9 springs of Kaliroi was also a few meters further on. Courtesy of Tyrant Piesistratos who set the stage for so much of todays western civilization.
Both residents and visitiors need to exercise care crossing the busy 3 way interchange. Those unable to scramble are better off getting on the proper side of the road from Constitution square with its cross walks and lights. The side with the National Gardens on it.
Etched in stone it states the arch separated the original classical section of Athens from the new construction inaugurated by the Roman Emperor Hadrian, of the newly conceived Roman quarter, although these boundaries are hard to define.
There is an inscription to this effect on the arch itself comparing Hadrian's Athens to the Athens of Theseus, the cities legendary founder.
Hadrian was Grecophile.
He liked Greece and spent a lot of money on public works which is why the Greeks of today still revere and respect him and even have a quite a large street named after him in the Plaka called Adrianou.