At the Furlo Pass in the Apennines, the Via Flaminia passes through a tunnel built in 76 - 77 AD during the time of Emperor Vespasian, replacing an earlier tunnel.
The tunnel built during the time of Vespasian. Attrib: By AlMare (Own work) [CC BY-SA 2.5 (https://creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons
This earlier, smaller (first) tunnel is "apparently of the 3rd century BC" according to the Wikipedia articles on Tunnels and the Via Flaminia, with the former article stating this is "one of the first road tunnels" (my highlighting). The article on the Furlo Pass states this smaller tunnel dates "from Etruscan times", as does the article on the Apennine Mountains
Did the Etruscans really build a tunnel here and, if so, is it the first known road tunnel built?
If not, what is the earliest known road tunnel built under or through a natural formation (i.e. not under or through a man-made construction)?
In his answer, Alex makes the valid point that it is not exactly clear what a road is. There are various definitions depending on what source one looks at.
- Merriam-Webster has "an open way for vehicles, persons, and animals"
- Oxford offers "A wide way leading from one place to another, especially one with a specially prepared surface which vehicles can use" (Collins and Cambridge have similar definitions).
- Wikipedia: "A road is a thoroughfare, route, or way on land between two places that has been paved or otherwise improved to allow travel by foot or some form of conveyance".
For the purposes of this (historical) question, any of the aforementioned definitions are acceptable. However, I would exclude tunnels built for mining.