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The small tunnel is irregularly shaped and let just one chariot at a time to pass. Many elements suggest that it was an emergency intervention being located almost on the edge of the cliff. This reduced and probably precarious tunnel constitutes a kind of "by-pass", which provides a narrow emergency passage to the Flaminia to circumvent the steep ridge of the cliff.
The passage is about 8 m long and the width is an average of 3.30 m. The entrance from the east reaches a maximum width of 3.90 m, while the eastern one is smaller, about 3 m. The highest point is 4.45 m high, but in correspondence of the passage to the east is close to 4 m. The same irregularities detected in the measurements of the peculiar elements of the tunnel can also be noticed at the starting point of the vault. On the rocky floor, the parallel grooves formed by the wheels of the chariots are still recognizable, but, being not that deep, they are due to a short period of usage.
There are no distinguishing features to formulate a dating hypothesis of the small tunnel. According to local tradition it was the work of «Umbri o altri popoli primitivi». This fanciful attribution (there is mention of "Etruscan" too) is based on a misinterpretation of a passage of Procopius.
People believe that if the Byzantine historian would have like to attribute the construction of the gallery to the Romans, they would have named it explicitly. Procopius describes events of the 538 a.D., that is 5 centuries after the realization of the gallery. Probably he did not refer to the inscription over the two entrances of the great gallery because they were already so rugged, or hidden by the ruins of the fortifications built by the Goths to bar the passage.
The time of the small tunnel opening has to be chronologically placed between the building of the adjacent massive substructure, implemented in the Augustan age, and the realization of the large gallery, open at the time of Vespasian.