The road soon became a route of European importance, complete with viaducts and embankments at its most critical points. Near the Abbey of Petra Petrusa there is still a finely built viaduct while a little further on, in the Furlo Gorge, the remarkable supporting walls of the road can still be seen.
Subsequent improvements to the Flaminian Way were made under the rule of Emperor Vespasian between 70 and 76 AD, when a new tunnel was cut through the gorge to ease passage through the gorge.
Furlo has four tunnels, each of which represents an important step in Italian road history.
The first tunnel, dating from the Umbro-Etruscan period, was built in 400 BC out of a need to develop trade links throughout the area. This tunnel was 8 metres long, 3.3 metres wide and 4.4 metres high. The technique employed involved heating the rock with fire and then immediately cooling it with water or vinegar before working it with a chisel and removing the rock that had been chipped away. It probably took around two years to complete the task but this was an important breakthrough for the area. From then onwards, it was possible to pass through the gorge even when the waters of the River Candigliano were high.
Later, it was Emperor Vespasian, remembering the difficulties experienced by his army while passing through the gorge, who decided to make the route less onerous by excavating a new tunnel. Its construction took six years and thousands of slaves were employed in the task. On its completion the tunnel measured 38.3 metres in length, was 5.4 metres wide and 4.8 metres high.
In the 1980s, the Italian State road authority began a new project for the creation of two parallel tunnels through the mountain, bypassing the gorge. The first of these was opened to traffic in 1985, the second in 1991.
The Flaminian Way constitutes a true open-air museum, with bridges, tunnels, road side columns, inscriptions, archaeological sites and drainage systems etc..
The route begins at Fano (Fanum Fortunae). With its Arch of Augustus (9-10 AD), this city retains some of the best conserved Roman remains along the route of the ancient road. At Tavernelle, near Serrungarina, various finds have come to light, including bowls, amphorae, coins and a rare marble head of the god Attis, which tend to indicate the existence of an ancient halting post here. Here also are the remains of an ancient sacred shrine.
Further on towards Fossombrone, at San Martino del Piano, is the archaeological site of the ancient town of Forum Sempronii. A short distance away we find a domus (or villa) with the remains of a Roman bath. Many objects from these sites can now be seen at the Museo Civico at the palace of Corte Alta in Fossombrone. A little further along the road, at Calmazzo, are two cippi (inscribed pillars) in an enclosed tomb area from the Roman period in memory of the Cissonia family. After Calmazzo we arrive at the magnificent Furlo Gorge, with its tunnel built by the Emperor Vespasian (76 AD) which is still open to traffic.
After the gorge, in the district of Acqualagna, we arrive at the ancient abbey of San Vincenzo. Just by it is a viaduct built with stone from Furlo during the Roman Republican period to protect the Flaminian Way when the River Candigliano was in flood.
At Cagli (the ancient city of Cale) we find Ponte Mallio, a bridge built at the end of the Roman Republican period which once crossed the River Bosso. Two more bridges lie a little further on, near Cantiano. At Pontericcioli along an ancient stretch running parallel to the modern road, we find a bridge known as Ponte Grosso, dating back to the Augustan period, with two arches separated by a water divider, as well as numerous other Roman remains. A second bridge by the same name still takes traffic across the River Burano over two arches, each around 7 metres wide, with a central pier and water divider.