Tour des Fouilles (18): How to build a road through the mountains
The valley of the Romanche river, in antiquity, looked a little bit different from today. Probably, there was a large lake close to where Le Bourg d'Oisans is now situated: most of the lower valley floor, which now is agricultural land, was inaccessible. This made the valley less attractive for humans, and as a consequence, there were no major roman settlements in the area. Still, it was not empty: like most valleys running in east-west direction, it had its Roman road, and some small settlements were clustered around it, one of which, Catorissium, known from the Peutinger Map, is often associated with Le Bourg d'Oisans, though there is actually little hard evidence found on the spot.
Yet the road is there, and it can still be seen at several places. The most spectacular remains are found a bit further up the valley, close to Bons, where the road had literally to be carved out of the mountains. The so-called 'Porte de Bons' that you see here is what remains of a small tunnel made by the Romans: you can still see the wheel ruts in the pavement, as well as the profile of the tunnel wall. It probably was a short tunnel, but think about it: making it was a major investment - imagine the amount of time and people needed for construction, and the logistics of keeping these people well-fed and well-equipped for their task.
Moreover, given that we are in the high mountains - this probably was not the only place where major and complicated interventions were necessary. From here, the road went all the way up to the Col du Lautaret, from where it descended to Briançon. From Briançon, one could cross the Col de Montgenèvre to Turin. Why did the Romans build this expensive road? Part of the answer is that the road connects Vienne and Lyon with Briançon and Turin, and that it was the shortest connection between Lyon and Italy.
Yet, still, of course, such a mountainous road, is not fit for large-scale transport - if you want to bring large amounts of wine or olive oil from the lower Po Valley to Lyon, you'd probably go all the way by boat. This would take a bit longer, perhaps, but it would be a lot cheaper. Yet, for information, or for small and more expensive stuff, this is much less true, and road transport can become a competitive alternative - it is probably a bit quicker - especially if you have a horse. Indeed, as the wheel ruts that you can see on the above picture indicate, a lot of the traffic consisted of cars or carts.
For the Roman state - who after all built and maintained this Road - probably it is the information aspect that was decisive: a good connection between Lyon and Italy meant that information needed on both sides of the Alps arrived quickly where it needed to be. As Lyon played a crucial role in communication with Roman Europe, this road may have been a lifeline for the Roman government. That it subsequently also made trade a bit easier, was a consequence - but probably not an aim.