Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
Leaving out of view the coast route, which Hannibal abstained from taking not because the Romans barred it, but because it would have led him away from his destination, there were only two routes of note leading across the Alps from Gaul to Italy in ancient times:(3) the pass of the Cottian Alps (Mont Genevre) leading into the territory of the Taurini (by Susa or Fenestrelles to Turin), and that of the Graian Alps (the Little St. Bernard) leading into the territory of the Salassi (to Aosta and Ivrea).
3. It was not till the middle ages that the route by Mont Cenis became a military road. The eastern passes, such as that over the Poenine Alps or the Great St. Bernard--which, moreover, was only converted into a military road by Caesar and Augustus--are, of course, in this case out of the question.
The former route is the shorter; but, after leaving the valley of the Rhone, it passes by the impracticable and unfruitful river-valleys of the Drac, the Romanche, and the upper Durance, through a difficult and poor mountain country, and requires at least a seven or eight days' mountain march. A military road was first constructed there by Pompeius, to furnish a shorter communication between the provinces of Cisalpine and Transalpine Gaul.
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Reference address : https://www.ellopos.net/elpenor/rome/3-04-hamilcar-hannibal.asp?pg=45