The bulk of information we have to go on about Hannibal's route is a pair of histories run through the wringer of multiple translations and copying as well as some slim fragments here and there in other sources. Archeological evidence is rare and suspect.

The entire controversy would disappear in an instant if, one day, an aerial survey spotted a couple hundred Carthaginian corpses tucked in a ravine. Not only would it confirm the route, it would also provide extensive information about Carthaginian armor, weaponry, and equipment. I suspect it will eventually happen, much the way the Neolithic hunter was found by Alpine hikers several years back. Until then, it's ancient accounts, geographic matching, and a dose of intentions.

The two main histories come from Polybios and Livius, and unfortunately they do not agree. Polybios (born 203BC, died 120BC) wrote his history relatively soon after the war (160-140 B.C.E.) and used Roman eyewitness and access to Greek sources from Carthage as well. It is believed that his is the more accurate account because of his command of time, location, and being versed in the writings of Greek historiography. Livius was an annalist. Nevertheless he used many sources also. It is apparent that Livius used Polybios as a source for his histories, as well as Q. Fabius Pictor's "History of the Punic War." One cannot dismiss Fabius that lightly. He took part in the war as a legatus and was a member of the Senate, so he was privy to the Senate's meetings during the war. While Fabius would be pro Roman, his books would be invaluable to an annalist writing about the Punic War.

However, between the two surviving works there remains indecision. Indeed to exasperate the situation, there is a fairly long passage in Livius that shows the controversy of which route and pass existed even when Livius was writing--between 29BC and 17 AD. "I cannot understand why there should be any doubt about the route he [Hannibal] followed over the Alps." [Livius XXI, 38]

But within each history are the clues of the journey--assuming, and this is a big assumption--that the translations are correct. With that, one can start to piece together at least a theory, as many historians have done over the centuries, about timing and distance. The trick is to figure out which information to use from each history, and it is from here that opinions diverge.

Distance and Timing

Polybios notes that the distance from the Ebro River to Emporaie, a city in the foothills of the Pyrenees was 1600 stades (about 280km). From Emporaie to the crossing of the Rhone River is another 1600 stades. "From the crossing of the Rhone, if one follows the bank of the river upstream as far as the foot of the Pass from the Alps into Italy, the distance in 1400 stades (about 245km). The length of the pass which Hannibal was to cross to bring him down into the plain of the Po is about 1200 stades (about 211 km)." [Polybios III, 39] That gives a reference point for the start of the main march into the territory.

Now timing. Polybios says from the Rhone crossing to the "Island" was a four day march. Here, he was "joining forces with this prince [Brancus] and driving out his rival."

At this point, routes vary. Polybios says Hannibal's army marched 10 days along an unnamed river, covering about 140km, to a spot described as the ascent of the Alps.

Livius says Hannibal "turned left to the territory of the Tricastini, proceeding thence past the borders of the Vocontii, to the Tricorii and finding nothing to stop him until he reached the River Druentia...From the Druentia Hannibal marched towards the Alps mainly through open country, and reached the foothills [of the Alps]..." (Book XXI, 31, page 55). However, Livius attached no tally of days to this account.

Both Polybios and Livius agree that the crossing of the Alps took 15 days.

Polybios now goes into detail about the timing. Hannibal pitched camp in order to send a unit of Gauls to scout the first defile. He then "discovered that it was the enemy's habit to remain under arms in their positions and guard them carefully during the daytime, but to withdraw at night to a neighboring town. So Hannibal revised his plans in light of this report and devised the following stratagem. He advanced with his whole army quite openly, and when he approached the part of the road where further movement would be threatened, he pitched camp only a short distance from the enemy. As soon as it was dark ... he led a force and captured the now unguarded defile." (Polybios Book III, 50)

At daybreak, Hannibal fought and routed the Allobroges, ultimately suffering heavy losses in the battle, but also taking their town and its provisions. Here, he rested for one day.

Livius recounts the same tale, but inserts a day while the Gauls explore. However, he does not mention a day of rest after capturing the town. So the times theoretically match.

Both agree that in the next three days, the Carthaginians marched in peace, and on the fourth day met with tribal elders who gave promises of peace, guides, and hostages to ensure good conduct.

Polybios notes that for two days the army marched as though at peace under the direction of native guides, but were then ambushed by the treacherous tribe(s). Livius doesn't say how long, but agrees with Polybios that after the ambush, Hannibal was forced to camp for one night with only half his army at a big rock. The next morning, the attackers withdrew and he reassembled his army to march, with some skirmishing that day. Here Polybios' narrative says "On the ninth day of his march, Hannibal reached the top of the pass." Note that if you count the days, this would be the ninth day in Polybios' narrative. Livius's account also tallies nine days. 

At the Pass

At the pass, Polybios notes Hannibal "pitched camp and halted for two days and rest ... by this date it was nearing the time of the setting of the Pleiades." Livius agrees with the two days, but says "it was the season of the setting of the Pleiades (Ed. note: in 218 this was late October) --winter was near and it began to snow." [Livius, XXI 35].

The next day, the Carthaginians started to descend, but ran into the area of landslide and the impassable path, and then came the fruitless attempt to detour around it. It took a day for the path to be wide enough for horses and mules. Back to Polybios: "Then he took the Numidians and set them in relays to the work of building up the path. After three days of this toilsome effort, he succeeded in getting his elephants across ... After he had reassembled all his forces, Hannibal resumed the descent and three days after leaving the precipice I have just described, he arrived on the plains" [Polybios III, 55-56]

Livius agrees and uses the word "precipice" to describe the landslide area. Here, some of the numbers seem off. If Hannibal reached the pass on the ninth day, then day 10 and l 1 were days of rest. Day 12 he started to descend but reached the landslide and could find no alternate route around. Day 13 was spent fixing the path for horses and mules. Day 14 and 15 were spent fixing the path for elephants (assuming Polybios' "After three days of this toilsome effort" includes that first day). Day 16, 17, and 18 completed the descent to the plains of Italy. Note that it would be 19 days if the precipice required three additional days to clear a path for the elephants besides the one day for the horses and mules.

Livius notes that "four days were spent in the neighborhood of this precipice." The troops were "given three days rest to recover from the fatigue of their road building. Thence the descent was continued to the plains" [Livius XXI 37].

Polybios says Hannibal's army descended into the territory of the Insubres. Livius says it was the territory of the Taurini. They cannot agree on the pass used .

Hannibal Crosses the Alps A Route Examined and a Proposed Alternate Route

Introduction
The Sources
Hannibal's March: Polybios and Livy
Polybios vs. Livy: Debate Over Details
A New Route Proposed: Begin at the End
A New Route Proposed: Start at the Beginning
A New Route Proposed: Where is the Druentia?
A New Route Proposed: The Pass in the Alps
Aftermath, Conclusion, and Bibliography