This section near the beginning of the Iter presents several problems (which is the reason that it’s been left until last). Venta Silurum is Caerwent, Aquae Solis is Bath; in between are the two stations Abone and Traiectus.
Looking at the other stages on this Iter, it seems that the Itinerary‘s distances are fairly accurate, especially on the shorter stages and where the route of a Roman road is clear or can be conjectured.
Both Venta and Aquae Solis were Roman towns. Their identification with Caerwent and Bath is certain, the distance between the two being 14 + 9 + 6: 29 mpm or approximately 43 km. But where would the Severn Estuary have been crossed (and why does the Itinerary make no mention of a crossing which would have been at least a mile by ferry)?
Theoretically, the crossing could be made in three places: the shortest would be the ‘Old Passage’ between Beachley and Aust, about 1.7 km (how did they measure a sea or river crossing in paces?). The drawback here is that the Itinerary gives the distance between Venta Silurum and Abone as 14 mpm (about 21 km). But if Abone is Sea Mills, the distance via Aust – even as the crow flies – would be nearer 27 km, which suggests an actual road (and ferry) route of nearer 30 kms.
The second possible crossing would be the ‘New Passage’: from Caerwent it is a mere 5 km down to Sudbrook on the estuary (where there was a Roman camp), and a 3 km crossing to Redwick. It would be, as the crow flies , about 21 km in total to Sea Mills, which makes the whole distance to Bath about 43 kms. Exact, though the road route down from Redwick to Sea Mills is not clear. And good luck to the ferryman because that entailed crossing by the treacherous narrow channel known as The Shoots, thus described:
“Sailing through The Shoots in the days before powerful engines was especially dangerous, and called for very high level of seamanship. The area is fringed by rocks on each side of the river and known for tricky whirlpools.”
The third possibility would have been a considerably longer crossing from Sudbrook, straight to the mouth of the Avon, and then upriver directly to land at the port at Sea Mills, portus Abonae. That would also be about 43-44 kms to Bath and there is a certain logic to arriving at a port by boat. Evidence of an important port has recently been found at Caerleon (Isca Silurum), where Iter XIV started. From Caerleon where would boats sail to?
There appears to be no incontrovertible evidence that Sea Mills was Abone/Abona/Abonae, other than that there was a Roman settlement there, on the Avon, dating from a very early period; and the identification seems to have been accepted as a given. So we have arrived at Portus Abonae.