C. The Date of the via Egnatia
At some point after the installation of a permanent Roman military presence in Macedonia, the via Egnatia was built, on a route that had been in use for centuries.[21] The construction of a great road is a major undertaking, and therefore it has bearing on the history of Rome's strategic commitments in the East.
Strabo tells us that the via Egnatia was
from Apollonia and Epidamnus to Cypsela on the Hebrus River in Thrace, and he reports a distance of 535 Roman miles, which he then proceeds to convert to stades using Polybius's rule of stades to the mile.[22] Another passage appears to indicate that Polybius himself convened this figure in miles for the distance from Apollonia to Cypsela.[23] Strabo further cites Polybius for a figure in miles of the distance from Apollonia and Epidamnus to Salonica.[24] It is surely very likely in view of these passages that Polybius gave distances in miles along the via Egnatia from its Adriatic terminus to Cypsela, but no farther, since Strabo seems to have from him no figures for more distant points. Although the argument falls short of proof, it is most probable that the road was built and marked out between Apollonia and Epidamnus and the Hebrus already in Polybius's lifetime, that is, before ca. 118.[25] We may probably presume that at the time of its construction the area of Rome's authority along the coast at least did not extend much beyond the Hebrus.[26]
A milestone recently uncovered near Salonica bearing the name Cn. Egnati<us> C.f. and the title 
in fine second-century lettering removes the doubt, occasionally voiced, whether the road was built by a proconsul who gave it its name.[27] Egnatius was very probably the witness to the senatus consultum concerning the Ambracian-Athamanian land dispute from around the middle of the second century.[28] His proconsulship in Macedonia cannot be dated precisely, but it would be hard to insert him into the crowded fasti of the 140s; thus Walbank's date for the road (the 140s) is unlikely.[29] Two other great viae publicae into the outer stretches of the imperium were built in the 120s and the beginning of the next decade: the via Aquillia in Asia[30] and the via Domitia through southern Gaul to Narbo.[31] It is attractive to associate these projects chronologically. But the via Egnatia should not be put quite as late as these other two roads. Since it did not run all the way through Thrace to Asia but stopped at the Hebrus River, it likely predates the war with Aristonicus. Though not impossible, it would not be easy to find room for a proconsul Egnatius between ca. 120 and ca. 106: the only gap in this section of the list of commanders in Macedonia is between Cn. Cornelius Sisenna (119-118) and C. Cato (114). The decade of the 130s is on the whole the most likely date for the construction of the via Egnatia .
The purpose of the road was of course strategic.[32] The via Egnatia was the direct line of communication between Rome and its legion in Macedonia, and movements along it of reinforcements, supplies, and other military traffic must have been fairly constant, particularly once Asia was being assigned to a proconsul as well. It will have allowed the small Roman
force in Macedonia to respond quickly to Thracian raids from the Axius to the Hermus valleys; furthermore, it will have been the route used for reinforcements and replacements, not only to Macedonia, but after 131 to Asia as well, on occasions when larger bodies of troops were needed there. Its construction—not a cheap undertaking, surely—confirms Rome's acceptance of a permanent commitment to the defense of the southern Balkans and a continuing military presence there.[33]