Many people know that the Via Cassia is the road on which Hilaire Belloc, somewhere south of the intersection with the Via Trionfale, first caught a glimpse of Rome — this at least according to Belloc’s account of the event in his celebrated travelogue The Path to Rome. As I understand, that was in 1901.
Fewer people — mostly Jesuits and devotees of St. Ignatius of Loyola — know that the founder of the Society of Jesus, who was also on his way to Rome, had a vision on the Via Cassia in which he heard the words “Ego vobis Romae propitius ero” (more or less: “I will be propitious to you in Rome.”). Ignatius interepted the statement thus: “I do not know whether we shall be crucified in Rome, but Jesus will be propitious.” This occurred in the village of La Storta, which is only a little north of where Belloc saw Rome for the first time. A simple chapel now stands in La Storta to commemorate Ignatius’s vision. The vision took place in 1537.
Probably only some grumpy old Aquinas scholars know that the Via Cassia also had an important place in the early life of St. Thomas. He had set out on foot with other Dominicans along the Via Cassia — traveling away from Rome — in 1244 just before Pentecost, heading for a general chapter in Bologna.
On hearing of this intended journey, Thomas’s mother, who, as is well known, did not want him to be a poor friar, sent his brother Rinaldo, who was camped nearby at Frederick II’s headquarters at Terni (a complex struggle was going on involving Frederick, Innocent IV, and Cardinal Ranieri), to intercept Thomas and bring him home to Roccasecca. This Rinaldo managed to do, meeting Thomas near Acquapendente (around 85 miles north of La Storta). Thus began the infamous episode of Aquinas’s “imprisonment” (probably too strong of a word) by his family, which ended in the Summer of 1245, when he was permitted to return to the Dominicans in Naples.
On your next trip to Italy you might consider a jaunt on the Via Cassia, Strada Regionale 2 (formerly Strada Statale 2). See the map above. The Cassia is the road marked “S2”. As far as I know, the route of the Cassia has changed very little since the eighth century, maybe a few deviations here and there. So, the stretch of it that you take may very well be the same stretch traveled by Aquinas, Ignatius, and Belloc.