Could you walk on the Via Appia Antica from Rome to Brindisi

I am not really an expert myself of this area of Italy but, searching on search engines in Italian language, I found out this brilliant website by the Italian ministry of culture and tourism which is available in English too: www.camminodellappia.it.

It is very possible that walking on this route is only recommendable approaching or leaving towns along it (e.g. around Rome as you wrote) while some other segments in between towns or cities may be primarily devoted to vehicles traffic. Indeed the Walkable Parts section describes

the stages that have the least safety problems and that could already be covered with low probability of risks

and talks about highways and rivers to cross if you walk.

Anyway the site provides a web app with the map of the route and a list of accomodations, cultural spots and sites of any interest through four of the twenty Italian counties between Rome and Brindisi (Lazio, Campania, Basilicata, Puglia).

Here they write about

• 600km walk in 29 days,
• cycling 50km a day for 13 days.

Not to be confused with the modern route Via Appia (Strada Statale SS 7), this latter being more an highway than a hiking track.

It has been done before. One traveler and writer, Paolo Rumiz, has walked along the entirety of the route, sticking as close to the original way as he could, and published a book, Appia, describing his journey. The linked publisher's website also has maps and GPS tracks showing his itinerary.

This website has maps of the route followed by the Via Appia (and other ancient Roman roads) and of specific locations (with photos) where you can observe ruins of the road itself or of structures built alongside it. Also, a project has revised and mapped the route followed by the road in its less well-preserved eastern portion (in Basilicata and Puglia), as well as the modern roads that follow it most closely and various other ancient roads and byways, using a variety of bibliographical sources. There are also plans to make an official walking and/or cycling route following the Via Appia, but nothing concrete has happened yet.

Keep in mind that the route of much of the road is not certain and has been greatly discussed by historians, archaeologists, and philologists. Also, the only substantial preserved ancient sections, known as the Appia Antica ("Ancient Appian" in Italian), are the one in and immediately outside Rome and that halfway between Fondi and Itri. Almost all of the rest of the way is now paved and part of modern roads, sometimes busy expressways; this, combined with the characteristic long, straight sections and with the overall length of around 600 km, probably makes it less than fun to walk along. A better option is to cycle it.

One last note: sometimes the Via Traiana, a variant of the original Appian Way built in 109 CE, is known as simply the Appia or Appia Antica, with some ancient sections still visible. This also reaches Brindisi but is not the same as the much older Via Appia, which was completed in 264 BCE and follows a different route.