Group of 10 interested in travelling the ancient Roman Road Cologne, Triers, etc.

Ten people plus luggage would be too many for a minivan, if you wanted a single vehicle you'd need a minibus (or in German Kleinbus). You can hire them self-drive, but you'd need to check if your driving licence would entitle you to drive a vehicle of that size in Germany. Also make sure that any hotel you book at has its own guest car park as finding parking in German city centres can be difficult and expensive.

Arles would be a pretty lengthy trip from the Rhineland/Mosel, I'd stick to that general region.

It's a reasonable idea, but I wonder if you have some experienced European travelers among the committee that will be organizing things. Lots of people have the mistaken idea that the major sites of Europe are each separated by a one-hour drive on (U.S.) Interstate I-95. It's not like that. And the picturesque villages from WW II movies are gone.

When I plan a trip, I try to minimize the faceless transport overhead time. There's also check-in and checkout for a large group at multiple hotels, and the issue of who's in charge? There's also RIck's general advice (I don't mean to a question as specific at this OP) to "Assume you'll return."

I understand the destinations, but I don't approve of the pleasant myth that the route itself actually means anything. It's like me going to the Bronx Botanical Garden (from Manhattan) and thinking about the fact that I'm ... er ... driving on the Dutch "Boston Post Road." C'mon. (I don't mean to sound rude. I'm trying to respond to your question, albeit with skepticism.)

As a practical solution, I would suggest drawing up a list of must-see destinations. Then plot them on a map, and do some Google Maps projections of auto driving times. Think about on-the-ground time at each place, and about related, but not Roman sites along the way. I mean, does a gorgeous medieval town warrant skipping just because the only Roman remains are invisibly used to build the town wall? Now, how much time do you want to spend crammed into a jammed van, and worrying about a break-in to the van while you are looking at Augusta Raurica (Switzerland) or Italica (Spain?)

Maybe after Aachen, Trier and Köln you'd want to head east along this designated route http://www.germany.travel/en-mobile/leisure-and-recreation/scenic-routes/german-limes-route.html?

Perhaps renting 2 cars could be a solution?

There is a website, mostly in German, that has a calendar of festivals and events related to Roman history in Germany. Once you have your beginning and ending travel dates set you could check this calendar to see if any of these festivals or reenactments might be close to your itinerary. Use the drop down menu to select your dates.

http://www.limesstrasse.de/index.php?id=388&L=1

Thank-you for these ideas

Do you mean Salzburg, in Austria, or Saleberg, in Switzerland?

I think they mean Saalburg outside of Frankfurt. It's just down the road from another good attraction, Hessenpark, although when I last visited a few years ago, it was only half completed.

I've only driven by Saalburg, but I believe Jo from Frankfurt knows more about it.

If you're really, REALLY into Roman stuff, stop by Miltenberg on the Main river. The town itself is lovely, and the Roman Limes ran through the mountains above the river. If you have a good hiking map (sold in town) and plenty of energy, you can see the remains of the watch towers and fortifications. Don't expect too much, however. It's mostly just the remants of the foundations with accompanying information placards. If I remember correctly, there were also a few scattered ancient stone columns that the Romans quarried in the area, but never used. It's a fairly rigourous uphill hike to reach these ruins, though, and they're not well signed. You really have to be pretty proficient at land navigation by map to find them, as well as in decent physical conditioning.

Another place to see Roman ruins in that area is Boppard, which was built over the ruins of a Roman forts. Some of the wall has been excavated.