Traffic in German Alps in late summer (Garmisch-Partenkirchen area)?

The bad traffic is on the A8 Autobahn between Munich and Salzburg, particularly near Rosenheim. Secondary roads can be busy, but rarely clogged unless there's some kind of event or roadwork. I don't think you'll have any problems, except as previously noted.

Some of the N-S Autobahn routes can be quite busy in the summer, as all people on vacation from Belgium, the Netherlands, northern Germany and Scandinavia head south to the Alps and beyond to Italy in the summer, but this is usually in July and August. And even here, traffic usually moves pretty well, unless there's an accident.

The only time I've experienced major traffic in this area was on the A8 between Munich and Salzburg and the A12/A13 (Innsbruck, Brenner Pass).

On the weekends when the weather was nice for hiking in summer you can get some slow traffic on the secondary roads but nothing to discourage you. During the week you should have no traffic.

gute Reise!

DJ

You should have seen the traffic jam coming north at Brennaro back when they had border crossings. I've never seen so many heavy trucks lined up.
You should have no problems driving throughout the Alps in September. See if the rental car office has the Austrian vignettes for purchase, because you'll need one to travel there.
Touring the West Tirol/Austrian Alps is one of my favorite trips. And the best drive is the Grossglockner High Alpine Highway--south of Zell-am-Zee to Lienz, Austria. You can go west to the motorway and north through the Brenner Pass into Innsbruck. It's simply the most beautiful place I have ever seen.

Two words, if you like your traffic stationary on steep inclines - Fern Pass.