One of the top ten complaints about Germany that Americans tend to utter....is staus.....traffic jams. If you travel around Germany by car in the summer period....you end up having to endear through some pretty tough episodes. My worst one was from a decade ago....making a trip from Frankfurt to Hamburg, It should take roughly 5.5 hours of decent driving speed to make the trip. Mine took almost ten hours.
The German Automobile Association in its monthly magazine....Motorwelt....did an excellent article on "Raus Aus Dem Stau".
Somewhere in the middle of the article.....they throw up a really great statistical display. So, here's the numbers and what it all means.
Throughout the summer, the worst period of travel is from 1100 to 1900 on Friday. From Saturday and Sunday....the worst period of the day on autobahn travel is 1000 to 1400.
My personal advice is that if you have to make some long miserable trip within Germany over the summer period....I'd either start early (5AM) and try to reach my destination by mid-morning.....or I'd start near 4PM and keep driving until midnight.
The idea about jumping off the autobahn and taking a secondary road? By the time you make this decision....there's at least 2,000 other folks who make the same decision and crowd up a secondary road that can't handle that kind of traffic flow. It's best to just find a decent rest-stop....pull off for three hours and chill out.
The renovation effect? If you go onto goggle and look at roads around Germany.....they will update construction zones and let you know of the issues involved. Between Stuttgart and Munich.....there's always a zone of renovation.....probably existing since 1945, so don't anticipate beating that situation.
As for the heat? If it's above 35-C.....it's just another reason to leave at O'dark-thirty and get ahead of the other folks.