The Austrian - German border near Lindau-Bregenz. When travelling towards Switzerland, you can avoid the Austrian motorway charge by coming off the autobahn at Lindau and driving around into Switzerland. This will add around thirty minutes to your journey but you can benefit from stocking up on cheaper Austrian fuel as well as getting views of Lake Constance. The cheapest filling stations are those next to the German border - ironically further into Bregenz and Hard the fuel becomes more expensive.
The customs post is now empty.
Incidentally don't drive driving a couple of kilometres into Bregenz on the motorway without buying an Austrian vignette. The police are onto this one and will show no mercy! The fine is quite hefty as a friend told me who got caught!