The Austrian Danube - THE long-distance cycle path in Europe
The Danube cycle path in Austria is possibly the most famous cycle route in entire Europe. Most of its travelers cycle downstream, starting in or near Passau and ending their bike tour in Vienna. Some go even further to Bratislava (Slovakia) as it is only another 35 miles downstream.
Just to avoid any confusion, Passau is in Germany. From here to a point 17 miles down the river, the Danube represents the natural border between Germany and Austria. Beyond that point it is all Austria. By the way: Since Germany and Austria signed the Schengen treaty, there is no border control.
Our recommendation: Bike the Danube downstream. Otherwise you would not be able to enjoy it as you had to look out for oncoming cyclists every minute. Approximately 38,000 cyclists bike the Danube from Passau to Vienna every year. Assuming that the majority cycles between May and September, that's about 300 cyclists per day. If you cycle the Danube downstream, you will face a fraction of those only. You will be able to relax way more compared to an upstream itinerary.
Options to extend your Passau to Vienna cycle tour (in Passau)
You may consider extending your bike tour at the beginning. If you start cycling further upstream, in Regensburg (Germany), you will enjoy another 100 miles to get rolling. Regensburg got a wonderful mediaeval town centre and the cycle path to Passau is flat and less traveled.
Another option is a day trip from Passau up and down the Inn River. It is about 21 miles to Schärding where there are plenty of cafés at the main square to sit down and relax. A shorter option is to cycle to Wernstein only (13 miles one-way). Cross the Inn River at the pedestrian bridge here and return on the other bank.