"Highway 84": When they landed planes on the Autobahn

In 1980, a NATO military exercise called “Highway 84” turned a section of the Autobahn into a landing strip.

Such capabilities were apparently fairly common during the cold war. This particular exercise was on the A29 at Kreuz Ahlhorn - but many other locations exist where you could theoretically create an airstrip.

The idea is, in the case that the Soviets bombed existing NATO air bases, pre-defined sections of road could be rapidly converted into a serviceable strip.

Military vehicles would jump into action, removing guardrails, the central reservation and putting up temporary facilities like this air-traffic control tower mounted to the back of a vehicle.

Other capabilities that had to be built were communications systems, perimeter security, radar systems, weather forecasting stations and even runway lighting.

The exercise lasted 3 weeks, and was the only time that the road was used for this purpose.

In the late 1990s, with the Cold War over, the German government started converting the strips back into conventional roadways. This one, Alhorn remained until 2006.

Source of lots of this information

And also Wikipedia!