Autobahn maintenance and improvements don't escape the German penchant for obsessiveness. As a result, construction zones (Baustelle) are frequent and widespread. The standard protocol for large projects is a traffic shift, the lanes for both directions are narrowed and crammed onto one side of the Autobahn so that the other side can be worked on in its entirety. Such situations are well-marked with signs and speed limits are usually reduced greatly in these areas.
Congested Autobahn with shoulder open to traffic
In the event that a segment of Autobahn must be closed due to an accident or other emergency, pre-posted provisional detours are ready to guide traffic around the closure. As you exit, look for the U-numbered detour sign on the exit ramp - this denotes the detour route for that exit. Follow the same-numbered route over the secondary roads and you'll eventually arrive at the next downstream entrance ramp. If that entrance is also closed, just follow the next sequential detour number to reach the next entrance after that. However, there is one small gotcha, odd numbers continue in one direction, even numbers in the opposite direction. So if you're following an odd numbered route, be sure to follow the next sequential odd number (and, obviously the same goes for even-numbered routes.) These routes also come in quite handy if your patience runs-out and you want to get around a Stau.