Bundesstraße
One distinguishing characteristic between German "Bundesstraßen" and "Autobahnen" is that there usually is a general 100 km/h (62 mph) speed limit on federal highways out of built-up areas, as opposed to the merely advisory speed limit ("Richtgeschwindigkeit") of 130 km/h (83 mph) in unmarked sections of the autobahns. However, a number of "Bundesstraßen" have been extended as expressways (dual carriageways) (colloquially called "Yellow Autobahns"). Many of these have speed limits of usually 100–120 km/h, others have only an advisory speed limit like autobahns. Most sections of the federal highways are only single carriageway with one lane for each direction and no hard shoulder pull-out area.
Bundesautobahn 4
The A 4 starts north-west of Aachen, where the Dutch A76 enters Germany. Initially it is 2 lanes each way with no speed limit. From Kreuz Aachen to Düren and from Kerpen to Refrath (between Refrath and Köln-Merheim westbound the hard shoulder is the 3rd lane and only open at peak times) it has 3 lanes each way. Between Kreuz Köln-West and Kreuz Heumar it forms the southern part of the Cologne Beltway (Kölner Autobahnring). The whole rest of the section between Kreuz Aachen and Kreuz Köln-West has a variable speed limit. Between Merzenich and Elsdorf, there is a speed limit of 130 km/h, that has been imposed in 2017, due to an increase of accidents. Between Kreuz Köln-West and Kreuz Heumar the speed limit is 120 km/h. From Kreuz Köln-Ost to Refrath there is a maximum limit of 100 km/h. The westbound section between Köln-Merheim and Kreuz Köln-Ost is restricted to 80 km/h. The section between Refrath and the current terminus at Krombach has no speed limit. Only between Untereschbach and Refrath westbound there is a 100 km/h speed limit from 6 - 10 am on weekdays. It is mostly 2 lanes each way, but between Untereschbach and Wiehl there are several additional climbing lanes in both directions.