[hn9780470248775]

The German Autobahn (equivalent to Interstate highways here) is entirely limited-access, high-speed (minimum speed limit 100 km/h; often no speed limit).

The German Bundesstrasse (equivalent to U.S. or state highways?) does have highly variable speed limits, as you describe, and I found them correspondingly maddening to drive on, due to the incessant need to accelerate or decelerate.

[CalRobert]

Indeed - in my view this correlates to one of the best places in the world to drive. Variable limits seem more logical than assuming one speed can always be the best for current conditions. Road throughput (vehicle-miles per hour) can be maximized at a lower speed when traffic is heavy, so limits are reduced. Conversely, when traffic is light the minimization of individual travel time can be prioritized, so limits can be raised.