[Light Speed]

Yes...it would seem their inability to improvise killed the offensive.
An American account of the first days, revealed that a U.S. tank regiment had driven by the Germans..only to appear miles away in the wrong place.
had the Germans reconned this..and acted..they could have busted inland quite aways.
This was mentioned as one of the key tactical mistakes early on...and led to things backing up on the germans in that sector..as you mentioned the road system.
Is it possible the Germans felt the U.S. was fleeing..and were waiting a bit so that the roads would be availble to them..with little contact in the process?.
another Bulge website commented about the lack of German artillery activity..tapering off dramatically after the opening of the offensive.

[SAMWolf]

Most of the German artillery was horse drawn and Armor had priorty on the road nets. Once the front line moved artillery had a hard time redeploying.

[SAMWolf]

Kampfgroup Pieper actually did breakthrough early in the battle, but bridges kept getting blown up in in face, his constant detours finally led to him being bottled up in the Ambleve valley in Stoumont and La Gleize, supplies weren't able to get through and he was forced to abandon his heavy equipment and breakout on foot.

The stand at St. Vith and the holding of the northern shoulder prevented any large scale followup of his breakthrough.