When a person is completely entranced by the monotony of - for example - a road

As Wikipedia describes it: Highway hypnosis, also known as white line fever:

a mental state in which a person can drive a truck or automobile great distances, responding to external events in the expected manner with no recollection of having consciously done so. In this state, the driver's conscious mind is apparently fully focused elsewhere, with seemingly direct processing of the masses of information needed to drive safely. --Wikipedia

I have heard it referred to as "driving on auto-pilot", taken from aeronautical usage.

It's not in any dictionaries that I know of (see Onelook), but I've found several uses of the word hypovigilance in the context of driving monotonous roads.

An article by Gregoire Larue, Andry Rakotonirainy, and Anthony Pettitt, and a 2011 PhD Thesis of Rebecca Michael, for example.

Aside, hypervigilance, the opposite phenomenon, oddly seems to enjoy much more popularity as a word: