Two Parts  

In the case of the  driving example you gave after 14 hours etc,  the FAA doesn't care as long as you are no longer flying. If you feel too tired to drive ( which would be expected ) it's personal responsibility. I have  had plenty of two hour drives back to base after 14 hours duty, but usually in day  time after night shift . I have also just called my RAM  and explained I am beat and plan to get a hotel room . 

Its the old story that you can get away with anything , until you have an issue like getting into a MVA or incident in the heli .. then all the Lawyers  come out..

My advice , that I now follow,  is I dont go over 14 and if it looks like that could happen.  I call my RAM or PAM and explain I am tired and plan to implement the F in  IMSAFE and call it a day and if away from Base get a hotel. So far I have never been called out on that decision. 

to the proof read peeps, go teach an English Class , this is a chat forum not a High School..