If I have a goal of at least 10 widgets per day, but for the first week happen to do 20 widgets per day, I am able to take a weeks break without danger.

For many goals this is undesirable - I may want to do "strictly at least 10 every day" and not "an average of at least 10 in a 2 week period".

I suggest either/or:
(i) A button to "raise" the road to your current progress, so that you lose your accumulated buffer. (Perhaps this is already implemented/suggested, but I cant find anything)

(ii) Set up goals with a hard bound of data entry per time period, or as a generalization have a "high" road and "low" road in order to combine "do more" and "don't do too much" goals. i.e. you must stay between the roads, and the roads may be configured independently. I realize this would significantly increase the complexity of the system and interface. Luckily the topology of a 2D graph precludes the possibility of more than 2 roads being useful.

[One day, perhaps I will be able to combine my "widgets per day" and "widget quality" into a 3D line graph, where the set of useful boundaries is a far more complicated surface. This is not part of my suggestion!]