I'm not sure, whether I understand you completely ...Well, in my case it makes no different because I'm adding the 'starting' plot manualy to the list and it's only for calculating the costs and the time for the road prodject.
example:
'starting' plot is hills, target plot is adjacent plains, calculated costs 2, correct 1
'starting' plot is plains, target plot is adjacent hills, calculated costs 1, correct 2
So I don't get how adding the 'starting' plot manually avoids wrong results ... The error occurs within executing 1 line of code - I suppose or even claim, this line has to be changed!
(in fact, this line has to be expanded, because the shown bug masks another, but then it works like a dream. 12monkeys did a great job: as a whole the mod is superb!!)
PS. maybe you build your "Imperial Roads" on existing "normal" roads ... (then there is no difference ... 1/2 or 1/3 in both directions) ??