The idea that "farther = physical and further = abstract" is not backed-up by established dictionaries, which actually allow for both uses for both words. Having said that, there does seem to be a bias in favour of 'farther' in the sense of physical distance in many parts of the US. In the UK you could use either to refer to physical distance, but 'farther' might sound a bit archaic/American - not that that's a problem!
TL;DR Use 'further' and forget about 'farther' and you'll be fine, as long as you don't mind occasionally upsetting some Americans :p
there isn't enough reason for them to build a highly expensive and dangerous road.