Learn English – “in front of” or “opposite”

As far as at vs for goes at specifies a point at which something is important. For specifies a length over which something occurs. I ran the car for a distance of 5 miles. I stopped at a distance of 10 miles from home.

As far as your requirement goes I'd propose something like this:

Road edge lines shall terminate at a distance of 50 meters plus half the ramp width measured from the center of the ramp on both sides of the ramp-road intersection.

(Requirements should be phrased in the positive rather than the negative)

Certainly. The ways in which prepositions are used in English - and probably in other languages - are numerous and often bewildering. I was about to add idiosyncratic, but there is probably a logical explanation behind even the most idiomatic usages, perhaps lost in time. Some grammarians have said that prepositions constitute a class of words both semantic and functional. Prepositional usages are sometimes graded:

central (locative, directional; temporal) (eg on the bed, to the park; before midnight)

semi-idiomatic (eg on the train, at a loss)

peripheral (eg on fire)

As you suggest, before has a locative sense, synonymous with the three-word (sometimes termed 'complex') preposition in front of. Using the word idiomatic in its other main sense now (in common use in the common register), in front of is the more idiomatic of the two choices - before sounds rather poetic, of a slightly refined register.