It would really help if you gave a sentence.
With roads and the use of "flat" and "level" the difficulty is that such adjectives are usually used to qualify the quality of the surface of the road. A surface of a road is, normally, called "the road". So any attempt to describe a horizontal stretch of road as "flat" or "level" will end up as a description of its surface.and if the streetlis level/flatdoes not go up or down, it seems logical to say 'go along this street...'"
With roads and the use of "flat" and "level" the difficulty is that such adjectives are usually used to qualify the quality of the surface of the road. A surface of a road is, normally, called "the road". So any attempt to describe a horizontal stretch of road as "flat" or "level" will end up as a description of its surface.
"If a street goes uphill, we can say 'Go up this street for about 200 metres'; if the street goes downhill, the directions could be 'Go down this street...'; and if the street is level/flat, it seems logical to say 'go along this street...'"
To me a flat road would either be one that lacked camber (level from side to side) or that had a good surface (free of potholes and other defects).Perhaps flat is slightly better or at least more common