I think each lane should be typically 12' wide. I don't know your era, but modern trucks are 110" wide. If they are wider than 110", then they need special oversize load signs and permits. I model in ho so I don't know what n scale measures out per foot. One trick to use is to make your roads slightly narrower than scale to save real estate for buildings, scenery, etc. If you are doing a city street with parking on the street, don't forget to allow an extra 8' on each side of the street for parking spaces.