During a discussion of bridge loadings, someone suggested use of a slug and mother to avoid putting too much weight on the structure in question. I was under the impression that slugs are well-ballasted for traction and would probably not offer much weight savings. What is a typical weight for a road slug? With the prime mover etc removed, and ballast added, are they lighter, heavier, or about the same as a comparable unit with insides intact? I realize slugs vary from road to road, but I'm looking for general info.