Although there is a osm wiki page that attempts to shed light on the different tables which are created within a postgres/postgis database there is something that is not quite made clear:
Is the data in table
planet_osm_roads duplicate? (i.e. would it provide anything that could not also be retrieved however indirectly from the other tables
Bonus: If as assumed this is duplicate stuff is there a way to prevent its creation in the first place (assuming it increases the costs both in time and memory and disk space)?