It's probably overkill but I typically design to have 4 levels of road hierarchy: Highway > Avenue > Road > Streets. Only interacting with levels directly above/below, streets all utilize underpasses to go under avenues. All commercial zones on avenues/roads, all residential zones on streets, all industrial zones bordering rail (industry exports using rail when bordering it), subway, busses etc. In my experience its pretty good at handling a fully zoned map with millions of sims, you just need to use roads for the street level at higher amounts of traffic, but still follow the schema. Beyond being effective it's a pretty good representation of American cities road networks and zoning philosophy
