That's determined by the roadtypes in the grf.wallyweb wrote:Is it intentional that a vehicle built in a road type's depot be restricted to travel only on that road type?
In the Road Hog case, yes, I've enforced separation between road (ROAD) and heavy haul road (HAUL), because I prefer the game balance that way.
Other newgrf authors could take a different approach.
There are however some limitations due to the way the railtype spec was designed - roadtypes and tramtypes follow broadly the same spec. Notably, the compatibility is determined by the railtype/roadtype/tramtype and not the vehicle, which has some side effects. George discovered them here for railtypes, and the limitation continues to apply as of Dec 2016: https://bugs.openttd.org/task/5006
Specifically the case of the supplies truck you mentioned, I want to fix. However it's not top priority. If you don't mind an ugly hack solution, you can build tram over both types of road with no limitation
Also FWIW, the tramtype labels would be wise to follow the railtype labels in many cases, as it avoids repeating much debate for similar end result. Road Hog provides RAIL for unelectrified tram, and ELRL for electrified tram. Tramtypes can effectively be treated as a class of light rail. https://newgrf-specs.tt-wiki.net/wiki/RailtypeLabels