nomenclature:
 - street: useable by cars
 - tram: useable by trams
 - road: either street or tram

baseset provides:
 - walkway
 - lamps, trees
 - roadworks

roadtype properties:
 - allow walkways/lamps

roadtype provide:
 - underlay for grass or city (without walkways)
 - track without underlay
 - optional (default from baseset): oneway for streets
 - optional (default from baseset): wires, pylons

drawing order for ground:
 - baseset: grass or walkway (walkway is drawn using baseset roadsprites; the roadgrf is supposed to overdraw the road-parts)
 - roadgrf: underlay from street or tram (only one, street takes preceedence)
 - roadgrf: street track without underlay
 - roadgrf: tram tracks without underlay
 - roadgrf/baseset: oneway (only for streets, not for trams)
 - baseset: roadworks
additional with bounding box:
 - vehicles
 - baseset: lamps, trees
 - roadgrf/baseset: wires, pylons from street or tram (only one, street takes preceedence)