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)