(posted in the TE-forum by marc aarts)
This is the list of hardcoded ID's in GP3_2000.EXE (GP3.EXE is very much the same).
The behaviour is most likely determined by the second value (0 = tarmac, 4 = gravel).
My assumption is that everything else mapped to the track or verge is considered
55 - 4 - ?not found in relation to track/verge (often used for adverts)?
60 - 4 - red gravel, melbourne
61 - 4 - blue gravel, melbourne/sepang
62 - 4 - yellow gravel, melbourne
848 - 4 - green gravel, melbourne
866 - 4 - tag heuer advert, monza / gravel edge on many tracks
951 - 4 - gravel, barcelona (bargravl)
952 - 4 - gravel, barcelona (bargravl)
978 - 4 - normal gravel (darksand, magsandb)
33 - 0 - ?used in gp2 for road?
134 - 0 - edge of road with yellow line, melbourne (used for adverts on other tracks)
837 - 0 - light asphalt (like in the pitlane)
866 - 0 - tag heuer advert, monza / gravel edge on many tracks
1229 - 0 - dark asphalt (tunroofd)
1230 - 0 - edge of road with with line (roadsign)