Sorry. I don't know techniques about that engines, but I have a idea. In the real life a track is just a place where the car must go. We can use the same interpretation. We can draw a imaginary line representating the way of the track. That imaginary line is not just only imaginary, the engine can interpret it but don't need to show it to the player (only the track creator can see it).Now, that imaginary line can show the way of a road. What is a road in real life? A road is a modification in the terrain. We can make the engine handle that kind of things. Just "droping" a road above some terrain (just as when we put a carpet on the floor). The engine too can "drop" some other model things same as tree, houses, wires, etc. The colision detection can be done simply. In the terrain is the same as the terrain form. Mathematic calculations about the side or edges of the terrain and we can determine if it just is colisioning.So, a less poligon terrain will result in a less work for the colision detection.Trees and other things can be considered just cubic prism for colision detection (we only have 6 sides for check and 8 vertices).Colision in the road is same, because the road will be droped in the terrain. We can simply the work for the colision detection just caching the track (where the road is droped it can be fusioned with terrain). We can simply that work too determining distances between vertices and we can skip the near ones (and only check colision with the most importants). So we can too just cache a colision map.Friction factor about a kind of terrain is easy to determine, using the material. So the ground is a material grass and earth and the road is concrete or some other. We can make a material list too so the road creator can choose. The engine must handle that too.Anyway we can "drop" water. The water just will low the friction and will produce a "splash" effect when the car just touch it.Handling the things in that manner we can have a very versatile physics engine. (We can drop oil in the road too
).What do you think about that?
