Hi Hoorah,
One of the ways of creating terrain type objects is from a height map ... a 2D greyscale image of the terrain from above. So first off you would need to have or create this "plan" of the circuit (in some graphics program such as PaintShop Pro etc.).
Once you have the 2d image it can be converted to a 3d object by various means.
For AC3D specifically there are two ways:
Dennis's *Image to heightmap converter* available here :
http://bellsouthpwp.net/a/t/atomicd/...ug/img2hm.html
This is a little standalone program that produces an .ac file ready for use (I believe as I have never tried it).
Or, Livewire's AC3D plugin available here (again never tried it) :
http://www.ac3d.org/forum/viewtopic.php?t=800
If you have a program like Bryce3d you can also produce terrains based on 2d images or directly in the program then export them in .obj or VRML1 format. This program also will allow you to create a texture for the terrain.
Finally one last option ... Terragen .... which is free for personal use :
http://www.planetside.co.uk/terragen/productmain.shtml
Not sure if this kind of technique is subtle enough for what you have in mind .... but hope it helps. You may also want to look at the two"road" tutorials here (scroll down):
http://calyxa.best.vwh.net/~calyxa/pearl/tutor.html
Regards,
griff