I think you could make the lanes narrower, but maybe widen it up a bit for the corners. Actually, that would be good, wide corners, but then goes think down the straights, so the only over taking to be done would be on the corners.

Logs is a really good idea. People seem to forget about the enviroment in this hobby... If not logs, you could try some wood, stick it in the ground upright, and use that. But remember when using wood for the jumps and the sides, it WILL rot after time. If you do use wood, paint it in the cheapest gloss you can find. Give the bits that tough the ground 2 or 3 coats, and it WILL make your wood last a whole lot longer, I say double its life, at least. Worth it for some real cheap paint down the hobby store. I would make the jumps out of soil, but that will take a lot of digging, so it would be a lot easier to use wood. Depends how devoted you are.