erm,, how can I explain it,,,, step out of the box and think of the track as a bunch of pixels on a screen, forget that it is a 3d model, because in the end, it comes to your eyes as a 2d picture on a monitor screen.
Ok, this is not a 100% accurate example, because you cant have a 100% accurate example when you have different monitors and different resolutions, but I hope it will give you the idea.
When your looking out of the cockpit, the road you see ahead of you takes up about 1/4 of your screen, meaning that the rectangle you see going off into the distance stops about halfway down the screen and has grass on either side, so you make the road texture 512 pixels wide.
The grass on each side takes up about the same, 1/4 (split in half). so infact the grass on the left takes up 1/8th and the grass on the right takes up 1/8th, so your grass texture needs to be 256 pixels wide.
When you put them together you have 256(grass)+512(road)+256(grass) which make 1024 pixels., now if your monitor screen was 1024 wide, then it will all fit in perfectly with no stretching or compressing of the image.
But for your screen at 1920, you are enlarging the texture almost double so you can fit it in, your going to start to see pixel bleeding on edges of colours etc, which is going to make it look blurry and messy.
Lets say you want the best quality for your screen which is 1920 wide. then you might want to make the road texture 1024 wide and the grass textures 512 wide, doubling them in size basically, so the you would have, 512(grass)+1024(road)+512(grass) which makes 2048 pixels wide, a bit bigger than your screen, but much closer to your screen size than a 1024px wide tex.
Now, the other scenario, if you take a 2048 picture and shrink it down to almost half the size, you are going to start to see jaggered lines, which will cause shimmering when you move around the 3d environment.
From her you get into antialiasing and anisotropic filtering and so on which is not my field so I can't explain that part properly. But basically, they filter out jaggered lines and blurry images to give you the clearest picture they can.
What your seeing with Toban is a stretched image, your system is enlarging it so it will fit on the screen.
Your lucky here, because it just so happens that SLN has made a texture pack for Toban, go to SLN's site
http://updates.slnart.com/ click on tracks and scroll down to Toban. Add those textures and test it again.