For example shaders are used to fix flashing tarmac.
nkasphalt and tangent are the shaders usually used.
nkasphalt is nice, but also tough on fps. tangent is easier on performance.
As a basic rule:
give all flat objects nkasphalt shader. So asphalt, grass, sand, kerbs
give all vertical objects tangent shader. So walls, some small objects around the track.
no shader on trees and big objects like boxes.
To apply a shader select an object and select the correct shader from the combo box