1. You need to go to "My Computer" Then on the left there should be a folder named "Documents".
2. When u open the folder u need to find a folder called "Euro Truck Simulator 2".
3. When u open the folder find config.cfg open that file. It might ask you how u want it to open find "notepad" in there and select it.
4. When u open the config file you need to find "uset g_lod_factor_parked" ; "uset g_lod_factor_traffic" ; "uset g_traffic" they all should be next to each other. You can find it faster by pressing "ctrl + F" then you can copy one of the things u need to find and it will show up.
5. Change the Numbers next to the "uset g_lod_factor_parked" ; "uset g_lod_factor_traffic" ; "uset g_traffic" from 1.0 to 9.9. 9.9 Is the max it will go and bellieve me their will be a lot of cars. DON'T FORGET TO SAVE THE FILE.

P.S it only works on Singleplayer. Good luck.