The site recently did a major update so there were problems, be all have been resolved as of now from what I can tell.
Anywho, for terrain textures (I agree a custom one is a MUST), I like to to the LEX on SC4Devotion. They have a category called "Mods-Graphics" that has tons of water and terrain mods.
For highway decorations (signs, lights, etc), check out mrtnrln's stuff here on the STEX. He has made dozens of sign and light packs reflecting styles from various countries.
Sadly, I do not currently know of any car traffic generators.
good luck ,guys .