Highways/Freeways = YB Gravel/YB Asphalt. there's not really a difference between them as Gravel/Asphalt are thought to be the same genes, just different lines.
i'm not good w/ technical definitions/descriptions, but i would describe the difference between a Highways/Freeways and Super Gravel/Super Asphalt as the former being lighter as the YB adds more colors; the dark colors will appear more along the eye stripes and dorsals making up the Highways/Freeways striping. YB also adds noise or pixelation in the side pattern, where the Super's appear cleaner.