Billboards aren't litterly on the highway, which these signs are. They're actual road signs. You're supposed to look at them, but you're not supposed to be unduly distracted by them.

The point is that signs that are actually on the highway need to follow strick rules so they're uniform, simple, and provide necessary information in an understandable, straightforward way.

You should get what you need from them at a glance, without much thought.

Billboards are junk off to the side of the highway that can be ignored. You aren't trained to pay attention to them the way you're trained to pay attention to road signs.