>it stands to reason that if phones are distracting
The action of using your phone is distracting, not that phones are distracting. You can absolutely use your phone as a GPS. The process of interaction is the distracting part.
>literally designed to get your attention
The NIH article actually says literally the opposite, of what you're saying. Billboards aren't distracting enough to completely draw your attention.
>a simple internet search yields all sorts of research into this topic supporting the common sense expectation.
No... No it doesn't. As evidenced by the results you provided. One talks about safety billboards and the other literally says the opposite of your claim.
And I quote: "Generally, billboard-related distraction appeared to be minor and regulated by drivers as the demands of the driving task changed."
>As a community, we should strive for higher information commentary maybe?
Yes. You definitely should note just make un-appealable claim without knowing the reasons for it. Your argument that billboards are a safety hazard is just your opinion, that has more evidence against than for.