It's a standardized part of the highway system in the same way as the standardized lines painted on the pavement, not just something to support or not-support based on current fashion. The linked article does a very poor job conveying this with its framing on radio-as-entertainment and on corporate consolidation of radio stations. Key phrases: “Highway Advisory Radio” or “Travelers Information Station”, neither of which are mentioned in the article.

Here are the technical requirements and implementation details of California's (CalTrans') system, for example: https://dot.ca.gov/-/media/dot-media/programs/research-innov...

And some general history: https://aairo.org/history.htm