Driving is a captive audience, I don't have the option to "close my eyes/plug my ears" to avoid your "free speech" but with free speech comes the right to avoid hearing your bullshit. I can avoid buying a book, I can turn the channel on the radio, but a billboard doesn't offer that "right" to be free FROM your bullshit speech. There's more obligation on billboards in that regards, and it's kinda horseshit that you're allowed to hold me captive because you have enough money to spend on a campaign (whether it's commercial, political, religious IDGAF)
Wish "freespeechers" could understand this. I'm not saying we should just ban everything, and I'm not even sure I agree with a billboard ban (I would have signed up 25 years ago on that, adbusters reading chud that I was). I'm just saying it's really pathetic that people cry "free speech" when there are two things at play and the SCOTUS did a disservice on differentation between amplitude of property vs signal of speech.
I mean, let's make exceptions for events where everybody came to hear the thing, where consent for the amplification can be assumed because we all bought tickets or something, but if you're standing on the corner with a bullhorn shouting at passers-by, that's hostile behavior in the same way that billboards are. Please don't do it.