IANAL, but my understanding is that there are different levels of "scrutiny" that can be applied to a statute when analyzing its First Amendment compatibility, and which one is appropriate depends on several factors. Typically a judicial opinion walks through the logic of which one it applies before applying it. And the level of scrutiny determines how strongly the law is constrained.
Under the strictest scrutiny--applied when the law is restricting the expression of a viewpoint or opinion--it doesn't matter why the law is there; free speech wins. Under lower scrutiny, as seems applicable here, the government must show it has a "rational basis". That's what allows them to, say, require you to communicate your intention to turn via a turn signal. I believe that rational basis test is what's failing here.
This comes from Footnote Number 4 [0], which was literally a footnote in a case where a Justice articulated what the Court was already doing in analyzing its cases. The Court generally applies one of three levels of scrutiny [1]:
* Strict scrutiny - The Government must prove there is a compelling state interest behind the challenged policy, and the law or regulation is narrowly tailored to achieve its result.
* Intermediate scrutiny - The law must serve an important government objective, and be substantially related to achieving the objective.
* Rational basis review - A challenger must prove the government has no legitimate interest in the law or policy; or there is no reasonable, rational link between that interest and the challenged law.
While I don't believe the Supreme Court has taken up a case of light signaling, it is likely that the Court would apply strict scrutiny due to the clear First Amendment considerations. If so, thus the government would need to prove that it's narrowly tailored solution to a compelling state interest. It would need to answer questions such as: is it okay to say flash your headlights for road hazard? Would the state prohibit the defendant from telling others at a gas station about the police officer? Etc. I'd suspect that it would be a hard case to win for the Government.
Under the strictest scrutiny--applied when the law is restricting the expression of a viewpoint or opinion--it doesn't matter why the law is there; free speech wins. Under lower scrutiny, as seems applicable here, the government must show it has a "rational basis". That's what allows them to, say, require you to communicate your intention to turn via a turn signal. I believe that rational basis test is what's failing here.