Flashing headlights to warn oncoming drivers that the police are waiting to catch speeders is protected by the First Amendment. So held a federal trial judge in Monday’s Elli v. City of Ellisville (E.D. Mo. Feb. 3, 2014), and he wasn’t the first — see, for instance, this Florida trial court ruling, as well as State v. Walker, No. I-9507-03625 (Williamson Cty. (Tenn.) Cir. Ct. Nov. 13, 2003).

Whether this is the right answer is not clear. The situation is a special case of warnings to hide one’s illegal conduct because the police are coming — “abort the plan to rob the store” or “flush the drugs down the toilet.” True, here that is done by a stranger rather than by a lookout who’s in league with the criminals, but it’s not clear why that should make a constitutionally significant difference. And this “police are coming!” scenario is in turn is a special case of what I call Crime-Facilitating Speech (see 57 Stan. L. Rev. 1095 (2005)), which is to say speech that conveys information that makes it easier for people to commit crimes or to get away with crimes. The Supreme Court has never squarely confronted when such crime-facilitating speech is protected by the First Amendment and when it’s not.

When I’ve blogged about this in the past, some people have argued that flashing headlights should be protected because it’s encouraging legal behavior (slowing down) rather than illegal behavior, but I don’t think that can dispose of the issue: Many lookouts do the same, e.g., when a lookout warns would-be robbers to abandon their plans because a police car is driving by. [UPDATE: Remainder of paragraph added.] Moreover, a headlight flasher’s warning to speeders seems likely to (and probably intended to) slow them down only until they get past the police car — they’d just be postponing their illegal act by a few seconds. That’s why the “he’s only trying to get people to be law-abiding” argument strikes me as weak; what he’s actually doing (and trying to do) is decrease the cost to drivers of breaking the law.

For an interesting similar question though one that doesn’t involve encouraging people to temporarily act legally), this story from several years ago:

An advocate for immigrant and civil rights has started using text messages to warn residents about crime sweeps by a high-profile Arizona sheriff.

Lydia Guzman, director of the nonprofit immigrant advocacy group Respect/Respeto, is the trunk of a sophisticated texting tree designed to alert thousands of people within minutes to the details of the sweeps, which critics contend are an excuse to round up illegal immigrants.

Guzman said the messages are part of an effort to protect Latinos and others from becoming victims of racial profiling by sheriff’s deputies ….

What’s the First-Amendment-relevant difference, if there is one, between this and a lookout who alerts criminals when the police are coming? (Assume that the lookout isn’t getting a share of the loot, but is just helping his friends avoid getting locked up.) Should it matter, as one expert who’s mentioned in the article suggests, whether Ms. Guzman’s real goal is preventing lawful arrest of illegal immigrants (as opposed to preventing racial profiling, assuming such profiling is unlawful)?

I think there may indeed be a difference between such revelation of facts to the public and individualized communications to a small group of criminals, and I don’t think it should turn on jury inferences about the speaker’s true purpose; my article discusses the question at length. But in any event it’s helpful to think about what the difference might be.