The stopping of random drivers to check for sobriety without cause is a search that barely meets constitutional muster. In 1990, the U.S Supreme Court ruled in a 6-3 split decision that sobriety checkpoints are legal (Michigan Dept. of State Police v. Sitz, 496 U.S. 444 (1990)), but part of its decision considered that drivers were free to avoid those checkpoints by making a u-turn. About a dozen states have outlawed the practice.
If a part of the constitutionality of a search depends on one’s ability to avoid it (just like the “if you don’t like it, just don’t fly” argument in favor of TSA nonsense), then a means for obtaining knowledge of an impending search is necessary. Subsequent cases have made clear that citizens have the right to warn others of police activity: for example, judges in Florida, Kansas, and others have ruled that there is a constitutional right to flash headlights to warn other drivers of speed traps.
Yet, New York police have always had a thing for arresting citizens for “interfering” with them verbally, despite New York law (and possibly the Constitution, absent some pretty compelling circumstances) quite clearly stating that such a charge requires physical interference, intimidation, or interference with telecommunications systems (case ruling against the police when interference is non-physical, listing other cases where police lost the same arguments time and time again). It’s therefore no surprise that an NYPD attorney sent a cease-and-desist letter to Waze, the driving directions app that allows you to flag checkpoints:
Individuals who post the locations of DWI checkpoints may be engaging in criminal conduct since such actions could be intentional attempts to prevent and/or impair the administration of the DWI laws and other relevant criminal and traffic laws. The posting of such information for public consumption is irresponsible since it only serves to aid impaired and intoxicated drivers to evade checkpoints and encourage reckless driving. Revealing the location of checkpoints puts those drivers, their passengers, and the general public at risk.
Irresponsible? Maybe criminal? Given that the Supreme Court has, to date, not allowed secret checkpoints. that seems far-fetched.
The letter was written by Ann Prunty, who lists herself as “Acting Deputy Commissioner, Legal Matters.” A review of State Bar records shows that she is a licensed attorney, which means she has the duty to avoid sending out letters like this without a good-faith basis for her assertions.
Of course, Ms. Prunty declined to respond to my request for clarification. I’ll take that as an indication that she’s full of shit.
One silver lining: if the police do actually arrest someone for using Waze, this letter makes not just the individual officer, but the City itself, liable for the false arrest, since it’s now the official policy (“pattern or practice,” if you will) of the government. Perhaps Ms. Prunty should think before she speaks and puts the taxpayer’s dollars on the line.
Professional Troublemaker ® 
I think flashing headlights is used for several reasons. Here is a few of mine. Check points that are just over a hill and dangerous to begin with. Imagine topping a hill and there sits a line of stopped vehicles and the speed limit is 55 MPH. No warning or anything. Yes, they do stupid crap like that around here at times. Another reason, accidents. At least it alerts people to focus and some even slow down even if they are not speeding or anything. I notice when I flash my headlights, I almost always see brake lights in my rear view mirror.
Now some city folks may not quite get this next one. Deer during mating season or the rut. Where I live, December and January are peak times. I use a couple back roads and if I’m out at night, I don’t care that the speed limit is 45 MPH or even 55 MPH. I’m going 30 or less and anyone who is stupid enough not to understand why, they can pass and clear the road for me a bit. I’ve actually had people pass me only to see brake lights just a little further up, they saw a deer in the road or close to it, or once, even hit a deer. I might add, totaled the car. Ruined the front end, hood and was laying on the windshield. It also bent the top of the car as well. The shape of the car acted like a scoop. Speed didn’t help much either I would guess. When I see deer on certain roads, I warn oncoming drivers. Most around here get what the warning is for. You only have to hit one deer to get the point. Don’t get me started on wild hogs. Deer are a walk in the park compared to a wild hog. Two words, brick wall.
You are correct about SCOTUS rulings. It is considered protected speech when warning others of any type of hazard ahead, whether it involves cops or not. It may not be a technical spoken word but most people know what is being said. At the very least, it means to pay attention and slow down regardless of what is ahead. I not only consider it a right to do this, I consider it a duty. It’s no different than warning someone of a cliff ahead on a dark moonless night.
I smell a lawsuit. Jon, you on this yet??? ROFL