If I saw a radar trap (or “speed checkpoint” as cops say) and I went home, got a sign and spray painted “COPS WITH RADAR AHEAD” and stood on the road 400-500 yards before the speed trap and warned traffic, what could be done to me by the police if they saw me? Especially if there is no law against standing or walking on the road in my area? I would guess I could get charged with interfering with an investigation or something, right?
Depends on the jurisdiction. I have heard unconfirmed anecdotes of drivers ticketed for flashing their lights to warn drivers of enforcement. In some areas it could be some sort of interference with police activity, as you suggest.
Now here’s a twist–a turf battle results in the *local * police flashing their lights to warn drivers of *state * police enforcement.
I’m afraid I don’t have the authoritative SD on the law, though.
I do it all the time. But I always flash when I PASS the cop. So how could they tell I was flashing someone or not?
Day or night? If it’s day and your headlights are off, when you flash your headlights on most cars, the taillights flash too. At night, well, it’s perfectly obvious even from well behind the car doing the flashing.
Why should be illegal to warn about police officers being somewhere, unless you are part of some kind of conspiracy, and keeping a look out for your colleagues while they do something criminal?
If I see a stranger trying to break into a building, and say to him, “Watch out – there’s a police officer just coming round the corner and he’ll see you!” – have I committed an offence? I don’t think so.
It seems to me that making it illegal to warn about speed traps is done because speed traps are about revenue-raising, and not about crime-prevention.
Precisely. Who cares, as long as people are slowing down? The cops with a quota to fill, that’s who.
Heh. There’s a well-known speed trap along on of the state highways here in rural northwest Ohio. And right near that speed trap is a kind of take-out ice cream/hotdog stand. The store has recently been renamed “The Speedtrap,” and a very large new sign has been erected on the roof.
Speaking of warning drivers that they should slow down:
The Register NSFW!
A Danish method.
Edit: fixed coding
In NJ, USA, it’s now illegal to flash your headlights. The spin on this was that it was road rage related behavior.
Before daytime driving lights were common, just seeing headlights was the warning to slow down. Now, you need to see blinking lights, but now that’s illegal, so it is difficult.
The reasoning behind making it (warning others) illegal is that for the short term (getting someone to slow down) the warning is effective, but in the long term you are creating an environment where people can speed and use others to help get away with it. If more and more drivers could depend more upon others and various warnings from motorist to motorist become regular, speeding would get worse overall AND harder to control.
I’m not following you. By saying that, don’t you arguably thereby enter into a conspiracy with that stranger?
I think you could fill in the details of your story to make it come out different ways, but just as you’ve told it, my intuition is that you’re doing something wrong when you warn the criminal.
-FrL-
In your example, you see someone doing illegal, and you might be charged with creating a conspiracy at the time you warned them. But when you flash your lights, you have no way of knowing whether the oncoming car is speeding or not., thus there is no way you could be accused of abetting a crime.
In some places the police notify local radio/TV stations where they are conducting speed and drunk driving patrols. The stations then broadcast these locations as a deterent.
I was in long haul trucking for over 20 years and I’ve never known anyone to get in trouble for warning of a cop in the area. I recall one ocassion when I was stopped for exceeding the 55 limit and the trooper asked if my CB was on the fritz, I told him that I had it turned off. He said he wondered how I was unaware of his presence.
Commercial rigs are forbidden to use detectors and bear warnings are common on the two way as well as by headlight signals.
A quick search of the New Jersey statutes fails to turn up any such law. Do you have a citation for it? 
Oh good lord, give me a break! This has been a standard communication (at least in my neck of the woods) for much longer than the quarter-century I’ve been driving. It’s actually much less common now than it was when I started driving.
I’m fairly sure that if this little trick was going to result in a massive, out-of-control rampage of speeding drivers, it would have already happened sometime in the past 50 years that drivers have been doing it.
I’ve never heard anything about it being illegal, but generally people wait to flash until they’re past the cop. Maybe over a hill, if you’re paranoid.
Every car I’ve had for the last 15 years has had a separate “flash headlights” on the controls. The “flash” does not trigger taillights. (I checked.)
http://www.doylestownpa.org/police/html/askacop_edition_3.htm How I read it , it is not illegal to do warning other drivers. It is a pull over offense, malfunctioning headlights. In PA that is.
Years ago I heard or read and interview with a state trooper (don’t remember what state) who was asked if it was illegal to warn about speed checks by flashing the headlights. His response was that it was a violation to flash the lights. The interview went on to ask about a few more methods of signaling oncoming drivers, and the cop pointed out that each particular method was some sort of violation. But he never did say that giving a warning, in and of itself, was illegal, even though he had ample opportunity to do so. The interviewer wasn’t sharp enough to ask in such a way that the cop had to say yes or no to that, but it seemed pretty clear that while common methods of signalling oncoming traffic were not legal, there wasn’t a law against giving the warning per se.
I often flash my headlights to tell someone that it is OK to merge in front of me. Sucks that in NJ I would be a scofflaw.
Just another reason to avoid driving in New Jersey if at all possible.
Are you sure you aren’t echoing this urban legend:
In actuality, the relevant statute is: Title 75 §4306 (Pennsylvania Vehicle Code)
The use of a high beam to signal an officer ahead would not fall under the exemption for warning of dangerous or hazardous conditions ahead.