http://gma.yahoo.com/texas-woman-arrested-warning-drivers-speed-trap-160551293-- abc-news-topstories.html
     Do you flash you lights or as in this case hold up a sign to warn other drivers of speed traps or if you don't drive yet, would you?  I don't speed.  Personally I consider speeders a menace.  I want the police to catch them and never give oncoming drivers warnings about speed traps.  Speeding is illegal and dangerous.  I'm on the cops' side in this.

I warn others selectively. In metro Atlanta the speed limits are ridiculously low and the traffic pattern is (usually) ridiculously fast. The cops tend to ignore people moving with the flow of traffic and only stop the people moving slightly faster than the speed limit. This behavior pisses me off.
 
If I pass by a speed trap and I see someone moving slower than the other cars then I will flash my lights at them. If everyone is moving the same speed then I won't do anything (and unfortunately neither will the cop). I don'twarn others outside of Georgia as a rule.
Edited:

 If I drive or see a cop, I'll tell the driver and they usually warn whoever they see next. I live out in the country so the only 'speeders' on the roads are usually 4 wheelers, not very dangerous.
    Accidents with deer are very common and we usually warn anyone for those becuase it could be their life at stake. Most of the speeding tickets around here are given to people going just over ticket range because they are in a hurry.
 
To tell the truth, I've never actually had the occasion to pass both a real speeder and have just passed cop so I can't really say.

Not sure.
Speeding is illegal and I had a bad car accident due to high speed, that lead to the death of a close friend.  
But here in Italy this is the situation: empty suburban roads with speed limits of 30 km/h, then 70, then 50, then 70 again, and 30 and so on and so on. With well hidden police cameras at every turn. All this because years of bad government led the towns to have less fundings, and they have to make money one way or another.
And I forgot police cameras put at the beginning of straight roads. At the beginning, leaving kilometers of straight road to increase the speed of your vehicle.
So I occasionally warn about speed traps, but just motorcycles  
Greetings,
Mauro.

Being in the biz, I think they should pay the wages of sin, as it were...
 
However, that opens the whole can of worms about traffic enforcement and whether or not the way we do it is a good idea...
I have read rather a lot on this subject, and we discuss it pretty frequently on the Randi forum.  A great deal of traffic enforcement does little or nothing to accomplish it's stated purpose; that of accident prevention.
Further, it has become a lucrative source of revenue production in many areas.
 
Just as a for-instance... The city of St. Louis contracted with an outside firm to install the "red light" cameras at intersections all over the city, and set up a special court to handle the cases.   The company receives a percentage of the fines/fees.  
An investigation by one of the local newspapers a year later found that of the 10 "most-dangerous" intersections in the city....None had been equipped with cameras.   In fact, all the cameras had been installed at the busiest intersections, dangerous or not.
This of course makes folks see the whole operation as geared to revenue production rather than safety...
And in fact St. Louis is now considering dropping the whole thing because of the public outcry.
 
It's been shown that just speeding is not a large contributing factor to accidents.   Driving too fast for conditions is....
Right now, "inattention" ranks almost alongside drunken driving as a prime factor for accidents.   Cell phones, GPS, fiddling with the sound system, fighting with the kids in the back of the van, etc, etc.

Not a driver, but my dad is always 10mph faster than the speed limit (although it's usually in the fast lane). I don't think drivers should be warned, and I would somewhat consider it impeding a policeman. I dunno. But then again, it's your own stupid fault for not noticing the speed trap in the first place. If you're driving fast and you consistently notice speed traps, then hell, maybe you are a semi careful driver (or darn lucky). If you can't notice them while driving fast, then maybe you aren't paying close enough attention to the road... Usually the highways have a grass strip/divider in between the directions, so that a cop can be behind a bend or near a ditch, harder to see, but not near impossible.  
 
On the other hand, in China speed traps are pretty comical. Usually cars will drive at around 80kmh, when the legal limit is 70 (on a Shanghai highway at least). There are radar stations every amount of kilometers. There's always a radar sign a few hundred meters in front of it, so drivers will guess at where the radar station is and slow down....to a crawl. It's ridiculous because they'll go down to walking pace for a brief moment, before rapidly speeding up again. Then again, Chinese roads are very dangerous. Traffic rules are mostly ignored, and going at 80kmh you'll have bumper to bumper (around a meter in between cars) traffic on extremely crowded highways. scares the nuts out of me when I sit in the front row of seats on a bus.  
 
On a side note, on a really crowded Beijing highway two kids who just graduated high school with really good scores went on a race during rush hour, that aforementioned 80kmh traffic (New York and DC got nothing...), only with even less distance between bumpers (hairbreadths). Note that in China testing and graduating with very good scores is crucial. For a month they'll have endless testing, most of them complete bullS trick questions. If they do poorly then in China they have little chance of getting even a mediocre job, there's such a massive population that not everyone is needed, and seniors and experienced people are more likely to be hired. Likewise, colleges are extremely competitive and hard to get into. Doing even slightly poorly may mean you'll be stuck the rest of your life doing very piss poor jobs (few workers rights), maybe 16 hours a day, everyday on a back breaking assembly line. When they graduate with good scores, it's very relieving. Anyways,  they were caught by police, and sentenced to a month of jail. As soon as they were released they were immediately employed by a fairly prestigious racing company to be racecar drivers. Lucky son of a gun.

As I've only been driving for less than a year, I've yet to encounter one, but would probably do nothing to warn others.

Quote from Masiakasaurus on Jun 29^th, 2012, 10:50am:
 
 
Yep. I lived in the Atlanta area for several years. The best commentary on the asinine speed limits in the atlanta area is this: http://www.youtube.com/watch?v=OoETMCosULQ   they add a lot of chaff, but the heart of it is worth saying. I don't think I've ever seen someone do the limit on 285, I've seen 80 and 5 miles an hour a lot more often...