It has always struck me that warning of a speed trap is a violation no different to being an accessory to a crime by warning the bank robber that the police are coming. Certainly very difficult to enforce.
And to take it a step further, what about radio stations that warn drivers over the air of speed enforcement areas. Could they increase the scope of what they perceive as public service to include DUI stop checks?
Living on the Malahat I often flash my lights when there is no speed trap if I see an obvious speeder coming towards me. Without exception, I see the brake lights come on in my rear view mirror.
There are a number of schools of thought about this. On the yes side, go ahead and warn, the idea is to have drivers perceive that police are busy and to slow down. The slow down behaviour can persist and the aim of enforcement, to slow drivers down, is met.
On the no side, I don't like the idea of warning drivers who are doing things that could potentially hurt me and I am against warning of drinking and driving checks. A common tactic now is to position a police vehicle at the chance to exit and checking all those who avoid the check and that would catch the "flashed" drivers, but not those warned by radio, text messages or Tweets. However, it does raise awareness that enforcement is there and you should consider not driving after drinking.
The problem with flashing headlights like this is that technically speaking, it is illegal to do:
4.06 (5) A person who drives or operates a motor vehicle must not illuminate the upper beam of a headlamp if another motor vehicle is within a distance of 150 m from that vehicle, unless the driver has overtaken and passed the other vehicle, so that the high intensity portion of the beam does not strike or reflect into the eye of the other driver.