Would you say that using an app to avoid speed traps is obstruction of justice? What about radar detectors?
For a start, in the U.K. it's illegal to use radar detectors [Edit: this first is apparently false; they publically stated that they were going to, but never changed the law. Other countries apparently do ban them, according to wikipedia.], and people have been given criminal convictions for "Flashing their lights" in order to warn incoming motorists of a speed trap.
I'd be interested in a link to the "flashing lights" case too.
RE: Headlight flashing, I can only find sources from a few years ago - http://www.bbc.co.uk/news/uk-england-humber-12823922 was the one I remember hearing about, but several other instances seem to have taken place; i.e. http://www.telegraph.co.uk/news/uknews/law-and-order/9324722...