Is it illegal to warn oncoming drivers of speed traps?

Are the police allowed to fine you for flashing oncoming cars to warn them of a speed trap? I seem to recall that they have to prove the drivers you flashed were actually breaking the speed limit or about to break the speed limit.

Asked on 8 November 2012 by DB, via email

Answered by Honest John
Correct. If they issue a fixed penalty and the driver accepts it, then that's effectively pleading guilty whether he has committed an offence or not. If it went to a proper court, case law (Glendinning) has held it not to be an offence unless the police can prove that the cars the driver flashes were actually speeding. See: www.honestjohn.co.uk/faq/flashing-other-drivers/
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