Is it illegal to warn oncoming cars of a speed trap? Can you be fined for flashing oncoming cars to warn them of a speed trap? I seem to recall you mentioning that the police have to prove the drivers they flashed were actually breaking the speed limit (or about to break it). Correct. If they issue a fixed penalty and the driver accepts it, then that's effectively pleading guilty whether or not an offence has been committed. 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 drivers flashed were actually speeding.