Flashing your headlights to warn other drivers about a speed camera could land you with a fine of £1,000 and a criminal record, as police in England and Wales treat the practice as obstructing a constable in the execution of their duty. While many drivers believe they are being helpful, the law takes a different view, and prosecutions have already been brought.
What the Highway Code says
Highway Code Rule 110 states: "Only flash your headlights to let other road users know that you are there. Do not flash your headlights to convey any other message or intimidate other road users."
Rule 112 also warns that the horn should be used "only while your vehicle is moving and you need to warn other road users of your presence," adding: "Never sound your horn aggressively."
The legal basis for prosecution
There is no specific criminal offence of flashing headlights to warn others, but police in England and Wales can prosecute under Section 89 of the Police Act 1996, which makes it an offence to "wilfully obstruct a constable in the execution of his duty." Because officers are actively trying to catch speeding motorists, warning drivers so they temporarily slow down is legally interpreted as interfering with police duties.
The maximum penalty for this offence is one month in prison or a fine of £1,000. Sentencing guidelines say punishments could range from a fine and imprisonment to a conditional discharge or community order.
Police do not take the matter lightly. In a weekly newsletter from a local primary school earlier in 2026, the school warned that "several people are warning other motorists" about the presence of a Cheshire Police speed van that had been placed to monitor traffic speed outside the school. The school said police were warning that this behaviour contravened both Highway Code Rule 110 and Section 89 of the Police Act 1996 and that they "could charge anyone who appears to be obstructing their ability to conduct speed checks on other motorists by warning them," adding: "The maximum fine for obstructing a police officer is a penalty of £1,000."
Official guidance and a key court case
The Crown Prosecution Service (CPS), in its official guidance on Public Justice Offences, lists "giving a warning to other motorists of a police speed trap ahead" as an example of an action that would count as resisting or wilfully obstructing a constable in the execution of their duty.
The case of DPP v Glendinning from 2005 involved a lorry driver who was seen flashing his headlights at approaching drivers after spotting a speed camera van on a dual carriageway. Police believed he was deliberately warning drivers and prosecuted him for obstructing police.
Initially, the driver was convicted, though the conviction was quashed at the High Court, where judges said there needed to be evidence of actual obstruction, not just the possibility that obstruction may have occurred. In this specific case, the High Court said there was no evidence that approaching vehicles were speeding, that drivers had changed their behaviour as a result of being flashed or that police enforcement activity had genuinely been disrupted.
The prosecution leaves no doubt that police will take drivers to court for flashing their lights to warn of speed cameras up ahead.
