Motorists who flash their headlights to warn other drivers about a speed camera ahead risk prosecution and a fine of up to £1,000, with police in England and Wales treating the practice as obstructing a constable in the execution of their duty.
While there is no specific criminal offence of flashing headlights to warn others, officers can prosecute under Section 89 of the Police Act 1996, which makes it an offence to wilfully obstruct a constable in the execution of their 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 is one month's imprisonment or a £1,000 fine, with sentencing guidelines allowing for anything from a fine and custodial sentence to a conditional discharge or community order.
Highway Code rules on flashing lights
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 cautions that the horn should be used "only while your vehicle is moving and you need to warn other road users of your presence" and adds: "Never sound your horn aggressively."
Police and CPS position
Police treat the matter seriously. In a local primary school newsletter earlier in 2026, the school cautioned that "several people are warning other motorists" about the presence of a Cheshire Police speed van deployed to monitor traffic speeds outside the school. Officers warned that this behaviour breached 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."
The Crown Prosecution Service, 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 conduct that would constitute resisting or wilfully obstructing a constable in the execution of their duty.
The court case: DPP v Glendinning
In a 2005 case, a lorry driver was observed flashing his headlights at oncoming motorists after noticing a speed camera van on a dual carriageway. Officers witnessed the behaviour, concluded he was deliberately alerting other drivers, and charged him with obstructing police.
The driver was initially found guilty, but the conviction was overturned at the High Court, where judges ruled that concrete evidence of actual obstruction was required rather than merely the potential for it to occur. The High Court determined there was no proof that approaching vehicles were exceeding the speed limit, that motorists had altered their behaviour in response to being flashed, or that police enforcement operations had been genuinely disrupted. Nevertheless, the prosecution makes it clear that police are prepared to take motorists to court for flashing their headlights to alert others to speed cameras ahead.
