IT is a simple act of common courtesy shared among most drivers, but by trying to prevent fellow road users from being caught speeding, good Samaritans could be at risk of being fined themselves.
Flashing your headlights to warn other drivers of a speed trap on the road ahead could attract fines of up to £1,000 if caught by police.
As part of the Highway Code, drivers are instructed to use their headlights "only to let other road users know that you are there," and not to attempt "to convey any other message".
According to the Department of Transport, flashing your headlights to warn of a speed camera or roadside police speed check would be in breach of Rule 110 and 111 of the Code, which could stand up in a court of law as evidence.
Conviction for such an act, however, comes from a breach of section 89 of the Police Act 1996, which dictates that it is an offence to "wilfully obstruct a constable in the execution of his/her duty".
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Police are able to charge drivers with this criminal offence if they perceive the person has obstructed their ability to conduct speed checks on other motorists.
Obstructing a police officer is an offence capped at level three on the fine scale, with a maximum penalty of £1,000.
In the most extreme cases, obstructing a police officer can attract a maximum of one month's imprisonment, however it is highly unlikely a driver would ever be given a prison sentence for flashing their lights.
Read about other hidden UK motoring offences that carry a fine for drivers
Laura Newton, motoring law expert at Rothera Sharp, explained: "I’ve certainly seen several notable cases of this type recently and drivers should be paying particular attention to those cases that have been challenged in the High Court.
"The Court made it clear that it will be only classed as an offence if the prosecution can actually prove that those being warned were speeding or were likely to be speeding.
"But I think people need to realise just how serious this can be treated in terms of the maximum penalty though.
"The headlight warning flash has become commonplace in driving but motorists need to think twice and consider the consequences. Is it really worth the risk?"
In January 2011, 64-year-old Michael Thompson was convicted of wilfully obstructing a policewoman in the execution of her duty after a trial at Grimsby Magistrates' Court, following an incident where he had flashed his headlights to warn drivers of a police speed trap.
He was slapped with a £175, and ordered to pay £250 in court fees, as well as a £15 victims' surcharge.
Neil Greig, IAM RoadSmart director of policy and research, said: “It is illegal and unnecessary behaviour.
"Speed traps should not be hidden as they are there to slow down traffic not issue tickets, so anyone aggrieved by the way a camera is being used should take it up with their local safety camera partnership rather than taking the law into their own hands."