Little-known rules could see people driving under 30mph hit with a £1,000 fine if they do one thing wrong.

The speed cameras that line the highways and byeways of the UK are designed to keep speeding motorists in check. They can be found on all types of roads, where limits vary from 30mph up to 70mph.

Road users will also know how many different types of camera are out keeping a watchful eye on them. Whether they're fixed on the roadside, or atop motorway gantries, or even hidden in the back of a police mobile speed camera van.

As it stands, motorists caught speeding face a minimum £100 penalty, and three license points. One type of camera can attract more attention that others. These are mobile speed cameras, which can suddenly pop up on the side of a road for a few hours.

There's been a long standing habit among drivers to warn other approaching motorists of the mobile camera by flashing their lights. This is to stop other drivers getting snared by the camera.

However, you could actually be caught breaking the law by doing this, despite your good intentions. Using your lights in this way is actually not permitted in the Highway Code and also carries a separate punishment under UK law.

As per the UK government website, rule 110 of the Highway Code 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."

Flashing your headlights in this scenario could also see you breach section 89 of the Police Act 1997. Under this law, it's an offence for members of the public to wilfully obstruct a police constable in their execution of duty. Doing so carries a maximum penalty of a £1,000 fine.

So even if you pass through a mobile speed camera below 30mph on a residential road, you could end up with a fine if you're caught alerting other drivers with your headlights.