Before taking your driving test in the U.K., you must pass a theory test.

Motorists must learn about the laws of the road – including road signs – as well as take the hazard perception test.

Despite this being mandatory, it seems that most drivers have forgotten what one common sign means.

Found on roads all across the UK, this particular road sign is round and has a white backdrop with a thick black diagonal stripe through the middle.

It’s a road sign that every driver has to be aware of – yet it seems a lot of people do not have a clue what it means.

Posting on Reddit, one person says: “I’m convinced 99% of drivers don’t know what this sign means.”

People have taken to the comments to provide their own guesses, with one person joking: “National drifting zone.”

Someone else says: “National speed limit, but what that limit depends on what you’re driving. For example, if you’re driving a BMW, it’s 100mph and excuses you from using indicators.”

Another person pens: “Oh, that’s the ‘go as fast as you want and overtake on a bend if you feel like it’ sign!”

One user comments: “It means ‘minimum speed – 80,’” leading to someone responding: “In my experience, it’s ‘max 40.’”

“Flat out is what I see,” insists a fourth.

“I’m pretty sure it means you’re driving in the countryside,” jokes another.

So if you are wondering what the sign actually means, it is an indicator to let you know you can drive at the national speed limit.

According to the RAC: “A white circular sign with a single black diagonal stripe through it tells you that the national speed limit applies on the upcoming stretch of road.

“It supersedes any previous speed limit signs you may have had to adhere to, such as passing through temporary roadworks.”

However, the national speed limit depends on the area you are in.

In a built-up area – a road where street lamps are placed no more than 200 yards apart – the speed limit for all vehicles is 30mph. On a single-carriageway, the national speed limit is 60 mph for cars.

However, for vans or vehicles that are towing a trailer/caravan, it is 50 mph.

Similarly, on a dual carriageway and motorway, the speed limit is 70 mph for a car or 60 mph for vans and vehicles towing a caravan/trailer.

“Of course, even when the national speed limit applies, it might not always be safe to drive at that speed, so use your common sense on this,” the RAC warns.