Before taking your driving test in the U.K., you must pass a theory test, covering road laws, signs, and a hazard perception section. Yet, despite its importance, many drivers seem to have forgotten the meaning of one common sign. This particular sign is round, white, and features a thick black diagonal stripe. Found across roads in the U.K., it indicates the national speed limit, but online, motorists have shown remarkable confusion.

On Reddit, one person claimed, “I’m convinced 99% of drivers don’t know what this sign means.” Comments ranged from joking guesses like “National drifting zone” and “go as fast as you want” to incorrect interpretations of minimum or maximum speeds.

So what does it actually mean? According to the RAC, the white circular sign with a black diagonal stripe signals that the national speed limit applies from that point onward, overriding any previous limits, such as temporary roadworks.

The national speed limit varies by location: in built-up areas with street lamps no more than 200 yards apart, the limit is 30 mph for all vehicles. On single-carriageways, cars may drive up to 60 mph, while vans and vehicles towing a trailer are limited to 50 mph. On dual carriageways and motorways, cars can travel at 70 mph, and vans or towing vehicles at 60 mph.

Even with the national speed limit in place, drivers are reminded to use common sense and adjust their speed for safety. So next time you see the white circle with a diagonal black stripe, remember—it’s the national speed limit, not a free-for-all.