A motorist has sparked a surprising debate online after claiming that “99% of drivers” no longer recognize one of the U.K.’s most common road signs.

Before anyone can take their practical driving test, they must pass the theory portion — learning road laws, road markings, and safety rules. Yet despite this being a universal requirement, it seems many drivers forget the details soon after passing.

The sign in question is simple: a round white circle with a bold black diagonal stripe. It appears on roads across the country, but judging by the reactions online, its meaning has become something of a mystery.

One Reddit user wrote, “I’m convinced 99% of drivers don’t know what this sign means,” prompting a flood of lighthearted guesses.
Some joked it marked a “national drifting zone,” while others teased that it was the sign allowing BMW drivers to “skip indicators and go 100mph.” A few people insisted it meant “minimum speed 80,” while others replied, “maximum 40, in reality.”

Behind the humor, though, many genuinely seemed unsure.

The real meaning is far simpler — and more important. The sign indicates that the national speed limit now applies on that stretch of road.

According to the RAC, “A white circular sign with a single black diagonal stripe tells you the national speed limit applies. It overrides any previous temporary or lower limits.”

What that limit is depends on the type of road and vehicle:

• Built-up areas (with street lamps under 200 yards apart): 30 mph for all vehicles

• Single carriageway:
  • 60 mph for cars
  • 50 mph for vans or vehicles towing trailers/caravans

• Dual carriageways and motorways:
  • 70 mph for cars
  • 60 mph for vans or vehicles towing trailers/caravans

However, experts stress that even when the national speed limit applies, conditions don’t always make that speed safe. Drivers are still expected to use good judgment — slowing down for weather, traffic, or road conditions.

In a way, this small debate serves as a reminder: even the simplest rules can fade from memory, and revisiting the basics is part of keeping ourselves and others safe on the road.