Drivers urged to learn meaning of black stripe road sign ‘most don’t know’

You will have seen this — but do you know what it actually means?

It’s easy to take the Highway Code for granted once you’ve passed your test. But years down the line, how many of us can honestly say we remember every road sign and its meaning?

One common road sign features a plain white circle with a single black diagonal stripe running across it. You’ve likely seen it on country roads, dual carriageways and even just beyond roadworks — but do you know what it actually means? According to Ben Raffell at BigWantsYourCar.com, an online platform for selling used cars in the UK, this sign is “far more important than many drivers realise”.

The sign signifies the national speed limit applies. But drivers have gone online to ask why we have the sign rather than just the exact number for the speed limit depending on the road (for example, 60mph or 70mph). One asked: “Does anyone actually know the reason we have national speed limit signs with a diagonal black line on a white background rather than just the number 60 or 70 depending on the road? I’ve looked around and can’t find the history or reason for it anywhere.”

The basic reason the sign shows a black stripe rather than a number is because the speed limit “depends entirely on the type of road and what vehicle you’re driving”, explains Ben. Here’s how it breaks down:

For a heavy goods vehicles (HGV) the national speed limit is 50mph on single carriageway roads and 60mph on dual carriageway roads. The new national speed limits for HGVs came into force in 2015 but soon afterwards government research found that while all HGV drivers consulted were aware of speed limit changes on single carriageways, not all were aware of the changes on dual carriageways.

And Ben stresses that this isn’t a green light to hit top speed: “The sign doesn’t mean ‘drive at the limit’ — it simply means that the previous speed restriction has ended and the default national limit now applies. Drivers should always consider road conditions, visibility, and surroundings. On narrow country roads, for instance, doing 60mph could be incredibly unsafe.”

Unlike numbered speed signs, the national speed limit symbol relies on drivers remembering the associated rules — something that can fade over time.

“It’s not as intuitive as a clear 30 or 40 in a red circle,” Ben says. “That black stripe assumes you know what’s expected of you, which is fine in theory, but in reality, many drivers forget the details years after passing their test.”

He says this gap in understanding could potentially lead to dangerous driving, especially in areas where road conditions don’t safely allow for high speeds, and advises drivers to take a quick refresher.

“Revisiting the basics of the Highway Code is a great idea for any driver,” says Ben. “It’s not just about avoiding fines; it’s about keeping yourself and others safe on the road.”

The national speed limit sign is far from the only common sign on UK roads that drivers might not know the meaning of — there’s also this ‘blue and red cross’ road sign ‘everyone should know’ the meaning of. And drivers have also been confused by this huge road sign, saying they’ve “no idea what it means”.