Motorway signs keep getting bigger

There is a new trend on British roads, and you are unlikely to have noticed it.

Road signs... they're getting bigger.

Everybody understands that motorway signs need to be big. The industry uses the term 'X-height', which in simple terms is the same as a font size, but they get it by measuring the height of a lower case letter 'X' when drawn in the correct font. The rest of the sign, including the space between letters and the size of any symbols, stems from that X-height. The X-height for a typical motorway sign would be 400mm - that's big!

According to the Department for Transport, you can determine exactly which X-height you need from "the 85th percentile speed of vehicles using the road". In very simple terms, we are saying that you should measure how fast cars will actually be going when they see your sign, because you may need to account for people speeding as they approach a dangerous bend, or you could save some money and have a smaller sign if it turns out people are actually driving very slowly (perhaps because they've just come off a roundabout) when they see your sign. This is why roadworks can have smaller signs, and that is one reason why a lower speed limit is needed through roadworks.

As wide as a lane: traffic leaving this small, three-arm roundabout is probably doing 20mph and knows where it's going, but this new sign is ready to shout at them. Image © Google.

This is where the problem unfolds. Motorways, by default, have a speed limit of 70mph. But there are lots of slip roads and link roads where it's simply not possible to get anywhere close to that, even on a clear day. That's where, in the past, you'd want to know exactly how fast people are really going, so you can make a sign that is of the right scale. It would also affect the size of the road markings, and where you place the sensors for the traffic lights.

In the case of a 'start of motorway' sign, they recommend it always have an X-height of between 100mm and 300mm, which offers a bit of variety for different situations. (Past versions used to say that the whole sign face should be 700mm tall, which is actually quite small.)

Calculating the 85th percentile speed takes time, costs money, and requires an understanding of the road sign regulations that is in short supply. So what if you don't bother? You'll be spending more on the sign - much more, because size impacts the posts and foundations - but it's possible that the corners cut here will appear to be worth it, especially if you aren't sure what you should be doing.

So we end up with an increasing number of situations across the UK where the road signs assume that you will be driving past them at 70mph, even when it's not possible. Look out for these giant signs on your next journey.

This roundabout with traffic lights on it is a motorway so technically has a speed limit of 70mph, but you're more likely to be stopped at that traffic light, which looks tiny by comparison. The signs all contain other errors too, includingt the missing arrow in the background. Image © Google.

This is a non-technical explanation written for armchair enthusiasts. Those who are actually making road signs should, of course, refer to official guidance.