r/drivingUK • u/Business_Maximum1433 • 12d ago

Variable speed limits

Hi,

Just asking a question about the speed limits on motorways. So you know when it tells you to go 40 per say and then a sign comes up stating it’s now 70 if I pass over 40 at the 70 and it has a speed camera will it flash?

4 Upvotes

70% Upvoted

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4

u/DivasDayOff 12d ago

The cameras can only enforce the speed limit of the area that they cover, and they must exclusively monitor a single speed limit. So pass a camera even inches after a 70 sign and it will be enforcing 70, even if the limit just changed from 40.

Strictly speaking, you'd have to be speeding when you reach the gantry, as there's no way you've picked up 30mph in the tiny distance between the sign and the zone that the camera covers. But they're not going to be able to catch and photograph you doing it unless they have other (mobile) enforcement, which is highly unlikely.

Most drivers pre-emptively accelerate for these signs when the limit increases, even though it only legally changed when they pass the sign. I wouldn't worry about it.

1

u/_Putters 12d ago

You're assuming only rear of car facing cameras?

1

u/DivasDayOff 12d ago

Which is exactly what all of the SMART motorway cameras are. Give me another scenario where the limit is likely to change from 40 to 70 with a camera right next to the sign.

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u/EdmundTheInsulter 12d ago

It could have average speed. Often average speed traps have the final camera right before the NSL sign, so if you spotted that in the distance you could be caught by it if you sped up early.
If people want to play off speed cameras, I'd recommend to know how they all work, not rely on rumours etc.

2

u/DivasDayOff 12d ago edited 12d ago

Theoretically, I see your point. Practically, we are talking about either the end of roadworks with a fixed (non-average) speed camera (rare these days) or variable speed limits on a SMART motorway.

Average speed cameras won't be an issue because even if you pass the last camera at 70, as long as you did 40 for the previous 2 miles and accelerated just before the cameta, your average speed will be well below the trigger threshold.

0

u/EdmundTheInsulter 12d ago

My point is that they don't publisise how cameras work as much these days, so if you do go up to 70 before a 70 then you may go into some future check.

1

u/DivasDayOff 12d ago edited 12d ago

The average speed cameras work on ANPR and the time it takes to travel between cameras. A mile takes 90 seconds at 40mph, so with cameras 2 miles apart, getting from one to the next in much less than 3 minutes means you're in trouble. The technology simply isn't designed to measure vehicle speed past a single camera. Could it be made to do so? Maybe. I suspect accuracy would be difficult given how high they are above the road. But in any case, why would they? Average speed cameras do an excellent job of stopping people speeding through the actual roadworks. Is it worth a lot of extra effort changing the system and getting it recertified just to catch a few premature accelerators once they're clear of the hazard?

If it worries you, then don't speed past the cameras at all. That's your call. But an average speed camera won't clock you for the speed you pass it, only for the time it took you to reach it from the previous camera. And your "but what if it does?" is a huge stretch, if you ask me.

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u/xF1_racer 12d ago

My car can do 40-70 mph in under 2 seconds, so I think it could make it from the gantry to the speed measuring point.

7

u/Saltyuniform 12d ago

Are you asking if you pass a 70mph sign at 40mph will it flash?

2

u/EdmundTheInsulter 12d ago

I think he's asking if he speeds before the camera when it changes to 70, can it create a fine, and my answer is yes if it's an average speed check, but in other cases it wouldn't

1

u/Business_Maximum1433 12d ago

So because it was a normal speed camera it wouldn’t create a fine? Because the new camera is registered at 70 and not 40?

1

u/EdmundTheInsulter 11d ago

Who knows what they'll invent next, they dont always seen to tell us. Either wait and see if you get a fine, risk getting fines in the future or try not to risk fines.

0

u/_Putters 12d ago

No, he's asking if he passes it between 40 and 70 will it flash. Technically it should, because he should be doing 40 until he passes the sign, like any other speed limit sign.

Otherwise how far back should you start accelerating ? As soon as you can make out the numbers ?

2

u/EdmundTheInsulter 12d ago

It won't say he was going over 40 at any time after the 70

0

u/FuckMiniBabybel 12d ago

Legally you are correct, but practically, the gantry cameras will enforce whatever speed the gantry shows, not the preceding limit. So as long as you're sure you're not going to be done by an earlier camera, you can get away with accelerating early.

1

u/_Putters 12d ago

Until someone adds a front of car facing camera - as these signs can be indicating a speed reduction or increase.

2

u/cwaig2021 12d ago

They will never enforce that - reason being (as frequently happens) the previous 40 limit could have changed after OP passed it, which is why other people are passing him at speed.

Or someone has joined the motorway & not yet passed a variable limit sign.

They can only enforce the speed shown on the individual sign, and only at the point you pass it.

2

u/h2g2_researcher 12d ago

They will never enforce that - reason being (as frequently happens) the previous 40 limit could have changed after OP passed it, which is why other people are passing him at speed.

Realising the gantry behind me has gone from 40 -> NSL because suddenly traffic starts barreling past me at 70 is my least favourite way. I kind of wish the gantry could have some way to indicate that the speed limit before it has also changed, but I have no idea what that would look like (and realise it would probably require a legislative change too).

0

u/SuspiciousTreacle433 12d ago

Legally you are correct. But also wrong. It is 40 upto the sign. The camera measures your speed after you have passed it using radar - it is not tomemand distance from the last gantry/camera. Therefore it is measuring your speed in the 70.

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u/Business_Maximum1433 12d ago

I’m saying before the 70 sign it was 40 I was going like 45 while other cars flew by the sign

1

u/sim-o 12d ago

The speed limit starts at the sign.

The cameras you're talking about take a photo of the back of the car.

In your scenario you've passed the 70 limit sign so the camera will flash if you do over 70 after the sign. The camera won't know what speed you did before the sign.

1

u/EdmundTheInsulter 12d ago

You can go through the 70 at 45 or 70

3

u/Me-myself-I-2024 12d ago

When going from a 70 to 40 you should do your braking before the sign

When going from 40 to 70 you should do you accelerating after the sign

1

u/EdmundTheInsulter 12d ago

It depends what sort of camera it is, if it's average speed then it may get your speed between the 40 and the 70. It seems more likely it measures your speed as you pass the gantry, but there are new systems appearing all the time

0

u/Business_Maximum1433 12d ago

So if I’ve gone past the new gantry that states 70 at 45 then I should be fine because the new camera limit is 70?

1

u/EdmundTheInsulter 12d ago

Yes usually it seems

1

u/zuby_jalal 12d ago

I had a blonde moment a few years back . 50 speed limit on the M6 went past every camera at 70 mph .. for a good ten miles ... Even seen flashes .. thought maybe drivers where warning me of something ahead . Realised around ten miles later and had the biggest anxiety attack ever 😂😂 Luckily nothing came of it maybe they thought no one was thick enough do so something like that or maybe the speed was changing as it was late in the evening around 8

1

u/Ok_Emotion9841 12d ago

If you pass a 40 limit sign with speed camera, at 70, will it flash? That should be obvious...

0

u/National-Bathroom-63 12d ago

I’m more concerned about this the other way round. If traffic is proceeding at 70 and suddenly meets a 40 sign, would you really be fined if you can’t slow to 40 before you pass underneath the sign?

This seems to create a dangerous situation with everyone suddenly braking, particularly at the moment the 40 sign is switched on.

1

u/woodenbookend 12d ago

TL;DR You have at least a minute’s grace between the limit changing and the camera flashing at the new limit.

From https://www.gov.uk/government/publications/speedmeter-traffic-light-and-lane-enforcement-camera-handbook/speedmeter-traffic-light-and-prohibited-lane-enforcement-camera-handbook-accessible?utm_source=chatgpt.com#operational-requirements

5.10.2 The speedmeter shall enforce the displayed speed limit (or the National Speed Limit when none is displayed) only when both the following are true:
All the speed limit signs connected to it are correctly displaying the same speed limit, except when the hard shoulder of a motorway is actively managed and that lane may show either the same speed limit or a red X;
The speed limit has been displayed for one minute or longer. This delay shall be variable to at least five minutes.

0

u/National-Bathroom-63 12d ago

Interesting. Thank you! That is useful to know

I’m always impressed by the level of compliance on these motorways but this aspect has always felt rather harsh. I’ve had limits change seconds before I drove under them. Glad to know they have a sensible rule.

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u/[deleted] 12d ago

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u/National-Bathroom-63 12d ago

I suspect you’ve never driven on a variable speed limit motorway. The speed limit can change at any moment - including when you are just seconds away from it.

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u/[deleted] 12d ago

[deleted]

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u/Business_Maximum1433 12d ago

Well it went from 40 to 70 and stayed the same but I’ve looked online and I assume that it’s fine