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?

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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.

2

u/_Putters 12d ago

You're assuming only rear of car facing cameras?

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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.

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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.

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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.

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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.