If I’m driving on the M1 motorway in the UK, and the caution speed (smart motorway) is lowered because of some congestion ahead perhaps, say 60 mph, will the camera trigger if I'm going at 68mph? Or will it only trigger if I’m going above the national speed limit, i.e. 70mph?

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    @Mahmood smart motorways can and do have average speed enforcement, as can other motorways Nov 4, 2023 at 12:27
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    This question would have been easier to answer if the reduced speed limit was 50 rather than 60. With 60 you might just get away with 68 [depending on local authority]. If it had changed to 50, you'd be in for a fine, no doubt. I doubt the OP is referring to averaging systems, they are advertised separately. Also, your average gets a whole lot more leeway in changing conditions. Most authorities operate 'benefit of the doubt' except notable ones like Northamptonshire.
    – Tetsujin
    Nov 6, 2023 at 9:37
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    Why are people using miles on a British road? I thought the whole world except USA and Liberia used kilometers. And Myanmar but they switched. Nov 6, 2023 at 18:36
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    @Harper-ReinstateMonica all of our speed limits are in miles per hour, and British people would typically always talk about driving distance in miles too. In general conversation, km would typically not be understood very clearly while people have an intrinsic "feel" for how far a mile is. Would it surprise you to learn that we also drink in pint measures too, not litres?
    – roganjosh
    Nov 6, 2023 at 18:47
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    @Harper-ReinstateMonica - Why are people using miles on a British road? Simply because road distances, and speeds, are still measured in miles. Google could have answered that. Everything but roads & beer is in metric - though we buy fuel in litres, we still use miles per gallon. Beer in pubs, but not in pre-packaged bottles/cans, comes in pints. We never adopted the €uro as currecy either - we can be stubborn that way [for good or bad].
    – Tetsujin
    Nov 6, 2023 at 19:52

4 Answers 4


It's probably safe to assume that on a smart motorway the cameras are as smart as the variable speed limit. Note that many of the cameras don't measure your speed, but record the time that you passed, which will be networked to the next camera to check your average speed.

But anyway, the true speed is usually less than what your speedometer says. The UK Construction & Use Regulations require that a speedometer may not under-read at all, but may over-read by up to 10% (See Hansard). So manufacturers typically calibrate the speedometer so that it over-reads by 5% to give them the greatest flexibility of being within the law.

So if the speedometer reads 68 mph, your true speed might be 64-65 mph. But perhaps your satnav was telling you the 68.

And then there is the leeway that the speed cameras allow. Advanced Driving UK says:

Even though you must adhere to all speed limits at all times, many Police forces have some tolerance for their speed cameras. For those that do, the tolerance is usually 10% over plus 2 mph.

This means that in a 60 mph restriction some police forces may allow you to travel at (60 + 6 + 2) = 68 mph. They say some forces and I am told that in Scotland there is no leeway allowed.

If they do allow the leeway, and your true speed was 65 mph, you may "get away with it".

  • Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Travel Meta, or in Travel Chat. Comments continuing discussion may be removed.
    – Willeke
    Nov 6, 2023 at 18:48

There are two types of display you can find:

  1. The ones that look like a proper speed limit signs, just digital:

Mandatory speed limit signal

These are mandatory speed limit signs and are considered the same as a "real" speed limit sign, so you need to take them into consideration, see the other answers on how they are measured and what leeway you might get away with.

  1. The ones that are only yellow numbers:

Advisory speed limit signal

These are only advisory, e.g. you should reduce your speed if it's above this value, but they are not measured and going over it will not get you a speeding ticket.

  • Do you have a source for that? Is the difference in presentation not just a matter of generation of the displays?
    – jcaron
    Nov 6, 2023 at 7:25
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    The first image is from a smart motorway where there limits are not only for safety but for congestion management. These are always camera-enforced. You can see the speed camera symbol to the right of the gantry. The second is from a "traditional" motorway - these are for safety only (though since congestion causes a safety issue they are usually applied in the case of congestion anyway). I can't corroborate that they are only advisory but they are frequently ignored (people generally slow down but not by that much) and I've never heard of anyone being prosecuted for that.
    – Rodney
    Nov 6, 2023 at 9:29
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    @jcaron - this covers compulsory vs advisory limits - drivingtesttips.biz/advisory-mandatory-speed-limit-signs.html - the 'old' advisory limits on motorways were if not because of, then a direct result of the cameras being 'dumb'. They were set to 70 [-ish, the ones on the M25 wouldn't go off at 85] & couldn't be changed without climbing up the gantry. Modern ones are switched with the same data network the signals use.
    – Tetsujin
    Nov 6, 2023 at 9:31
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    It’s just as well the advisory limits are only advisory — at least around Glasgow it’s quite common to see wildly different limits on successive gantries! Nov 6, 2023 at 14:05
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    Indeed, the absence of a red ring on the speeds in the second photograph indicate the speed given there is advisory. These gantries are normally controlled by highway control centres in response to incidents or congestion detected ahead. The Traffic Signs Regulations and General Directions 2016, section 10, part 2. The advisories are given in section 11, part 2. legislation.gov.uk/uksi/2016/362/contents/made ... the advisories, given they can be activated manually, can sometimes be erroneously left active some time after they are no longer required! Nov 7, 2023 at 16:11

You should assume it will trigger at the displayed speed limit. From RAC What are the rules on a smart motorway?

The same laws and sentencing applies for speeding on a smart motorway, but with more cameras on smart motorways and variable speed limits, motorists typically have a much higher chance of getting caught and fined for speeding.

This is further fuelled by the fact that many drivers do not realise cameras on smart motorways that enforce variable speed limits can still catch you travelling over the national speed limit, when a variable limit isn't in place.

Highways England has stated that: "If no special speed limit is displayed then the national speed limit applies. Speed cameras are in operation on smart motorways. If you don’t keep to the speed limit, you may receive a fine."

  • Thanks, @traveller. Really helpful.
    – Mahmood
    Nov 4, 2023 at 12:29
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    @PeterM There’s no national minimum speed, however there are certain roads that will display the slowest speed that you can drive. moneyexpert.com/car-insurance/uk-speed-limits-explained I interpret ‘to’ in the Highways England advice in my answer as ‘within’.
    – Traveller
    Nov 4, 2023 at 15:41
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    @PeterM British English usage :-)
    – Traveller
    Nov 4, 2023 at 22:22
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    I would interpret 'keep to' the speed limit to mean 'obey' the speed limit. Nov 5, 2023 at 17:11
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    @SteveMelnikoff why shouldn't they? The systems know who the registered keeper is (who gets the ticket), and the class of vehicle. Nov 5, 2023 at 21:55

If the motorway is 'smart' - indicated by compulsory speed limits above each lane [ie there is a red circle around the number] then the cameras are also equally likely to be 'smart' & will change their trigger speed based on the posted speed limit.

From Advisory And Mandatory Speed Limit Signs

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These small cameras are 'smart'. You should be able to see the cameras as you approach. They must be painted bright yellow…

enter image description here

… though sometimes even if the letter of the law is obeyed, they get a bit sneaky. This one is very hard to spot on approach, as it sits a couple of meters the far [driving] side of the gantry, rather than directly alongside.

enter image description here

Based on observation over many years, if the speed limit changes, then you are allowed approximately 5 seconds to comply. Many a time I've seen the first few past the camera get flashed just after a change. I have no idea whether those few actually got tickets, or whether a human decided against it afterwards.

One thing to point out is that not all gantries have cameras & not all cameras are on gantries, some are stand-alone at the side of the road. They can capture traffic on all 3 or 4 lanes from that vantage point.

The older 'dumb' cameras do not change when the speed limit chnges. These are usually on motorway sections that haven't yet been updated to 'smart' status, and are usually accompanied by the advisory speed limit signs [because they can't be practically enforced]. Some of them used to flash once not twice as a warning, if you were over but not by enough for a ticket. This seems to no longer be the case.
You can just spot these on approach [though Google Streets seems to not be able to catch them from their viewing angle], as large grey boxes on brackets, hanging on the far side of a gantry. You cannot rely on absence of cameras on the opposite carriageway meaning corresponding absence on your side.

Picture looking back…
enter image description here

Picture showing gantry & brackets. You can usually see these as you get close - some still have the brackets but no cameras.
enter image description here

Some of them wouldn't even trigger at 85mph [whatever the speed limit said]. Few of them seem to be active at all any more. Many of the early ones were loaded with actual film which required changing periodically. Highly impractical in this day & age, but it's easier to leave them up as a visible deterrent than take them all down.

Vehicle speedometers are allowed to read high, but not low - so in all possibility, you were not going quite as fast as you thought.

Practically, at 68 in a 60 zone, you are borderline. Some authorities allow you to do speed limit + 10% + 2mph. Some do not. Few authorities actually publish their limit allowance, so much of this is hearsay. Scotland & Northamptonshire are notoriously tight on their limits. There may be others.

If you triggered the camera, you should have noticed the double camera flash, one second apart. Just because you triggered it does not necessarily mean you will get a ticket. It may have been someone else who triggered it, so you get away with it. They may decide you were so close to their allowance that it's not worth a ticket. The only way to know for certain is keep watching your post box.

From comments…
Anyone not familiar with the capabilities of the latest UK camera network should look up "ANPR" (automatic number plate recognition). Different types of camera can track where and for how long you park [retail parks etc], whether you travel within specified zones [ULEZ, Congestion Zone], traffic light, bus lane & yellow box violations; whether your MOT, tax & insurance are up to date; average speed along road segments…
Smile, you're on camera wherever you go.
The motorway cameras are actually primitive by comparison.

  • 1
    Thanks, @Tetsujin , your last para is exactly describing my situation :D. Fingers crossed.
    – Mahmood
    Nov 6, 2023 at 10:49
  • If you ever drive abroad you will not be warned as much for speed cameras. Most are not announced at all.
    – Willeke
    Nov 6, 2023 at 11:24
  • Sure - but when I don't know the roads, I stick to the limit anyway. I've driven in much of Europe & the US… carefully, and used to do 1,000 miles a week in the UK. When I know where all the cameras are, then it's a fun game to play ;) [I already knew where to look for each of these camera types on Google Streets, because I've passed them all a thousand times.]
    – Tetsujin
    Nov 6, 2023 at 11:36

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