Safe Speed Forums

ok, apologies for what are probably stupid questions.

Having the misfortune to spend most of my days driving on the M25 i was wondering if the gantry cameras actually work for the variable speed limit?

Also do average speed cameras, such as those currently on the M1 work, as i chug along at 50mph, but some people seem to pay no regard to them at all.

Yes. I believe they are owned and run by real police, not money-making partnerships so their use hasn't been as widespread as it could be. They are only allowed one camera per gantry (guess which lane it is usually in) and I don't think they have too many running at the same time. I guess it takes effort to get up on the gantry to change them out...?
A lot of the gantries have white lines on the road but no boxes for the cameras to go in. I guess this is to scare you and cause more panic braking?


Yes. They read front numberplates so motorbikes are exempt. So is anyone who has a missing/fake/cloned or un-readable front numberplate. I'm not sure what they make of numberplates registered overseas.
Also they are type approved for use on a single lane, so if there is more than one lane and you change lanes between a pair of cameras you are (legally) safe.

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Andrew.

Thanks for that.

So change lanes does actually work, i always thought that was just an old wives tale!

So are you soom to be seen doing 70mph in a 50mph zone covered by average speed cameras?

Nope, just interested to know why some were obeying the limit and some weren't.

Lots of reasons why people might know they can ignore a speed camera.

Unregistered or false plates (especially rife around the congestion charge areas).

Also with average speed cameras and the right GPS software you can make use of any holdup by travelling faster for the rest of the section.

And those using a GPS speedometer will probably be driving at 5 to 10 mph faster than those using the cars speedometer due to the tendency of a speedometer to over read (legally a speedometer can read up to 61mph at a true 50). Add to that the 10%+2 prosecution threshold and you have quite a speed margin between someone with a bad speedometer and someone who wants to push the boundaries.

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Time to take responsibility for our actions.. and don't be afraid of speaking out!

Thats 10%+6.25mph on the true speed I believe, which for a true 50mph would give in the region of 61mph at the maximum deviation.

Be careful though. That's what a speedo that has been approved under the Single Vehicle Approval scheme has to manage. For an EC Type-Approved car it's only 10% + 2.5MPH max. over-read. Also, I got a shock the other day as to just how accurate my 807 speedo was - barely 0.5MPH over-read at 70 compared to two GPS sets! I'd go so far as to say that most modern cars with electronic speedos that take their signal from the ABS sensors won't be anywhere NEAR the upper limits of the tolerance.

No one is saying all speedos are that inaccurate, merely that that is the outer limit permissible, and therefore must be considered.

yep my slk is also only 1mph out up to about 3 figures

Don't forget that tyre wear will increase the reading of the speedo at any specific speed. It can be about 2.5% over-read when tyres are down to 1.6 mm tread.
Be careful about the lane changing with the SPECS units. Since there are often several sets, there is nothing to stop one set being unused and the measurement being taken between, say, set 1 and set 3. If you are really close behind an HGV at the first set, your number cannot be read, so you are exempt until the next measured section.
Foreign vehicles, motor-cycles, vehicles with obscured plates, vehicles with incorrect plates, vehicles with very dirty plates, vehicles with no front plates, vehicles with broken front plates or just partial plates, etc., are all exempt.
One of my rally cars has a very accurate average speed read-out on its Brantz Laser 3 computer. It is surprising how quickly one can travel along the A14 without exceeding an average of 76 mph between SPECS units. If you are held up behind an HGV for a mile or so, you can do really high numbers before you are up to a 70+ average again at the next SPECS unit. And to think, they spent over £2.5million on the SPECS between Huntingdon and Cambridge. What a waste of money!

It definitely works. However you never know which cameras are live and which are dummies. So you would need to be in a different lane for each camera point, because any two camera points could be paired. Therefore the only time this "trick" would be useful is when there are as many different possible lanes to choose from as there are camera points for you to pass. Otherwise you'd have to be in the same lane for at least two different camera points, which could be in a pair. Hope that makes sense.


Would that be automatically dangerous at, say, the small hours in a temporary roadworks speed limit on a 6-lane motorway when there were no workmen, a few cones on the hard shoulder and no other roadworks whatosever?

Have you never seen a roadworks speed limit which you have thought inappropriately low for the road and/or conditions at the time? In my experience many of the most ludicrous speed limits that the country's ever seen are temporary ones. They bring cameras and legitimate speed limits into disrepute, as does being sanctimonious about observing them.

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Paul Smith: a legend.

"The freedom provided by the motor vehicle is not universally applauded, however: there are those who resent the loss of state control over individual choice that the car represents. Such people rarely admit their prejudices openly; instead, they make false or exaggerated claims about the adverse effects of road transport in order to justify calls for higher taxation or restrictions on mobility." (Conservative Way Forward: Stop The War Against Drivers)

It's true that as tyres wear, the speedo will read higher than it did with new tyres, but the legislation doesn't allow for that. If it EVER over-reads, it is ILLEGAL. That's why the tolerance is all the other way and most manufacturers will leave a bit more than 2.5% for tyre wear PLUS whatever other natural variations in the rest of the equipment they would expect so that a random "conformity of production" check on a vehicle off their production line would never get dangerously close to under-reading once that vehicle had worn it's tyres away. That's why I was so surprised at how close to the limit my car was - but maybe they really can control their production process so tightly that they can afford to go right to the wire and ONLY allow for tyre wear!

Isn't that the wrong way around?
To 'over-read', which most speedos do and will do more as the tyres wear, they are reading a higher speed than the vehicle is travelling. Hence you will be going slower than you think. The tolerance is always +something and -nothing. In other words, they are illegal if they 'under-read', but legal up to a 10% over-read. So they start off reading correctly or slightly over the true speed, maybe 5% or so, then read increasingly over the true speed as the tyres wear.

Oh dear! ...and I can't even use the excuse that I posted that in the middle of the night and wasn't really awake! !

You're quite right, that last post was completely the wrong way round and what I SHOULD have been concerned about was whether the speedo was under-reading when the tyres were NEW, not over-reading whe they are WORN!