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Mobile speed cameras, how do they work?

Discussion in 'Motoring Forum' started by the_fall_guy, Nov 27, 2006.

  1. the_fall_guy

    the_fall_guy Member

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    This morning on my way to work, I was driving along the motorway which marges with another motorway. As I was about to join the new motorway, I noticed a police van with a mobile speed camera sign on the back. I was alongside (i.e. parallel and not in the same lane) this van for a couple of secs and doing approx 57mph in a 50mph zone.

    Can the camera get you from this angle? Is there any way of knowing they have caught you speeding?
  2. mjcairney

    mjcairney Active Member

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    If you were running alongside him, I can't see how the camera would see your number plate. Anyway, it's a very sad day if anyone gets done for 57mph in a 50mph limit - I know it happens occasionally, but don't believe it's common practice.

    As for knowing for sure, you must be notified with 2 weeks of any charges, so unfortunately you'll need to wait that long before being 100% sure that you're in the clear.

    Fingers crossed.

    Best of luck.

    Cheers,

    Martin.
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  3. the_fall_guy

    the_fall_guy Member

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    Cheers that man.

    PS I am just down the road from you in Kirkintilloch!
  4. Dave

    Dave Well-Known Member

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    Don't worry about it, it's normally 10% of the speed plus 3mph they start to prosecute at and you'll probably find the speedo in your car reads over by 3 mph anyway.
  5. terawua

    terawua Guest

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    Quite right, when checking the speedo in my car I found that at a GPS registered 70 mph, the speedo was reading 74mph. The result, I get places a couple of minutes earlier nowadays! :D

    If you want to read more about it, then take a look at www.pepipoo.co.uk as they have many articles about speed cameras etc...
  6. sinizterguy

    sinizterguy Member

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    I have noticed that with TomTom gps units, the speed reading on the GPS is always a little bit lower than the car - I have noticed this in about 5 cars, couple of different makes. Dont know if the speedo on cars is intentionally set a little higher - but it probably helps to reduce unwanted speeding tickets.
  7. mjcairney

    mjcairney Active Member

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    Used to live in Kirkintilloch myself - then one day, in a moment of madness, we bought our current house in Airdrie - love the house but can't say the same for Airdrie.

    Cheers,

    Martin.
  8. mjcairney

    mjcairney Active Member

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    By law, the speedo cannot be slow, and therefore they are all set to be fast.

    Cheers,

    Martin.
  9. eddieo

    eddieo Member

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    In simple terms:

    They read your number plate, to find out if you're an everyday tax paying kinda person with a likely roadworthy car and that you have money.

    Their aims (as always) are to make you poor whilst fattening the treasury, so new pointless schemes and get-in-the-way jobs can be created.

    A licence... to print money!
  10. mjcairney

    mjcairney Active Member

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    God, and I thought I was cynical - I'm a learner by comparison. :eek:

    Not that I disagree with you.

    Cheers,

    Martin.
  11. Geege

    Geege Active Member

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    A camera in a mobile speed camera van would simply record the speed of the vehicle at a set point as it past. This is calculated obviously by the distance you travel in the time travelled. This would have to be calculated as the vehicle travelled away or towards the van, as the van was static (parked on a verge or layby). It couldn't calculate the speed of an object travelling parallel, and especially when the van itself was moving.

    Other cameras calculate over a longer distance (e.g. 200m) for an average speed. The camera in a mobile speed camera van, wouldn't "flash" recording the picture. You would have to wait for the N.I.P to be received within 14 days.

    In general police allow 10% + 2 mph over the limit to initiate prosecutions, Static speed cameras are less forgiving as far as I am aware.

    Hope this helps.
  12. Abootnoo

    Abootnoo Guest

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    In terms of "how does the camera work" - it's dopler radar. The camera bounces radar signals off you as you approach it. It measures the delay between the signal being sent and the echo coming back - this tells it the precise distance.

    The unit sends these signals in rapid succession over a measured cycle, so as each echo returns it knows how much closer you got over the period it was measuring.

    Hey presto, they know how fast you were going.

    And as paul-gilmour quite rightly said, it can't measure the speed of a vehicle travelling parallel, as it is neither moving towards or away from the camera.
  13. dan653

    dan653 Member

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    in addition to the above, depending on the van/unit the also take 3 photos or video of the target; one distant front photo, one close front photo, a side on or acute angle pic as you drive past

    some areas now want to also take a photo of the rear, to afaik provide more evidence to prevent taking other peoples points and i guess ultimatley govern traffic in both directions simultaneously (and possibly to see if your brake lights are working?:D)

    also i think they want to integrate with the dvla/vospa license plate recognition cameras which automatically link to the new ownership/insurance/tax/mot databases
  14. alfiedog1978

    alfiedog1978 Guest

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    [​IMG][​IMG]
    [​IMG]
    Does anyone know what this camera is. Does it flash or not. Think I got caught with one yesterday but cannot find any detail about it. Speed camera van was out at 2030. Very stange time. I was doing 46 in a 40.
  15. eric pisch

    eric pisch Active Member

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    yes it is

    its illegal for car makers to sell a car that under displays the speed, ie 50 when doing 51 but they can go as far out as they like the other way so showing 50 when doing 30 is fine

    they also have to allow for tyre wear, the reduction in rolling radius does affect the speedo reading

    as for the 10% + 3mph, dont bank on it any more, that was only a guide line and since the new tougher penalties for speeding came in place in aug/sep 08 alot of the safety camera partnerships choose to ignore this and often do ppl for doing 3mph over the limit

    as to how it works depends on the van, many have a guy sitting in the back with a laser gun hooked up to a pc to record a movie / take stills of what hes doing

    sometimes they have a portable unit as well often at the front to catch ppl who slow down for the van and speed up (sometimes they position the mobile unit and then hide the van) these normally use radar. Alot of places have fixed sites they can plug a camera into (3 wires buried in the road, u can normally see the black lines close together) which use magnetic induction

    oh and FYI cameras have to be type approved by law and some of the cameras they have used recently are not and the case is currently in the courts, so if u get done this is worth checking
    Last edited: Oct 10, 2009
  16. setjellyfish

    setjellyfish Member

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    The feds don't use old fangled Doppler radar any more they use lasers guns with video equipment attached to it, the :censored:.
    Last edited by a moderator: Oct 11, 2009
  17. eric pisch

    eric pisch Active Member

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    hopefully if i get done again i hope its by the feds, ill just post the ticket back to america :D
    Last edited by a moderator: Oct 11, 2009
  18. FZR400RRSP

    FZR400RRSP Well-Known Member

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    Half right.
    A speedo is never allowed to under-read, you are right there.
    But it must also never over-read by more than 10%.
    So at 30mph, it must not read more than 33mph.:nono:

    Here's a snippet from the type-approval laws.

    Vehicle construction and use regulations require a vehicle speedometer accuracy to be in the range of -0->+10%. The implications are that it must never under-read - for obvious reasons - but may over-read.
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    Last edited: Oct 11, 2009
  19. Confucius

    Confucius Member

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    A Laser speed gun works on the same principle as doppler radar, the change in frequency indicates the speed at which the target vehicle is travelling.

    V=cFd/2Ft, V=target velocity, Fd=Doppler frequency, Ft=Transmitted frequency

    A 'ladar', as they can be called, has the advantage, to the police, of having a LPI, low probability of interception. Less beam spread means a smaller chance of signal detection by other vehicles - such as those equipped with speed gun detectors.



    I tend to get around the over-read by fitting slightly bigger tyres to my car, having calibrated the standard ones against GPS I found my speedo over-read by ~5%. On bigger (new) tyres it over-reads by ~1% against GPS. Going from a 205/55R16 to a 225/55R16 has a 3.5% difference (increase) on actual speed against indicated speed. Obviously as the tread wears the over-read will increase slightly.
    Last edited: Oct 11, 2009
  20. eric pisch

    eric pisch Active Member

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    was a long distant memory from when my friend SVA'd his kit car, probably been changed since then
  21. FZR400RRSP

    FZR400RRSP Well-Known Member

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    Dunno when it was changed, but I recall it being this way since I first started driving in the 80's

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