I've just passed a police trailer parked on the verge which indicated my speed. How accurate are they? On the basis of this one my speedometer is 7% fast at 40 mph, but can I rely on that?
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L\'escargot.
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I pass a permanent one every day which flashes if you are exceeding 30 - doing 30 it does nothing. I have passed a temporary one on a tripod - last time I was doing spot on 30 and it said 34, according to GPS at 30 I'm doing about 28. So in my single experience they are not to be trusted.
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Given that your speedo could be up to + 10% out, and the road side speed trailer could be +/- 10% out (they dont need to be calibrated, they are not used in evidence) you could be doing any speed.
7% fast at 40 mph sounds about right for a speedo. wouldnt rely on that in court tho.
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TourVanMan TM < Ex RF >
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there's one in roadworks on the motorway near me, it's a temporary 50mph limit but the sign is triggered by any speed over 40 - makes it a bit pointless I think.
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One near me for several months which is supposed to flash if you're exceeding 30. Went through it a number of times at that speed and was flashed. Tried it again at 20, when the road was quiet, and it still flashed.
Clk Sec
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Some bad boys around here have been racing through them to try and out max each other !
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There's one near me outside a school in a 20 limit. I've no idea why they've reduced the limit to 20. Yes, I know its outside a school but whenever I've been past there near school finishing time I haven't been able to manage more than 2mph, I'd be happy enough with 10.
i've yet to manage more than 32mph* on the board though I haven't really tried.
*4am, no schoolkids about then, visibility is better than the usual 30mph limit.
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I read often, only post occasionally
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If they can't be used in evidence, why doesn't someone try going past one at 100 just to see what it claims?
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They should be as accurate as SIDs (smiley/frowning face "speed indicator device") or hand-held "speed guns" or those flippin' things people poke out of the back of small white vans. The next time you see a SID, use it to double-check. N.B. it'll only be useful for a small range of values, typically a SID on a 30MPH road will be set to give a maximum reading of 45MPH - so if you go by at 60, it'll either read "45", or more likely stay blank. This is to discourage sundry herberts and boy racers from seeing what they can get it up to. The actual speed is (AFAIK) still recorded internally, for later analysis of traffic speeds/volumes at that location.
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