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#Post#: 116046--------------------------------------------------
Re: Slip error
DIR By: andy_foster
Date: April 16, 2026, 5:12 am
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Laser speedmeters (Lidar) typical work by measuring the distance
to the object through ~40 pulses over ~1/3 second. The
distance(s) measured are the beam length from time of flight
halved. 4 initial pulses will set the window for expected
reflected pulses and the remainder (including the last of the 4
initial pulses) will be used to calculate the speed.
Inconsistency will throw up an error.
"Slip" can be achieved by sliding the end of the beam along a
smooth surface, such as the side of a lorry. If the device is
clamped into position, slip would create a negative error - the
lorry would be moving, but the length of the beam could remain
constant. If the device is panned to track the vehicle, and the
beam moves from the rear to the front of a length of smooth
surface during that 1/3 second, it could add up to ~7mph per
metre of slip (3m/s).
#Post#: 116060--------------------------------------------------
Re: Slip error
DIR By: P3ty10
Date: April 16, 2026, 7:52 am
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Thank you Andy. So what could it be that theres such a big
difference. 18 miles is a lot. At the beginning I thought that
maybe the camera was set on kilometres by mistake but in not
sure if it's got 2 functions for km or miles. I wouldn't even
consider fighting this if I knew I was speeding but as I
previously stated there where evident things( the 4 people in
high vis observing) to make you "aware" that there might be a
possibility for police officers being with camera. And I was
towing the mini-digger downhill so depending on how far he was
there is a great possibility that having that speed and the
distance between me and him I physically wouldn't be able to
stop. I'm not sure what to do now as pleading guilty while
driving in a legal and safe manner has been haunting me for days
now. Had driving license for over 9 years and never been caught
speeding
#Post#: 116113--------------------------------------------------
Re: Slip error
DIR By: sparx
Date: April 16, 2026, 4:46 pm
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--- Quote from: P3ty10 link ---
>
> The officer alleges 47 mph from a front-on handheld, but my
raw Google Takeout logs (the JSON telemetry) show a
speedMetersPerSecond of 12.96m/s at peak, which works out to
about 28.99 mph. The log also shows a high accuracy rating of 3
meters.
--- End Quote ---
Were you actively using Google Maps at the time? What is the
time interval between the data? If you're not using Google maps,
is there a possibility that the data is averaged?
Does your vehicle/the towed equipment, have any reflective
surfaces or long, flat surfaces? If it can generate 7mph per 1m
of slip (per andy), then you're looking at 2 and a bit metres to
get to 18mph difference.
Is it actually possible for your vehicle (towing said trailer)
to reach the speeds allegedly reached? And if it did reach these
speeds, could you have made your turn into the junction?
#Post#: 116133--------------------------------------------------
Re: Slip error
DIR By: P3ty10
Date: April 17, 2026, 3:02 am
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Thank you Sparxy.I was using Google maps for the whole duration.
Exit roundabout 17:41:24 at 4.26m/s . Around mid point 17:42:06
at 12.96m/s . Just before turning into the side road where I was
pointed 17:42:48 at 4.51m/s. Total length of section is less
than 0.5miles/800 metres
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