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#Post#: 115804--------------------------------------------------
Slip error
DIR By: P3ty10
Date: April 14, 2026, 4:03 am
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Contesting 47 in a 30 (Band B) - Forensic GPS contradicts Police
Allegation
Hi everyone,
I am seeking advice on a potential "Not Guilty" plea for an
alleged speed of 47mph in a 30mph zone. I currently have a clean
license (0 points) and want to protect it.
I have a significant discrepancy between the police allegation
and my phones verified GNSS (GPS) telemetry for the time of the
stop.
The Case Context:
1. The Allegation: 47 mph (Band B).
2. The Setup: I was towing a heavy piece of plant machinery on a
trailer.
3. The Road: A downhill gradient with a very short total journey
distance (~800m/ 0.5 miles).
4. The Evidence: I was stopped at the roadside (front-facing
laser)shown 47mph. I have been told over the phone that no
photographic/video evidence exists.
The Telemetry Data (3m accuracy):
- My peak speed for the entire journey was recorded at 12.96 m/s
(approx 29 mph).
- At the timestamp provided on the police documentation, my
telemetry shows the vehicle had already been fully stationary
(0.0 mph) for over 30 seconds.
The Proposed Defense:
I believe the 47 mph reading is a technical error (likely "Slip
Error") caused by the combination of the downhill slope, the
vehicle pitching under braking, and the complex reflective
profile of the heavy equipment being towed. Furthermore, there
appears to be a clear time-synchronization error in the police
log.
Current Status:
The Justice Unit has declined to review this data at the
Conditional offer, stating that evidence will only be provided
at court.
The Conditional offer doesn't state the speed I was doing.
I've sent an email with screenshots showing the speed I was
doing on the stretch of the road from exiting a roundabout , mid
point , just before turning left to a side road where I was
pulled and stopping time.
I am prepared to go to court to keep my license clean. Does this
level of telemetry (3m accuracy) usually carry enough weight
with a Prosecutor/Magistrate to highlight a technical error? Are
there specific "procedural" questions I should be asking once I
receive the SJPN?
Any advice would be greatly appreciated.
#Post#: 115812--------------------------------------------------
Re: Slip error
DIR By: Southpaw82
Date: April 14, 2026, 4:32 am
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You’re essentially entering into a “battle of the experts”.
While your data is evidence, the court may feel that it requires
an expert to explain the meaning of the data to the court. I
would imagine that the prosecution will also want to deploy an
expert to show that the reading obtained by their device was
correct (and potentially an expert to rubbish your evidence).
That gets expensive very quickly.
#Post#: 115822--------------------------------------------------
Re: Slip error
DIR By: JustLoveCars
Date: April 14, 2026, 5:08 am
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The timestamp difference is unlikely to be relevant unless it's
wildly different. (Without knowing the specific device it may
not be GPS synchronised anyway - 'proving' you were stationary
at the 'exact' time isn't going to cut it)
It appears to be a corroborated measurement by a laser device
without video evidence. (That, in itself, is not fatal)
A virtually instantaneous laser measurement versus telemetric
system with low accuracy and averaged measurements. You would
likely need an expert witness from the system's makers - they
may well decline and may state the system is not designed for
such use anyway.
It seems a was a frontal ping, so not sure of the relevance of
items being towed?
A 'slip error' requires a smooth movement of the device across a
flat surface to fool the device into reporting a higher than
actual value - a 'complex reflective profile' (if even scanned)
would most likely trigger the reading to be abandoned. The
officer's statement will obviously be a factor.
#Post#: 115827--------------------------------------------------
Re: Slip error
DIR By: P3ty10
Date: April 14, 2026, 5:32 am
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So as I was driving downhill I had to use the break multiple
times to maintain the speed at 30mph. When I was braking the van
was pulled back and forth due to the trailer. That 800m / 0.5
miles was covered in 126 seconds from exiting the roundabout to
the stopping point. I'm not sure what's the distance from where
the officer used the gun but the road slightly bends and if I
would've had 47mph from where he could've physically see me the
distance would've been to short for me to be able to stop the
van and turn left safely into the side road where he pointed me
to go. There quite a few factors to be taken in consideration
but they didn't wanted to provide any type of evidence and been
declining everything that I was asking. The Conditional offer
didn't even state the speed that I was "caught" doing it just
says exceed 30mph on restricted road in England.
#Post#: 115839--------------------------------------------------
Re: Slip error
DIR By: NewJudge
Date: April 14, 2026, 6:46 am
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--- Quote ---
> That gets expensive very quickly.
--- End Quote ---
As this chap discovered:
HTML https://www.bbc.co.uk/news/uk-england-hereford-worcester-49641063
The court will have two lots of evidence:
One from a device that is approved by the Home Office and, as
the prosecution will say, was operated in the correct manner.
Another from a device that is not approved and which you have
operated.
The question for them will be to decide whether the measurement
from the unapproved device casts sufficient doubt on the
accuracy of that from the approved one (which is presumed to be
working correctly unless the contrary can be shown). If, after
considering that, the court cannot be sure that you committed
the offence (which is exceeding the speed limit, not travelling
at any particular speed) then you must be acquitted.
--- Quote ---
> There quite a few factors to be taken in consideration but
they didn't wanted to provide any type of evidence and been
declining everything that I was asking.
--- End Quote ---
The idea of the fixed penalty offer is that the driver accepts
the allegation as it stands. This sees the matter dealt with at
considerably reduced costs to both parties. Even if no expert
evidence is required, a conviction following a trial in the
Magistrates’ Court will see you walk away with little change
from £1,500. So the police will not enter into any
correspondence or debate about evidence and certainly not of a
technical nature such as you describe.
What do you mean by a “forensic GPS”?
#Post#: 115840--------------------------------------------------
Re: Slip error
DIR By: andy_foster
Date: April 14, 2026, 7:03 am
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--- Quote from: NewJudge link ---
>
> What do you mean by a “forensic GPS”?
>
--- End Quote ---
Indeed. The OP has used this as a term of art (capitalised the
initial letters), and this appears to be the technical backbone
of their defence, but they have given no indication as to what
exactly they are referring to.
#Post#: 116030--------------------------------------------------
Re: Slip error
DIR By: P3ty10
Date: April 16, 2026, 2:02 am
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Thanks for the advice so far. I’m trying to make sense of a huge
discrepancy and would appreciate your technical take on it.
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.
I’m not a tech expert, but an 18 mph difference seems like a lot
for a GPS error. I’m a bit concerned that it will just be the
officer's word/device vs my JSON data.
I wouldn't be debating this if I thought I was speeding, but I
know this road well. In fact, at that exact moment, I had just
exited a roundabout where there were people in high-vis green
vests taking notes, which made me even more conscious of my
speed.
Is it possible for a handheld to have a 'slip error' or misread
that could account for such a large gap (from 29 to 47)? Or is
the court likely to just ignore the GPS data entirely because
it’s not an 'approved' device?
#Post#: 116031--------------------------------------------------
Re: Slip error
DIR By: NewJudge
Date: April 16, 2026, 2:37 am
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--- Quote ---
> Or is the court likely to just ignore the GPS data entirely
because it’s not an 'approved' device?
--- End Quote ---
They won’t ignore it entirely but they will decide what weight
it carries. All mechanical devices (including yours) are
presumed to be working correctly unless there is evidence to the
contrary. That’s why so many sub-postmasters were convicted
under the “Horizon” scandal - because the computer said there
was money missing.
The court must be sure that you committed the offence and they
will weigh the evidence they have heard to decide that. There
are two aspects to your argument: the reliability of the device
itself and its method of operation. I suspect the second may be
more likely to be relevant, but I believe you will need expert
testimony to convince the court that you are correct and the
police are wrong. Simply turning up and saying "my satnav says
otherwise" won't cut the mustard.
#Post#: 116033--------------------------------------------------
Re: Slip error
DIR By: P3ty10
Date: April 16, 2026, 2:57 am
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Thank you new judge. There's a few things that don't really add
up. Conditional offer not specifying the speed . The huge
discrepancy between the actual speed and what the officer
states. The telemetry shows I did that distance in 2 minutes
which proves that I "took my time" let's say. I didn't see the
officer pointing the camera towards my van as he came out from
the side road into the middle or the road and pointed me to turn
left and stop so I'm not too sure from where he was checking my
speed but with the slight bend of the road it couldn't be too
far and if I would've drove with 47mph I wouldn't be able to
slow down and stop safely.
#Post#: 116034--------------------------------------------------
Re: Slip error
DIR By: Southpaw82
Date: April 16, 2026, 3:10 am
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The conditional offer doesn’t need to state your speed, so the
fact that it doesn’t is irrelevant.
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