Free Traffic Legal Advice

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.
 
 
 

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.

That gets expensive very quickly.

As this chap discovered:



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.

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 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”?

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?

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.

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).

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.

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?