Caught by Police using a handheld speed gun

Car_54

ontheroad1970 said:

[Deleted User] said:

ontheroad1970 said:

[Deleted User] said:

sand_hun said:

Grumpy_chap said:

Does anyone know whether, when there is a Community Speedwatch Area, manned by volunteers with a "gun", can they actually issue speeding penalties (NIP), or just warning letters?  

They will have been writing down your number plate details and the make of your vehicle. At worst, you will receive a warning letter (and a telling off from your wife).

Yes, you can safely ignore them. They rarely have any clue how to properly operate the speed gun, and even on the very best days it's unreliable and likely to produce inaccurate readings.

Get yourself a dashcam with GPS feature that records your speed on the video. Then if you have any issues in future you can review the video to see if you were actually speeding, and then decide what to do.

Again, as the GPS speed is not in real time, it is of very little use.

What do you think "real time" means?

Because GPS speed is real time, or at least as real time as the speed guns.

There is a lag between the time the device reads the speed and it being relayed back to the device, unlike a speed gun.  Next time you use a TomTom pay attention to how long it takes to reflect the speed you're doing.  There is a lag.  Therefore it is not reliable especially when faced with a bill for thousands for expert witnesses.  Unless you are prepared to pay people's costs for them, of course.  

The device doesn’t “read” speed. It calculates two positions from the GPS satellites and calculates the speed from those.

tobygarrod939

Legally you are never going to prove anything using GPS data, it might be supportive evidence but it will not usurp a calibrated speed gun.

On a b-road where your speed varies, with corners, etc. I wouldn't rely on GPS. It will be 'fairly' accurate but no more. On a motorway, travelling more or less straight or shallow angle corners - at a continuous speed - the way GPS works, it will very accurate.

I've been in a passenger in a car - few years ago now - which got done for speeding by an older radar gun, and I know the driver was not going the speed he was done for. Motorway, GPS and cruise control set for 75mph - he got done at 92mph. His GPS and Speedo were both not wrong, and not wrong by almost 20mph. He got absolutely nowhere with his protests.

That day he was driving a low profile and angular car (Cayman), outside lane, and police were in one of their motorway spots measuring from an angle. Seen countless arguements disputing similar over the years, usually to no avail, and various 3rd party tests demonstrating flaws with that equipment - but..... To no avail.

Car_54

Jenni_D said:

[Deleted User] said:

ontheroad1970 said:

[Deleted User] said:

sand_hun said:

Grumpy_chap said:

Does anyone know whether, when there is a Community Speedwatch Area, manned by volunteers with a "gun", can they actually issue speeding penalties (NIP), or just warning letters?  

They will have been writing down your number plate details and the make of your vehicle. At worst, you will receive a warning letter (and a telling off from your wife).

Yes, you can safely ignore them. They rarely have any clue how to properly operate the speed gun, and even on the very best days it's unreliable and likely to produce inaccurate readings.

Get yourself a dashcam with GPS feature that records your speed on the video. Then if you have any issues in future you can review the video to see if you were actually speeding, and then decide what to do.

Again, as the GPS speed is not in real time, it is of very little use.

What do you think "real time" means?

Because GPS speed is real time, or at least as real time as the speed guns.

There's a delay for the (radio frequency) signals to bounce back from the satellites (plural) before the GPS location can be determined. And then at least two such "transactions" need to take place before a speed can be calculated. So the speed on the GPS will be at least a second old - probably more. A laser speed gun only requires the (light speed) signal to bounce from the gun to the vehicle, again probably at least twice, which will be much faster.

Also the gun has been calibrated and is type-approved for speed measuring. GPS satnav has not.

Does this matter in the real world? Probably not. Does this matter in the legal world? Absolutely.

There are no signals “bouncing back” from the satellites. The satellites transmit their own signals.

Jenni_D

Car_54 said:

There are no signals “bouncing back” from the satellites. The satellites transmit their own signals.

Fair point ... but GPS still needs to triangulate using several satellite signals and will never give a speed calculation anywhere near as quickly as a laser speed gun.

So my technical analysis may have been incorrect but the basic point was still sound.  

ElefantEd

Isn't GPS only accurate to a metre or two anyway? Accuracy will also vary according to atmospheric conditions, proximity to other terrain features etc. Given that the speed calculation is based on calculating the time between two points, any inaccuracy in the positions will affect the speed. Even if you were to assume that any inaccuracies are likely to be the same for consecutive position measurements, this does introduce an uncertainty which would prevent it from being used as good evidence in court; the random error in a GPS system must surely be greater than that of a speed gun.

[Deleted User]

Car_54 said:

[Deleted User] said:

Ectophile said:

[Deleted User] said:

sand_hun said:

Grumpy_chap said:

Does anyone know whether, when there is a Community Speedwatch Area, manned by volunteers with a "gun", can they actually issue speeding penalties (NIP), or just warning letters?  

They will have been writing down your number plate details and the make of your vehicle. At worst, you will receive a warning letter (and a telling off from your wife).

Yes, you can safely ignore them. They rarely have any clue how to properly operate the speed gun, and even on the very best days it's unreliable and likely to produce inaccurate readings.

Get yourself a dashcam with GPS feature that records your speed on the video. Then if you have any issues in future you can review the video to see if you were actually speeding, and then decide what to do.

If it ends up in court, the police will be able to produce a training record for the officer who was using the gun, and a calibration certificate for the speed gun itself.

You will have a recording on a dashcam, which will come with no calibration information.

Which do you think the courts will accept?

The GPS data is compelling because it does not require calibration. It is the satellites which are calibrated, and if the calibration was off by even a tiny amount it would be a major news story as systems malfunctioned all over the place.

The satellites may be calibrated, but they don’t calculate speed. That’s down to the satnav, which probably isn’t.

The speed calculation doesn't need calibration. It's distance over time.

The GPS receiver uses timing information from the satellites, which have atomic clocks. That's how it determines location. There is no calibration because it's just maths, and any oscillators used are all disciplined by the signal from the satellites.

If any of it was off by even nanoseconds the position information would be way off too. Ideally you want to log the position data as well so you can demonstrate that it places the car on the road, proving that the speed calculation is accurate.

[Deleted User]

ontheroad1970 said:

[Deleted User] said:

ontheroad1970 said:

[Deleted User] said:

sand_hun said:

Grumpy_chap said:

Does anyone know whether, when there is a Community Speedwatch Area, manned by volunteers with a "gun", can they actually issue speeding penalties (NIP), or just warning letters?  

They will have been writing down your number plate details and the make of your vehicle. At worst, you will receive a warning letter (and a telling off from your wife).

Yes, you can safely ignore them. They rarely have any clue how to properly operate the speed gun, and even on the very best days it's unreliable and likely to produce inaccurate readings.

Get yourself a dashcam with GPS feature that records your speed on the video. Then if you have any issues in future you can review the video to see if you were actually speeding, and then decide what to do.

Again, as the GPS speed is not in real time, it is of very little use.

What do you think "real time" means?

Because GPS speed is real time, or at least as real time as the speed guns.

There is a lag between the time the device reads the speed and it being relayed back to the device, unlike a speed gun.  Next time you use a TomTom pay attention to how long it takes to reflect the speed you're doing.  There is a lag.  Therefore it is not reliable especially when faced with a bill for thousands for expert witnesses.  Unless you are prepared to pay people's costs for them, of course.  

The value displayed is for the last second. Most GNSS modules update position once per second by default, but many can do it faster if required.

But it's irrelevant anyway. If it shows you doing 10 MPH lower for a few seconds before and after the alleged incident there is no way that the car could have accelerated that much and then slowed down again within one second.

[Deleted User]

Jenni_D said:

[Deleted User] said:

ontheroad1970 said:

[Deleted User] said:

sand_hun said:

Grumpy_chap said:

Does anyone know whether, when there is a Community Speedwatch Area, manned by volunteers with a "gun", can they actually issue speeding penalties (NIP), or just warning letters?  

They will have been writing down your number plate details and the make of your vehicle. At worst, you will receive a warning letter (and a telling off from your wife).

Yes, you can safely ignore them. They rarely have any clue how to properly operate the speed gun, and even on the very best days it's unreliable and likely to produce inaccurate readings.

Get yourself a dashcam with GPS feature that records your speed on the video. Then if you have any issues in future you can review the video to see if you were actually speeding, and then decide what to do.

Again, as the GPS speed is not in real time, it is of very little use.

What do you think "real time" means?

Because GPS speed is real time, or at least as real time as the speed guns.

There's a delay for the (radio frequency) signals to bounce back from the satellites (plural) before the GPS location can be determined. And then at least two such "transactions" need to take place before a speed can be calculated. So the speed on the GPS will be at least a second old - probably more. A laser speed gun only requires the (light speed) signal to bounce from the gun to the vehicle, again probably at least twice, which will be much faster.

Also the gun has been calibrated and is type-approved for speed measuring. GPS satnav has not.

Does this matter in the real world? Probably not. Does this matter in the legal world? Absolutely.

The delay from the satellites is how GPS works. The signal is sent continuously so the actual delay doesn't matter, and does not mean that the location information is not instantaneous for when it is reported once a second. Speed is for the second before it is reported.

None of which matters because the speed gun is known to be janky so they always get several seconds of measurements with it. The display only updates every second or so as multiple measurements are averaged.

You will have GPS data for minutes before and after the measurement period and can show that your vehicle didn't exceed the limit at any time.

[Deleted User]

tobygarrod939 said:

Legally you are never going to prove anything using GPS data, it might be supportive evidence but it will not usurp a calibrated speed gun.

On a b-road where your speed varies, with corners, etc. I wouldn't rely on GPS. It will be 'fairly' accurate but no more. On a motorway, travelling more or less straight or shallow angle corners - at a continuous speed - the way GPS works, it will very accurate.

I've been in a passenger in a car - few years ago now - which got done for speeding by an older radar gun, and I know the driver was not going the speed he was done for. Motorway, GPS and cruise control set for 75mph - he got done at 92mph. His GPS and Speedo were both not wrong, and not wrong by almost 20mph. He got absolutely nowhere with his protests.

That day he was driving a low profile and angular car (Cayman), outside lane, and police were in one of their motorway spots measuring from an angle. Seen countless arguements disputing similar over the years, usually to no avail, and various 3rd party tests demonstrating flaws with that equipment - but..... To no avail.

Yes, to win you need to have the evidence and a decent lawyer, go to court and make your case. Pleading with the police won't work, you will have to go to court and it will probably cost you more than the fine anyway.

That's why I said only do it if you have a lot to lose, otherwise just do the speed awareness course and keep your eyes peeled for speed gun scams.

TooManyPoints

I haven't read all of this as I've been away. But to challenge this you need to prove that the measurement taken by the device the police used cannot be relied upon. What you have in the way of SatNavs or anything else is immaterial. An approved device operated in the correct manner is assumed to be accurate unless the contrary can be proved - and you have to do the proving. Proving your SatNav may be right does not prove the other device was wrong, and that's what you have to do. You will need expert assistance and a good motoring specialist lawyer to present your case and it will cost you £Thousands, which you will not be able to recover even if you are acquitted.