Caught speeding, but was it 74.4mph or 106mph

Hiawog

Firstly, there seems to be little to doubt, I was speeding.

The essential question here is: Are Garda speed guns equipped to measure the angle of a cars trajectory to their static position and compensate in order to give an accurate reading of speed?

The photograph sent to me on the fine sheet shows the car at an angle of appromiately 45 degrees. As both radars and lasers work in straight lines, I investigated whether the reading could be accurate from such an angle. I discovered 'Cosine Error Geometry' as written in a book called 'Police Traffic Speed Radar Handbook' by Donald Sawick. Chapter 2.1 is here, http://www.copradar.com/preview/chapt2/ch2d1.html .

I have been informed that I was doing 120kph in an 80kph zone or 74.4mph in a 49.6mph zone. Cosine Error Geometry (against my advantage) suggests that an uncompensated radar at a 45 degree angle will read only 70% of the actual speed. This suggests that I was actually travelling at over 106mph.

I can emphatically deny that I was not travelling at that speed. And now I'm puzzled because, if the Garda radars and lasers do not have the technology to measure angle and compensate against Cosine Error Geometry and I know I was not travelling at 106mph, then this calls into question the 74.4mph(120kph) that I am accused of.

It's hardly possible that I can argue my way out of this one but the level of potential inaccuracy, either way, is astounding. If you know more about the actual equipment used by Gardai then please let me know.

Anan1

Good question. Might the speed be measured when the car is coming straight towards the gun and the photo be taken at a different time?

kirving

Interesting alright. I had a physics lecturer in college who thaught us a bit about how these radars work. He was telling us about one if his old college friends who went on to do law. Apparently he specialises in things such as this. Explaining how the measuring device is subject to all kinds of interference usually gets the driver off.

Our man in Havana

Anan1 wrote: »

Good question. Might the speed be measured when the car is coming straight towards the gun and the photo be taken at a different time?

There would be a constantly running video. It is very possible the pic sent out is not the money shot.

Stationary camera or speed van ?
Either way I can't see them setting up to have oncoming vehicles subject to such error.

pa990

The camera vans radar operates at an angle of 22 degrees, and the sw compensates for the cosine error.

Either way.. Best of luck in court

Imo.. Your best bet is to take the hit of €80 and 2 points.
When a judge hears that you were doing 100+mph you won't get any sympathy

Chimaera

Have you any idea what a 45 degree slope is like? It's so extreme I doubt that any road anywhere would be allowed to run at such an angle. It amounts to a 1 in 1 slope.

For an observer to be at a 45 degree angle to the observed car, they'd need to be much higher or lower which is only likely if they're on an overpass monitoring traffic below them.

(Basically, I'm calling shenanigans on the OP's claim that his car was at 45 degrees to the speed gun).

colm_mcm

I think what the op is saying is that the camera was way off to the side so not pointed directly at them.

ZENER

Chimaera wrote: »

Have you any idea what a 45 degree slope is like? It's so extreme I doubt that any road anywhere would be allowed to run at such an angle. It amounts to a 1 in 1 slope.

For an observer to be at a 45 degree angle to the observed car, they'd need to be much higher or lower which is only likely if they're on an overpass monitoring traffic below them.

(Basically, I'm calling shenanigans on the OP's claim that his car was at 45 degrees to the speed gun).

:rolleyes:

slimjimmc

Chimaera wrote: »

Have you any idea what a 45 degree slope is like? It's so extreme I doubt that any road anywhere would be allowed to run at such an angle. It amounts to a 1 in 1 slope.

For an observer to be at a 45 degree angle to the observed car, they'd need to be much higher or lower which is only likely if they're on an overpass monitoring traffic below them.

(Basically, I'm calling shenanigans on the OP's claim that his car was at 45 degrees to the speed gun).

He didn't mention slope. One would have thought it was blatantly obvious the OP was referring to the vector on the horizontal (xy) plane and not on the vertical plane.

coylemj

Surely the radar gun is measuring speed based on the Doppler Effect which means that if the gun is at the centre of a circle and the vehicle is travelling around the circumference of the circle, the gun would register a speed of zero because the vehicle's distance woud be constant relative to the gun and therefore it's relative velocity (towards or away from the gun) would be zero.

It follows that the actual speed of the vehicle is only registered correctly by the radar gun if the observer is stationary and the vehicle is travelling directly towards him (the collision course) and if there is an angle involved, it will be to the benefit to the driver i.e. the radar gun will register a lower speed.

So I can't see where there is a problem. The gun will never register a higher speed than the actual speed of the vehicle and in most cases it will clock a lower speed, unless the vehicle is on a direct collision course with the gun where it will register the true speed.

Chimaera

slimjimmc wrote: »

He didn't mention slope. One would have thought it was blatantly obvious the OP was referring to the vector on the horizontal (xy) plane and not on the vertical plane.

I guess I just see the world differently to everyone else

Surely the radar gun is measuring speed based on the Doppler Effect which means that if the gun is at the centre of a circle and the vehicle is travelling around the circumference of the circle, the gun would register a speed of zero because the vehicle's distance woud be constant relative to the gun and therefore it's relative velocity (towards or away from the gun) would be zero.

It takes a measurement of speed radially to/from the gun. (Working from the info at http://en.wikipedia.org/wiki/Doppler_radar assuming we can trust wikipedia). Unless the gun tracks the vehicle, the 'contact' point of the beam is going to slide along the front of the car if there's an angle between the two, until it reaches the corner of the car where it'll slide along the side. If the gun tracks, then the gun has to know how fast it's rotating to maintain accuracy - I doubt this is actually the case.

So if the gun is off to the side of the road, and measures the car at a 45 degree angle, it's measuring a speed along the hypotenuse of the triangle, which is larger than the actual speed, not smaller. At 45 degrees, the factor is 1/cos(45) or 1.414.

In the OP's case, assuming the measured speed of 120 km/h is along the hypotenuse, then the real speed would be 120/1.414 = 84.866 km/h. Still over the limit but not by much.

This is all contingent on the radar gun not being set up to account for the angle to the cars. I'd be curious to see what procedure is prescribed and/or followed for verifying the gun's angle to the road being monitored.

keithclancy

Hiawog wrote: »

Firstly, there seems to be little to doubt, I was speeding.

The essential question here is: Are Garda speed guns equipped to measure the angle of a cars trajectory to their static position and compensate in order to give an accurate reading of speed?

The photograph sent to me on the fine sheet shows the car at an angle of appromiately 45 degrees. As both radars and lasers work in straight lines, I investigated whether the reading could be accurate from such an angle. I discovered 'Cosine Error Geometry' as written in a book called 'Police Traffic Speed Radar Handbook' by Donald Sawick. Chapter 2.1 is here, http://www.copradar.com/preview/chapt2/ch2d1.html .

I have been informed that I was doing 120kph in an 80kph zone or 74.4mph in a 49.6mph zone. Cosine Error Geometry (against my advantage) suggests that an uncompensated radar at a 45 degree angle will read only 70% of the actual speed. This suggests that I was actually travelling at over 106mph.

I can emphatically deny that I was not travelling at that speed. And now I'm puzzled because, if the Garda radars and lasers do not have the technology to measure angle and compensate against Cosine Error Geometry and I know I was not travelling at 106mph, then this calls into question the 74.4mph(120kph) that I am accused of.

It's hardly possible that I can argue my way out of this one but the level of potential inaccuracy, either way, is astounding. If you know more about the actual equipment used by Gardai then please let me know.

Same with the Gatso cams, they allow 10% as a Margin of error.

The handhelds are much more.

But 45 degrees is the max, no Copper would clock you from that angle.

sundodger5

The Vans are Radar but are the handhelds not laser?

Our man in Havana

You really need to see the full photo to be sure.

Was this a so safe or a Garda? These things you need to find out.

KamiKazi

OP can you post the picture here? It will cut out the debating on what might have caused your confusion (you can blank your reg if you want)

Anan1

Chimaera wrote: »

So if the gun is off to the side of the road, and measures the car at a 45 degree angle, it's measuring a speed along the hypotenuse of the triangle, which is larger than the actual speed, not smaller. At 45 degrees, the factor is 1/cos(45) or 1.414.

In the OP's case, assuming the measured speed of 120 km/h is along the hypotenuse, then the real speed would be 120/1.414 = 84.866 km/h. Still over the limit but not by much.

I'm not great at maths, but I would have thought that, if the gun measures how fast the car is closing on it, the speed would be under-read if the car was doing anything but coming straight at the gun.

I'd agree that the gun is unlikely to track the angle, so i'm going to guess the following:

1. The Gardaí set things up so that a reading is taken when the car is coming close enough to straight at the gun that the angle doesn't much matter. As coylemj says, it probably gives us an extra couple of km/h leeway.

2. The photo is taken from the roadside but much later, when the car is very close, which would account for the 45% angle.

Occam's Razor and all that.:)

Bohrio

I'd say you are lucky then, I would pay the fee and move on... you might get away with this but if oyu dont you could end up with a much higher penalty.

You said you were doing 170 km/h on an 80 km/h zone... considered yourself lucky! where I am from, you would have got 600 euro, 6 points and inmediate suspension of your license... and it will be up to the traffic policeman to determine whether it could be considered dangerous driving if so you could end up in jail!

Del2005

Does anyone of this even matter?

The OP has admitted that they where speeding and our laws don't require any proof that you where speeding to get a conviction. It doesn't matter if the detection device is calibrated or set up correctly.

keithclancy

Anan1 wrote: »

I'm not great at maths, but I would have thought that, if the gun measures how fast the car is closing on it, the speed would be under-read if the car was doing anything but coming straight at the gun.

I'd agree that the gun is unlikely to track the angle, so i'm going to guess the following:

1. The Gardaí set things up so that a reading is taken when the car is coming close enough to straight at the gun that the angle doesn't much matter. As coylemj says, it probably gives us an extra couple of km/h leeway.

2. The photo is taken from the roadside but much later, when the car is very close, which would account for the 45% angle.

Occam's Razor and all that.:)

Correct, I believe they all use Lidar now .. the Radar guns havent been used in years.

Anan1

Del2005 wrote: »

Does anyone of this even matter?

The OP has admitted that they where speeding and our laws don't require any proof that you where speeding to get a conviction. It doesn't matter if the detection device is calibrated or set up correctly.

Not only does it matter, but it's interesting.

Bohrio

First thing is to find out what type of gun was used and then move from there

I think you shoul pay the fee specially after oyu have admitted you were speeding.

If this would have been the case where you werent going over the limit and still got a ticket for it then I'd agree with you to fight until the end. Or if we were in a country where you get different fines depending of your speed then yes!

PCPhoto

Bohrio wrote: »

First thing is to find out what type of gun was used and then move from there

I think you shoul pay the fee specially after oyu have admitted you were speeding.

If this would have been the case where you werent going over the limit and still got a ticket for it then I'd agree with you to fight until the end. Or if we were in a country where you get different fines depending of your speed then yes!

to be honest ... even thats not worth the hassle for the amounts of money/penalties involved ..... takes months of energy to fight against "the man" so.....given the OP said he/she was over the limit but they dispute the amount, I would suggest they pay and move on.

Veloce

Anan1 wrote: »

2. The photo is taken from the roadside but much later, when the car is very close, which would account for the 45% angle.

I was under that impression too. Your speed is measured at a further distance away (which means less of an angle thus more accuracy) and the photo is taken when up closer to the van.

keithclancy

Veloce wrote: »

I was under that impression too. Your speed is measured at a further distance away (which means less of an angle thus more accuracy) and the photo is taken when up closer to the van.

I don't think so, I've seen them setting up the van here and they seem to use a point on the road to callibrate the thing.

I.E. It doesn't move, just covers a fixed area of the road.

You can see it in the UK where they mark the box that the Gatso fires at.

Wossack

Im going to go with Anan1's option 2 also. I'd say they give you the second photo so that you can see who was driving at the time, and pass points on if possible (if multiple people drive that car, the fine goes to the car owner)

Veloce

keithclancy wrote: »

I don't think so, I've seen them setting up the van here and they seem to use a point on the road to callibrate the thing.

I.E. It doesn't move, just covers a fixed area of the road.

You can see it in the UK where they mark the box that the Gatso fires at.

I have seen them in action too Keith. I saw them operating late one night on the old Cabra road. I saw a passat a good distance up the road that appeared to be well over the 50km/hr speed limit. However he spotted the van and put the anchors down to a near crawling pace. He continued at crawling pace for another 50 metres towards the van and it was only then the flash went off.

Chimaera

It doesn't matter if the gun is laser or radar - both use the same doppler shift effect to measure the speed. Laser has the advantage of a much narrower beam width so it's harder to pick up with a detector.

I've done up a small graphic to illustrate the error with the gun being at an angle to the road: the blue outline is the car initially, the red being the car at some distance down the road; the green line is the gun's line of sight to the car, and the yellow bits show the speed being measured.



The speed gun at a fixed point has no idea what direction the car is going in relative to the line of sight, all it sees is a surface moving towards it along the line of sight.

the culture of deference

Firstly request a copy of the photo, the details of where to get it will be on the charge.

Contact the superintendent of the station the gardai were from. Send him the photo if you have it, and explain what you said here.

The Super has the power to drop the charge.

You are not allowed an appeal.

Our man in Havana

You are not allowed an appeal.

Is that not what the courts are for?

Mycroft H

the culture of deference wrote: »

Firstly request a copy of the photo, the details of where to get it will be on the charge.

Contact the superintendent of the station the gardai were from. Send him the photo if you have it, and explain what you said here.

The Super has the power to drop the charge.

You are not allowed an appeal.

Our man in Havana wrote: »

Is that not what the courts are for?

Of course your entitled to your day in the district court.

Hiawog

Would you believe, I posted the question and went off and completely forgot about it. As I tell everybody, 'I have a memory like a....ehmm....ehh....what's that thing called again?'

I think I'll buy the book on download and read it. I would not be so worried about making a challenge, I've made three in my life. The first was for breaking a red light, the Garda was actually jay-walking. The second was for parking, I was actually on private property and the third was for receiving a fine for a charge not mentioned to me by the Garda. He was a bully who spent twenty minutes with me trying to elicit an apology and then he didn't turn up in court. The judge told me I'd have to return to appear again before him and that very rarely happens. He was a heavy finer against challenges, up to €600 to some people, so I paid the €80 and wasn't called back.

The suggestions that the photo is taken after you are checked for speed seems unlikely unless the law specifies that no photo of the car speeding is needed or indeed if another photo is taken while the car is speeding. With a static camera, there is no witness to say the car identified from the photo is the same car that was speeding at a different time earlier.

As for the science and trigonometry of the situation, I'll need to study it more. There is one point though, I have been informed that I was doing 120KPH in an 80KPH zone. I am not admitting to that nor am I admittting to speeding. What I am saying is that if no 'Cosine Error Geometry' correction technology exists and if the angle is greater than 45 degrees, then my corrected speed would be approximately 80KPH.

I would like not to have to pay only beacause I'm currently a 'jobseeker' and €80 is a real kicking financially but I'm actually pursuing this more out of curiosity than anything else.

Best Regards.