, showing my calibrated speedometer and my GPS, another .
I have yet to see any civilian vehicle that has stamped on it "calibrated speedometer" such as in purpose built police vehicles. Where does one go about obtaining the personal vehicle calibration? How much does it cost? I have asked at automotive dealers, not one will provide any calibration to me, and last was on a brand new vehicle in February this year (2011) on a 2011 vehicle. So is my speedometer accurate? I would think so, but I am not 100% confident either in the speedometer.
GPS -has been already tossed out of courts for a defence as it is does not have a precise enough location, most are accurate within 5-20m. We have all been there where we were sitting somewhere and our GPS puts us 5-15m away on a different road. Easy way to do that go to edge of a parking lot, all 3 of my GPS units think I am parked on the road.
As a civilian, I am not held to the same level of due diligence as police officers and other professionally certified persons working in the speed enforcement industry. Before the courts, I only have introduce "reasonable doubt" as to the veracity of the officers evidence. While I have worked with measurement/calibration concerns in the past, I am no longer certified or employed as such. As others have observed here, I'm simply "well informed".
So the particular "calibrated" speedometer I speak of is electronic with an aftermarket circuit that corrects it for certain variables in gearing etc. My other speedos are factory-supplied and unaltered. I took video while comparing my "corrected" speedo against two separate GPS units and simultaneously with a stopwatch over MTO measured kilometer makings. Held as near as possible to 100 kph for 3 km, the speedo, both GPS and the measured mile/timing calculation all agreed within +/- 1%. My other speedometers disagree by up to 10% faster than the GPS and measured/timed speeds.
That said, lets play... So for what tire condition is the "calibration" for police vehicle speedos taken? On new tires? Tires worn to the wear bars? In any case the closest this can be is +/- 5%, as that will be the minimum difference between new and worn tires. Do you take tread depth readings on the drive tires and present specific pavement/tire friction/slippage data for each time you write a "pace" ticket? Didn't think so...
"Certified calibrated" or not, the speedo in my vehicles show the speed measuring device I was using. Think your standard gas pump notation- "In the case of disagreement between the pump and register reading, the pump reading shall be taken as correct". Unless I am running seriously non-standard size tires or have tampered with the speedo, relying on a manufacturer-supplied standard speedo demonstrates no mens rea... I honestly thought the speed I was going was as my speedo showed. The video just shows what I saw. Besides, the video timing where permanent roadside landmarks like lamp posts and signs were passed give a tamper-proof indication to verify speed. Just go back, measure the distances and look at the video time code when each passes out of frame... viola, exact speed calculated.
IIRC, the main landmark case in the US where the GPS reading was thrown out was because the defense could not prove that GPS was in that car at that time. Hence my video cameras, which place that GPS with my vehicle at that time/place and also show the officer who wrote the ticket. Along with the pertinent excerpts for display in court, I will be bringing the GPS with the logs still stored, where they cannot be tampered with. Ditto for the video files, on the original memory media which uses a proprietary camera writing schema. The prosecution can make copies and look for tampering to their heart's content. I may be good, but not good enough to convincingly fake original GPS logs and video files simultaneously.
GPS speeds are calculated using many readings, typically 1 to 5 per second for modern units, not just single reading. That is why the lag occurs when speed changes or the GPS signal is lost. But like cosine error for radar/lidar units, the GPS thinking it is a meter or two from the actual point a is minor error, reduced to near zero by the large number of readings taken over many kilometers.
In my defense proposal, the GPS is not to show the actual speed reading at a specific time/place, it is to show that when the speed was held held constant at some time shortly before the speed stop, the speedo and GPS agreed closely. When the alleged speed offense took place, the speedometer is the pertinent number on the video, as it responds as quickly as possible for that specific technology. When combined with the roadside markers on video, the speed can be closely confirmed.
So all combined, the speedo/GPS/video evidence will either verify the officer's evidence, or not.
Honest officers need have no fear. Mahoney-Breur types may justifiably find themselves looking for another line of work. Can't see why this is a problem for you.
Unless manufacturers/police/prosecution know something about radar/lidar that the public doesn't. Such as these reading are taken over such a short timespan (about 0.1 second) that the potential errors cannot be completely eliminated by the software. Those 60 mph trees coming back to haunt again?