Accuracy of recent speed measurements
Discussion
Just noticed the thread in GG about the BMW driver claiming his throttle jammed etc etc.
A couple of people questioned the accuracy of claims the car was doing 135-140mph and it made me think of several cases lately where high speed figures have seemed iffy.
from dodgy memory
Case 1) PC Milton, Vectra GSi 153mph
Case 2) the lad in his dad's Boxster, think the claims were of over 150mph on a short road.
there is a few more i think but can't remember the details. Does anyone else know of some recent cases where the Police measured figures seem akin to those of Lamborghini in the late 80's?
If so, is it possible the calibration is deliberately skewed?
A couple of people questioned the accuracy of claims the car was doing 135-140mph and it made me think of several cases lately where high speed figures have seemed iffy.
from dodgy memory
Case 1) PC Milton, Vectra GSi 153mph
Case 2) the lad in his dad's Boxster, think the claims were of over 150mph on a short road.
there is a few more i think but can't remember the details. Does anyone else know of some recent cases where the Police measured figures seem akin to those of Lamborghini in the late 80's?
If so, is it possible the calibration is deliberately skewed?
Probably not delibarately skewed, but we have to take into account how these devices are tested each year.
As long as from sat on a tripod they can correctly measure the distance of a stationary test object 20, 40, 60, 80 and 100 metres away, then the device is tested as accurate to measure the speed whilst the officer has the device handheld at distances up to 1000 metres and to measure speeds I think up to 200mph.
It may very well be that the speeds are dodgy. If the laser can misread a car with a speedo on 30, and say it is doing 42 for example, then on a car doing 28mph, it is 14mph out, thus a 50% error.
Not saying that any or all of the erroneous readings are as far out as that, but its a good example. It is also worth remembering that the devices *can also* misread and lower the cars actual speed. Of course, we dont hear about those...
As long as from sat on a tripod they can correctly measure the distance of a stationary test object 20, 40, 60, 80 and 100 metres away, then the device is tested as accurate to measure the speed whilst the officer has the device handheld at distances up to 1000 metres and to measure speeds I think up to 200mph.
It may very well be that the speeds are dodgy. If the laser can misread a car with a speedo on 30, and say it is doing 42 for example, then on a car doing 28mph, it is 14mph out, thus a 50% error.
Not saying that any or all of the erroneous readings are as far out as that, but its a good example. It is also worth remembering that the devices *can also* misread and lower the cars actual speed. Of course, we dont hear about those...
justinp1 said:
Probably not delibarately skewed, but we have to take into account how these devices are tested each year.
As long as from sat on a tripod they can correctly measure the distance of a stationary test object 20, 40, 60, 80 and 100 metres away, then the device is tested as accurate to measure the speed whilst the officer has the device handheld at distances up to 1000 metres and to measure speeds I think up to 200mph.
...
I was thinking more of the in car measurements as in the PC Milton case
KB_S1 said:
Probably not delibarately skewed, but we have to take into account how these devices are tested each year.
As long as from sat on a tripod they can correctly measure the distance of a stationary test object 20, 40, 60, 80 and 100 metres away, then the device is tested as accurate to measure the speed whilst the officer has the device handheld at distances up to 1000 metres and to measure speeds I think up to 200mph.
...
I was thinking more of the in car measurements as in the PC Milton case
I see, as in VASCAR measurements?
My design these are made by operator measurements. There is also a conscious and subconscious desire to prove yourself right and bias to your own opinion. Obviously in these cases it depends on the length of road that the reading was taken from. I have heard of a case where a reading was taken over seven miles, so this would seriously even out any slightly out reading, if a reading is taken over say 4 seconds and the first reading is 0.4 seconds late and the next is 0.4 seconds early this would add a 20% sway to the speed reading. That would be enough to make 125 into 150.
justinp1 said:
Probably not delibarately skewed, but we have to take into account how these devices are tested each year.
As long as from sat on a tripod they can correctly measure the distance of a stationary test object 20, 40, 60, 80 and 100 metres away, then the device is tested as accurate to measure the speed whilst the officer has the device handheld at distances up to 1000 metres and to measure speeds I think up to 200mph.
It may very well be that the speeds are dodgy. If the laser can misread a car with a speedo on 30, and say it is doing 42 for example, then on a car doing 28mph, it is 14mph out, thus a 50% error.
Not saying that any or all of the erroneous readings are as far out as that, but its a good example. It is also worth remembering that the devices *can also* misread and lower the cars actual speed. Of course, we dont hear about those...
Justin - Are these figures correct? ie. calibrate at 20,40,60,80 and 100 metres?
My job is calibration and i would really like to see a results sheet from one of these devices.
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