Help me reply re: technical reasons against laser detectors
Discussion
Guys I wrote to my MP regarding laser hand held speed detectors and motorcycles after reading the article in MCM this is part of the reply
"There is no condition on the type approval of any enforcement device that it cannot be used on motorcycles. HOSDB(home office scientific development branch) do not carry out specific tests of laser speedmeters on motorcycles as there are no techical reasons to beleive they present new technical challenges not present with all other vehicles.
The laser speedmeter will only generate a reading when it recives back a signal of adequate strength to generate a reliable reading. it can take longer to aquire a reading from a motorcycle than a car but is no les accurate"
so can you help me word a reply stating why there are technical differences between motorcycles and cars.
Cheers
>> Edited by pesty on Saturday 26th November 09:22
"There is no condition on the type approval of any enforcement device that it cannot be used on motorcycles. HOSDB(home office scientific development branch) do not carry out specific tests of laser speedmeters on motorcycles as there are no techical reasons to beleive they present new technical challenges not present with all other vehicles.
The laser speedmeter will only generate a reading when it recives back a signal of adequate strength to generate a reliable reading. it can take longer to aquire a reading from a motorcycle than a car but is no les accurate"
so can you help me word a reply stating why there are technical differences between motorcycles and cars.
Cheers
>> Edited by pesty on Saturday 26th November 09:22
AFAIK, the LTI 20:20 gets a reading from a reflection off the target vehicle. Operators are told to target the number plate as it's a flat reflective surface so the beam should just bounce straight back. The 20:20 then does a 'beam distance travelled'/'time' calculation to work out the speed. (it's way more complicated than that and involves multiple readings, software assumptions involving least squares etc but I believe that's the jist)
How do they know that the beam hitting the bike hasn't reflected off an angular front panel, bounced onto a nearby vehicle/object and then travelled back to the camera? Since the beam distance travelled will then be much greater the speed reading would be higher than the actual speed of the bike.
They will of course tell you that the gun is too clever to allow that to happen, but how do they know if they've never tested for it?
Ask him to explain what a signal of adequate strength is and how it prevents the reflection scenario I've described. Ask him to explain how the camera knows that the beam hasn't slipped down the side of the bike.
On second thoughts, don't bother. The answer to those will also be "the gun is too clever to allow that to happen" Since they admit that Home Office testing doesn't include testing for slip I don't see how they can make that claim, but I'm sure they will.
>> Edited by MrsMiggins on Saturday 26th November 15:24
How do they know that the beam hitting the bike hasn't reflected off an angular front panel, bounced onto a nearby vehicle/object and then travelled back to the camera? Since the beam distance travelled will then be much greater the speed reading would be higher than the actual speed of the bike.
They will of course tell you that the gun is too clever to allow that to happen, but how do they know if they've never tested for it?
Ask him to explain what a signal of adequate strength is and how it prevents the reflection scenario I've described. Ask him to explain how the camera knows that the beam hasn't slipped down the side of the bike.
On second thoughts, don't bother. The answer to those will also be "the gun is too clever to allow that to happen" Since they admit that Home Office testing doesn't include testing for slip I don't see how they can make that claim, but I'm sure they will.
>> Edited by MrsMiggins on Saturday 26th November 15:24
MrsMiggins said:
AFAIK, the LTI 20:20 gets a reading from a reflection off the target vehicle. Operators are told to target the number plate as it's a flat reflective surface so the beam should just bounce straight back. The 20:20 then does a 'beam distance travelled'/'time' calculation to work out the speed. (it's way more complicated than that and involves multiple readings, software assumptions involving least squares etc but I believe that's the jist)
>> Edited by MrsMiggins on Saturday 26th November 15:24
I *think* the gun sends out a pulsed beam. It measures the distance of the object (in this case a bike), then a fraction of a second later measures it again. The difference between the two distances allows the gun to calculate the Speed = Distance/Time equation and then display the speed.
Ben
Yes. the gun takes the data set, messes it around it's circuitry for a while and supposedly rejects it if it doesn't 'look right'.
It's explained by John Brignall over on Numberwatch at www.numberwatch.co.uk/2005%20September.htm
Rather than exploit the Doppler Effect, it takes a series of readings over a period of about a third of a second. The speed is calculated by fitting a straight line to the data sequence. Only if the data do not form a straight line is an error flagged. Just fitting a straight line is fraught with difficulties; determining whether the line is actually straight is a whole new ball game. The major potential source of falsity is “slip error” in which the beam rakes the side of the vehicle.
Numberwatch is an interesting place and it's difficult for TPTB to rubbish his comments because he knows his stuff. He's not that impressed by Frank Garrett either.
>> Edited by MrsMiggins on Saturday 26th November 17:32
It's explained by John Brignall over on Numberwatch at www.numberwatch.co.uk/2005%20September.htm
Rather than exploit the Doppler Effect, it takes a series of readings over a period of about a third of a second. The speed is calculated by fitting a straight line to the data sequence. Only if the data do not form a straight line is an error flagged. Just fitting a straight line is fraught with difficulties; determining whether the line is actually straight is a whole new ball game. The major potential source of falsity is “slip error” in which the beam rakes the side of the vehicle.
Numberwatch is an interesting place and it's difficult for TPTB to rubbish his comments because he knows his stuff. He's not that impressed by Frank Garrett either.
>> Edited by MrsMiggins on Saturday 26th November 17:32
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