According to Mr Garratt, TeleTraffic, at the Preston Crown Court Appeal of Brian Wiltshire, the LTI 20.20 error trapping
allows a deviation from linearity of +/-20mm. i.e. 40mm overall.
There are about 40 distance readings in 0.3 seconds to get your speed, and it is allowed to drop 5 outliers (exceptional readings).
Considering the situation relative to the middle of the speed measuring interval, +/-0.15 secs,
but allowing 6 outliers in 40, this reduces to +/-0.1275seconds
X = 0.5 A T^2
X is total deviation from linearity (40mm)
A is acceleration
T is time (0.1275s)
hence A = 2 X / T^2 = 2 x 40E-3 / 0.1275^2 = 4.92m/s^2 = 0.5g
but don't believe all the claims made for the tolerances of the error trapping.
I understand modern cars can achieve braking of 0.8g, or more, but fairly heavy braking would be required.
Hence if you keep braking as soon as you see the camera van it should produce an error reading if they zap you.
You can see what amount of braking , A, in metre per second squared, is required for this by measuring your stopping distance, X, in metres, from some known speed, V, in metre per second, then;
A = 0.5 V^2 / X , divide by 9.81 to get the answer in g.
Given a speed of 30mph = 13.4m/s
and stopping distance of 18m
A = 4.99m/s^2 = 0.51g
However they often have the speed cameras about 400m after a left-hand bend in the road, or some obstruction such as a sign mostly on the left, so as to get your speed before you realise they are there, but they can't read your number plate at that distance, so they track you until you come close enough to read your number plate.
If you can, stop before you get within about 200m and then wait to follow a large passing lorry, or if possible turn around and drive away, or turn off, before you get close enough for your number plate to be readable. The video cameras used have fairly limited resolution, even with the powerful telephoto lenses used.
Obviously this doesn't help if you are going away, but you should have seen the camera van by then, but they may be concealed to make then difficult to spot.
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