On 2006-06-30, beerwolf (aka Bruce)
was almost, but not quite, entirely unlike tea:
Quoted message said:FWIW, I'm with Theo and Peter about fines for low range speeding, and
I regard them as little more than voluntary taxation.
on the stupid, which I heartily endorse 🙂
Quoted message said:Two or three times
a year I drive between Sydney and Melbourne, and always make a
point of checking my speed via the overhead readouts on the Hume Hwy,
if there's a clear piece of road ahead of me. I always hold the speedo
rock steady at 110, and invariably the readout shows me doing between
5 and 8kmh less.
As in your speedo was reading 110, and the camera was reading 102-105?
Yes, that is bad.
Quoted message said:When I first started doing it, I thought my speedo was
inaccurate, and compensated. But I have seen this now in 3 different
cars, and have also checked my speedo (while someone else was driving)
using a GPS. It's the overhead readouts that are wrong.
Somebody more inclined to conspiracy theories than myself might
think that the authorities actually don't mind a bit if you drive past their
cameras at 5kmh over the limit.
The speed detection devices used in enforecement measure your velocity
in the line of sight to the camera extremely accurately -- the 0.0001%
kind of accuracy limits that are inherent in the crystal circuits
used. Then they have to make a cosine angle correction to that
extremely accurate line of sight velocity detection, because you are
not travelling directly towards the camera. The closer to the camera,
the higher the perpendicular velocity component is, that can't be
measured. So it looks like you are travelling slower than you really
are. So if the cosine correction is not applied (presumably
automatically by the camera decting the angle relative to the fixed
tripod, with the tripod aligned relative to the straight road), then
the reading you get from the camera is a lower limit, which is
precisely what you want when it comes to enforcement. But after the
correcion, they will have more or less the correct speed, and will be
able to fine you accordingly.
The overhead camera on your bridge, that is not used for enforcement
(unless there's two cameras or two lines that act as a standard
ruler), isn't going to be pointed like a gun at the direction of the
car, and just reads the speed of the biggest signal in its field of
view. It won't really know the correction angle to apply, so just
gets the raw line of sight reading, which will be lower than the real
speed you are travelling. The further you are from the camera, the
smaller the difference between the real speed and the measured speed.
Up close, and the angle might be out by 20°, and the speed will be out
by 1/cos(20°)=6%.
This is fine, because they are not used for enforcement, but it does
mean that people may think their speedos are reading too fast, and
hence speed up. Oh well, they get fined, too bad. Trust the speedo -
the vast majority of them are adjusted quite a lot better than the 10%
they are required to be, by law. If they aren't, then you deserve to
be fined for having a speedo so badly outside the defined limits.
It's not that they don't mind you being over the limit, it's the
technological limits based on trying to manufacture the largest amount
of cameras for display purposes, for the cheapest cost.
--
TimC
Shame on you! Don't you love her? Girls don't want Kmart specials, they
want carbon - either as diamonds or as CF bikes! -- Tamyka in aus.bicycle