The accuracy of speed measuring equipment has often been disputed but
seldom has the cars speedometer been discussed.
All anologue devices accuracy is quoted as a percentage of full scale
deflection. ie if your speedo has a full scale reading of 200 kmph
and the accuracy is one percent then any reading, say at fifty kmph
between 48 and 52 means 50 kmph and this would be supportable in a court
provided the court was prepared to recognise the laws of physics.
No court has ever been able to over rule those laws, but they do
sometimes pretend they dont exist.

I am not sure what accuracy is considered normal for car speedos but
I doubt they would be as good as 1 %, around 3% would be my guess.
That would mean anything +- 6kmph would be acceptable and should be
acceptable to a court. Of course additionally you must add to this
the percetage accuracy of the police radar etc, so if it was 1% then
the overall accuracy in dispute is 3 + 1 = 4 %.

Additionally Federal law requires that any measuring equipment used in
financial trabsactions, selling, weighbridge charges, fines etc must
have a valid calibration certificate.
If booked you are entitled to ask the copper for the certificate.
He probably will say ehh ! whats that ?
But they will have to produce it in court.
You would need to have a good reason to go to the expense however.

Speedo accuracy is very easy to determine with a GPS, which gives an accurate speed based on satellite location, so long as you can sit on a constant speed for a short distance. You can very easily and quickly note the accuracy of your speedo this way.

I do this all the time and have done for years – compare GPS speed with speedo speed in the various different vehicles that I drive.

So given that this is easy and given the basic premise that accuracy and roadworthiness of all aspects of a car should be the drivers/owners responsibility, it should be a requirement.

GPS units should be hirable at RACQ, NRMA, Dept of Transport, council offices, etc.

Let’s get away form this silly notion that is alright for drivers to have pretty highly inaccurate speedometers and that there should be a large degree of slackness in the law to allow for it!

Let’s try that last line again shall we…

Let’s get away FROM this silly notion that IT is alright for drivers…..

Fahhhhk!!

It should be the responsibility of drivers to err on the side of caution regarding any inaccuracy in their speedos. I would also argue that any inaccuracy in police speed-measuring devices should be thought of in the same way, for as long as the police are forthright with us about this inaccuracy. However, I’m sure that any inaccuracy these days in this equipment is trivial.

The point is that the principle of law needs to be upheld. And with a law such as speed limits, which are 100% crystal clear on the signs and in the zones represented by the signs, those values surely must be what is policed and observed by all road-users.

For some reason which I have yet to understand, practically everyone that I have encountered in this debate, on this forum and elsewhere, disagrees with this and thinks that leeways are ok and that traveling a few ks over the stated limit should be acceptable.

Well if it is to be ok, then the definition of speed limits needs to change in law. They need to be called speed zones and the actual limit needs to be defined and publicized widely. If 60 signs actually meant that you could do 65 or 70 or 10% over (66) or 5% over or whatever, then fine.

Ludwig has missed the point.
Calibration accuracy is not something that can be dismissed as
irrelevant in court. Accuracy and its calibration percentage is
fundamental and is the result of the laws of physics.

Laws cannot be written that ignore it.

Also it is a legal requirement. I underestand without having read it
myself that the speedo accuracy is defined in the Australian design rules.
If I am wrong and it is not defined then all fines should be refunded.
Any decisions based on speed are illogical and furthmore are unprovable.

I have discussed this with an inspector of the Highway patrol and this
is why there is an allowance, mainly because of the accuracy percentage
of their own equipment but also the accuracy of the cars speedo.
Without being told I suspect that they allow 1 ot 2 Kmph for their equipment and
2 or 3 Kmph for the speedo accuracy.
The cars indicated speed varies with tire pressure and wear.

In the field of measurement it is normal practice to make allowances for calibration accuracy.

Black Spots By Stupidity. (to get back to one point of the initial post)

Roundabouts.
Probably 99% of roundabouts have reverse camber. There are instances on record of trucks falling over on roundabouts at low speed, usually because the load shifted, the load being cattle. Now they are confined but they can only be crushed so much, they must, by law, be given a little room to move.

Why the reverse camber when, again 99%, of other road curves have positive camber?

Very simple...drainage. If you put positive camber on a roundabout then you must provide underground drains and they are costly both initially and for maintenance.

So what price safety?

For some reason we have adopted the practice of very small roundabouts.
These roundabouts have many accidents because the time to make a
decision on whether the other driver is in the roundabout before you is
just too short.

Any reasonable vehicle speed does not have a great deal of affect
on the time to make a decision.