Safe Speed Forums

With the increasing introduction of 20mph speed limits in this country, and the recommendation of strict enforcement by PACTS, it begs the question how motor vehicles are supposed to know when they have exceeded this speed limit.

My understanding of Speedometers fitted to Motor Vehicles is that they need to comply with the relevant approvals. For example SI 2001 No.25 (The Motor Vehicle Approval Regulations) has in Schedule 2, point 19:

http://www.opsi.gov.uk/si/si2001/20010025.htm#sch2

3. For all true speeds of between 25 mph and 70 mph (or the maximum speed if lower), the difference between the indicated speed and the true speed shall not exceed -
V/10 + 6.25 mph
where V = the true speed of the vehicle in mph.

and

This item shall not apply to vehicles having a maximum speed of less than 25 mph.

So essentially if you are travelling below 25mph, your speedometer can give any indicated speed (and many will), there is no regulation covering it's accuracy below this speed. In some vehicles capable of exceeding 20mph you may not even have a speedometer!

This is fine for when speed limits are above 30mph, but when they start issuing fixed penalty notices for travelling at a speed that you have no means of measuring this to me seems grossly unfair.

Has ANYONE in Government considered the changes needed to vehicle regulations (both in the UK and EU) for enforcement of 20mph limits to be reasonable?

I emailed PACTS about this issue, but to date have not had a response. One for Paul?

Rufus.

I thought that this one of the reasons that 20 mph zones had to be self enforcing, I'd forgotten this when reading the PACTS proposal, it's a good point.

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Thanks. Can open. Worms everywhere. Shame.

I'll add this information (which I had forgotten) to the mix.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Hi RufusA

If I read your message right, I’m afraid there is a flaw in the reasoning.


So the speedo must never UNDER-read, hence it follows that drivers will know when they are within the limit (but not necessarily know when they are over, but that supposedly doesn’t matter).


Another angle of attack to is question why there is no provision to check the accuracy of the speedo once the car has left the factory (is it even checked then anyway?), given that speed compliance is just sooo important.

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

whats a false vehicle speed anyway ?

A speed reading taken with an LTI2020

[quote="smeggy"]Hi RufusA

If I read your message right, I’m afraid there is a flaw in the reasoning.

[quote="OPSI.gov.uk"][b]2. For all true speeds up to the design speed of the vehicle, the true speed shall not exceed the indicated speed.[/b][/quote]
So the speedo must never over-read, hence it follows that drivers will know when they are within the limit (but not necessarily know when they are over, but that supposedly doesn’t matter).

>>>>>>>>>>>>>>>>>>>>>>>>

Smeggy, I'm afraid you have got that the wrong way around.
If the true speed should not exceed the indicated speed, the indicated speed should not be less than the true speed.
That is to say the speedo CAN over-read, but must never under-read

The way I understand it, a speedo could start at 25mph (i.e. no needle movement below 25) and still be legal - is that correct?

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COAST Not just somewhere to keep a beach.

A young loner on a crusade to champion the cause of the innocent, the helpless, the powerless, in a world of criminals who operate above the law.

Cheers fixitsan. I accidentally said over when I should have said under. I've corrected my previous post.

I read it the same way. That kinda buggers up the principle of enforcing 20 limits - "am I doing less than or equal to 20, or more than 20?"

In the words of the Late and Great "Vivian Stansfield" RufusA, .......... "NICE!".........

devils advocate mode...
That would only get you out of 25mph in a 20 limit...
and seeing as the prosecution threshold is 20mph + 10% + 2mph they could not prosecute until 24.1mph
/devils advocate mode...

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Speed limit sign radio interview. TV Snap Unhappy
“It has never been the rule in this country – I hope it never will be - that suspected criminal offences must automatically be the subject of prosecution” He added that there should be a prosecution: “wherever it appears that the offence or the circumstances of its commission is or are of such a character that a prosecution in respect thereof is required in the public interest”
This approach has been endorsed by Attorney General ever since 1951. CPS Code

My experience of modern cars is that substantial damping is applied to the needle on the speedo. I wouldn't really trust the measurement during acceleration or braking. I do not think speedometers in modern cars use the spinning magnet technique.
Regularly I spot that the needle on my speedo catches up with my stationary position eventually.
Is this due to the fact that the car is full of microprocessors for which there is questionable software ? For example my car can sometimes flash the dash indicators for a second or two after turning the indicators off.

No they don't use spinning magnets any more! All electronic I'm afraid. The most modern ones just take a speed pulse from the ABS sensors.

Indicator telltales staying on for a bit is probably (on the most modern cars) a "CAN" or other multiplexed system. There is often a slight delay for things it deems "non-essential". The telltales await a signal from the main controller telling them to stop flashing but the main controller only "looks" for an incoming signal from the stalk that the indicators have been turned off every "X" milliseconds and then sends another one out to tell the telltales to stop. The cumulative effect of this and the natural period of flashing can combine to make a noticeable delay on some cars. On others, you might notice that when you turn the headlight switch off there's a short delay before the lights actually GO off!

Pretty much my guess as to what happens. It is a bit cack though, my wife is not used to my car and has to double check a left hand indication to then result in a right hand indication. I understand the CAN network concept but surely it is simply necessary to partition the network such that the lights are not routed through the safety critical stuff. Indeed lights are safety items but can be treated in human time frames.
The electrical networks help to reduce control wiring but power distribution is probably more than for decades, given that there is normally a centralised fuse box.

That's exactly what seems to be happening! The newest cars (at least the ones I've had any dealings with) seem to have now settled for three CAN systems - one for engine / transmission management, one for "chassis" (ABS etc) and one for "house" systems (lights etc).