Outside Lane Hoggers

[plod]

Similarly you can not accelerate until you pass the next sign signalling an increase in the legal limit.

I disagree

It's 24 years since I past my test, but I remember on one of my lessons, the instructer asking me what I was doing as we approached a 60 sign whilst in a 30, as I remained at 30, and he said you were allowed to increase your speed so you were up to speed when you reached the sign

[mjb]

Wrong.

The speed limit sign marks the beginning of a speed limit as delimited by a line passing the speed limit signs on either side of the road*, not the beginning of some magic grace period. Why do you think there's so many GATSOs mounted immediately after a speed limit reduction?

You are required to brake BEFORE a sign reducing the speed limit. If the sign is not placed in accordance with the legal guidelines to allow this (such as dropping the speed limit from NSL to 30mph on the exit of a tight bend with no prior warnings like "30mph in 100yds") then you have broken the law but you have good grounds to contest a speeding ticket. If it is a properly placed and visible sign and you get done for 31mph in a 30mph area a foot past the sign then you have no recourse.

Similarly you can get done for doing 31mph in a 30mph area inches before an NSL sign or on a motorway slip road before the blue motorway sign if there happens to be an arsehole in a scamera van nearby

If you care to disagree, please find the law which allows a grace period first.

* The exception to this is where a junction is concerned, in which case the demarcation point is the white line dividing the roads

[Welly]

You are certainly spot-on there m but I was kinda saying that in general day-to-day motoring we don't really need to correct the speed immediately just so long as it is done reasonably, of course if you were to fall foul of a speed trap then you wouldn't have a leg to stand on. Usually the change in speed restriction is done well before the 'hazard' so gives this 'transition time' I spoke about but purely in my own opinion.

I understood the law on speeding was an 'allowance' of +10%, + 2mph so 30 would be 35+ and 60 would be 68+ and so on.

Anyway, what about drivers who almost slam into the back of you when you creep over a speed bump so as not to damage your motor

[leestudd30]

When it comes to 31 in a 30, are car speedos really that accurate? I always thought that the filth have calibrated speedos so they are always accurate, but surely production cars and older ones aren't as accurate as the police?

[mjb]

I understood the law on speeding was an 'allowance' of +10%, + 2mph

Almost certain it's not law, just police guidelines for who to prosecute, and I've got a feeling it's 5%+2mph

When it comes to 31 in a 30, are car speedos really that accurate? I always thought that the filth have calibrated speedos so they are always accurate, but surely production cars and older ones aren't as accurate as the police?

As far as I'm aware (haven't read the law on this one), cars deemed fit for UK roads are not permitted to have speedos which read less than the actual speed, or more than 10% more. Most manufacturers aim for over-reading by about 5% to account for production tolerances and tyre wear.

This means if your speedo says 30mph and your car is legal, then you will be doing anywhere between 27mph and 30mph.

[gumby6371]

I would have to agree with speedos reading over, I've used different sat navs in several cars and the sat nav speedo always read under the cars speedometer by 2 or 3 miles an hour. In theory (unless you've done some major mods) if the needle is pointing at the correct notch on your speedo you should be just under the legally allowable maximum speed.

[waue1978]

I would have to agree with speedos reading over, I've used different sat navs in several cars and the sat nav speedo always read under the cars speedometer by 2 or 3 miles an hour. In theory (unless you've done some major mods) if the needle is pointing at the correct notch on your speedo you should be just under the legally allowable maximum speed.

Unless you are driving something fitted with a tachograph which is supposed to be as bang on accurate as the speedo in the police cars.

Has anybody ever looked at the speedo in a police car? I looked at an ex-police ST200 Mondeo & whereas the normal Mondeo speedo has a line every 10 or 5mph (can't rightly remember) the police one had a lot more markings on it.

[steve_earwig]

I had an ex-police Senator and it had what looked like the standard (analogue) speedo with a calibration sticker on it. I didn't have another to compare it for extra lines or enifink tho...

When I was in GB back in April I was relying on the scat nav to tell me how fast I was going (no mph) and was keeping it bang on the speed limit, overtaking masses of other cars coming round the old M25.

[plod]

Wrong.

The speed limit sign marks the beginning of a speed limit as delimited by a line passing the speed limit signs on either side of the road*, not the beginning of some magic grace period. Why do you think there's so many GATSOs mounted immediately after a speed limit reduction?

You are required to brake BEFORE a sign reducing the speed limit. If the sign is not placed in accordance with the legal guidelines to allow this (such as dropping the speed limit from NSL to 30mph on the exit of a tight bend with no prior warnings like "30mph in 100yds") then you have broken the law but you have good grounds to contest a speeding ticket. If it is a properly placed and visible sign and you get done for 31mph in a 30mph area a foot past the sign then you have no recourse.

Similarly you can get done for doing 31mph in a 30mph area inches before an NSL sign or on a motorway slip road before the blue motorway sign if there happens to be an arsehole in a scamera van nearby

If you care to disagree, please find the law which allows a grace period first.

* The exception to this is where a junction is concerned, in which case the demarcation point is the white line dividing the roads

I agree that when slowing down, that where the lower speed limit is idicated, it starts level with the sign
What I was saying, that when the speed increases from a lower limit to a higher, my driving instructer informed me I was in the wrong for waiting till I reached the sign before speeding up

I tried searching for a definitive answer when I posted my first post but couldn't

[Doggy]

Now here's a technical conundrum for you....

We've all noted a similar difference between speedo mph & satnav mph and generally the discrepancy 'fits' pretty well with the known inacuracy of yer average speedometer. But I think there's also another possible explanation.

A satnav computes where it is by triangulating the signals from multiple satelites, then integrates speed from the rate of position change. Will this calculation not undersate actual speed if you're travelling in anything other than a perfectly straight line? For that matter, don't you have to be maintaining precisely the same altitude too?


So many questions, so little beer.

[mjb]

A satnav computes where it is by triangulating the signals from multiple satelites, then integrates speed from the rate of position change. Will this calculation not undersate actual speed if you're travelling in anything other than a perfectly straight line?

Yes, but given it happens several times a second, and the speed you see is averaged over the last second or so (and typically only given to 1mph/1kph resolution anyway), it makes very little difference.

High end GPS receivers can calculate your speed up to 100 times per second. Unless you're strapped to a helicopter rotor in flight, the difference between a straight line and the curved path in 0.01s is as good as non-existent.

For that matter, don't you have to be maintaining precisely the same altitude too?

GPS receivers are inherently three-dimensional, although much less accurate when it comes to heights, however if there's a couple of satellites nearby, you should be able to get a reasonably accurate fix. TBH if your altitude is changing enough to impact the speed calculations, and you're focusing on the satnav instead of the road, you have MUCH bigger problems