How accurate are speed indicator signs ?

Retrogamer

The ones around my area vary. I've seen people going through the 30mph one, one day and it not flash at all when they're doing 35+ then other days the same one goes off when you're doing less than 30mph on the speedo (mines is almost 100% accurate via sat nav)

dannyrst

First post, nothing to do with saving money...weird

z1a

"(mines is almost 100% accurate via sat nav)"

Is it a Saab? Mine is extremely accurate with sat-nav pretty much right through the range.

AdrianC

coffeehound wrote: »

Probably another thread, but is the speed indication on sat-navs reliable?

Yes, absolutely.

GPS sends time signals from each satellite. The GPS receiver in your satnav then calculates the position from the miniscule time differences between each signal that it receives. So the two things it knows with absolute precision are the exact time and the pretty-damn-near exact location - usually within a metre or five.

From that, it's a doddle to give you the ABSOLUTELY bob-on precise speed.

Meanwhile, your speedo tells you the speed by measuring how quickly the wheels are rotating, then working your speed out from what it was once told about the diameter of the tyres that were fitted to your car when it was new.

The sign tells you your speed by shouting and listening for the echo.

Nilrem

AdrianC wrote: »

Yes, absolutely.

GPS sends time signals from each satellite. The GPS receiver in your satnav then calculates the position from the miniscule time differences between each signal that it receives. So the two things it knows with absolute precision are the exact time and the pretty-damn-near exact location - usually within a metre or five.

From that, it's a doddle to give you the ABSOLUTELY bob-on precise speed.

Meanwhile, your speedo tells you the speed by measuring how quickly the wheels are rotating, then working your speed out from what it was once told about the diameter of the tyres that were fitted to your car when it was new.

The sign tells you your speed by shouting and listening for the echo.

I hate to disagree but Sat nav's tell you the speed you WERE doing between the updates (it has no way of telling what speed you are doing all the time unlike the speedo).
There is a lag between it updating both when accelerating and when braking.
How much it lags depends on how often the sat nav updates, how many satellites it's locked onto, and if you're moving in a straight line on the flat, or going up/down or turning.

I've seen satnavs indicate I'm driving in a field 100 yards to to side of me, I've seen satnavs indicate I'm still slowing down after I've put the hand brake on, or when I've had to do a fast stop.
From memory my old TomTom One v2 would take about 2 seconds to show I'd come to a complete stop under some conditions despite being on a nice straight A road.

Don't assume that a Sat Nav will be more accurate for your speed under all circumstances, it should be more reliable on the flat if you're keeping a constant speed, but it can and will vary in it's accuracy under other conditions.

Consumer level sat navs are not all the same* so to assume they are all the same level of accuracy under anything other than ideal conditions is silly.

Personally I'd be happy to use the sat nav to work out how accurate my speedo is, but not under all conditions.


*Some especially older/cheaper ones may not have the ability to lock onto as many satellites, or maintain the lock on some of them and may not update the display as often as newer/better models.

Strider590

BeenThroughItAll wrote: »

So your car's speedo UNDER-reads?

Try again........

The signs are set too low, genius.

AdrianC

Nilrem wrote: »

I hate to disagree but Sat nav's tell you the speed you WERE doing between the updates (it has no way of telling what speed you are doing all the time unlike the speedo).

Yes, that's true.

How much it lags depends on how often the sat nav updates, how many satellites it's locked onto, and if you're moving in a straight line on the flat, or going up/down or turning.

True - with a caveat that the actual linear velocity isn't actually altered that much by the kind of radius you're turning through at anything but parking speed, or on anything but the very steepest of hills.

I've seen satnavs indicate I'm driving in a field 100 yards to to side of me

Which is as likely to be a mapping issue as a position-calculation issue.

Don't assume that a Sat Nav will be more accurate for your speed under all circumstances, it should be more reliable on the flat if you're keeping a constant speed

And, of course, the same applies for roadside speed indication signs - but with an even greater layer of likely error.

BeenThroughItAll

Strider590 wrote: »

Try again........

The signs are set too low, genius.

Is there any reason to be rude? I know you have a big of a chip on your shoulder about everything, but please do go back and read what YOU wrote. It is far from clear.