How accurate are Speed Indicator Devices

[dillm]

Just to come back from Worksop via Mosborough and travelled through 3 speed indicator devices

Going through two in Worksop the Speed indicated my Speedo was showing 2mph fast @ 30mph (actual 28 mph according to Sat Nav and the roadside speed indicator )

Going through Mosborough The roadside speed indicator showed the speed as 29mph the same as my speedo but sat nav said 27mph

How accurate are these signs any one any ideas

[Revel]

No speedometers are accurate. That's why speed cameras allow for the extra couple of MPH to account for the leeway between different brands of cars.

Personally, I wouldn't trust GPS either - I always go by my car. If the signal is weak or becomes obstructed, you'll be seeing the wrong speed.

[apelike]

The on-board car speedo I would think is the most reliable indicator as they are manufactured to a high standard.

 

Sat-nav is only accurate to around 3mtr so will have a lag, not sure about the roadside indicators but would think they would also be more accurate than a sat-nav as its in real-time.

 

Edit..

 

Just had a bit of a google and it seems that the sat-nav is the more accurate..

Edited June 18, 2016 by apelike

[Shef1985]

The on-board car speedo I would think is the most reliable indicator as they are manufactured to a high standard.

 

Sat-nav is only accurate to around 3mtr so will have a lag, not sure about the roadside indicators but would think they would also be more accurate than a sat-nav as its in real-time.

 

Sorry, you have it the wrong way. Car speedos overestimate the speed, a Sat Nav is far more accurate.

 

Your use of 'real time' is peculiar here - Sat Navs use real time to determine speed.

[apelike]

Sorry, you have it the wrong way. Car speedos overestimate the speed, a Sat Nav is far more accurate.

 

Your use of 'real time' is peculiar here - Sat Navs use real time to determine speed.

 

Yes it seems car speedos can over-read by a few %. I mention real-time as sat-navs have a delay and according to google can be inaccurate on roads that have bends.

[Shef1985]

Yes it seems car speedos can over-read by a few %. I mention real-time as sat-navs have a delay and according to google can be inaccurate on roads that have bends.

 

Interesting, have you a link to that?

 

Can't see why bends make any difference. The 'map' lives in the device, preinstalled. The Sat Nav is just triangulating it's position with satellites and popping it onto that map: it won't know it's a bend.

Edited June 18, 2016 by Shef1985

[carosio]

Yes it seems car speedos can over-read by a few %. I mention real-time as sat-navs have a delay and according to google can be inaccurate on roads that have bends.

 

And steep hills, so I've read.

[Shef1985]

And steep hills, so I've read.

 

Yeah, I can see why a steep hill makes a difference but not bends...

[apelike]

BTW I meant speed when I said it could be inaccurate on roads with bends and I believe that the satellites rely on GPS data and not the map when calculating speed. I also underestimated the accuracy as its around 5-10 metres on the ground.

 

All I can offer is this snippet from Money Saving Experts:

 

"On a straight, level road, at a constant speed, with a clear view of the sky, the GPS will be accurate. Any bends, hills or changes in speed and the GPS will be inaccurate. As the above poster states there is some "lag" or delay in the speed reading from a GPS."

 

And info about accuracy on level ground and height from here:

 

https://www.ordnancesurvey.co.uk/resources/sat-nav-data.html

[tlangdon12]

Steep hills and bends make a difference for the same reason; the GPS is measuring the time to travel between sets of GPS coordinates which it assumes are on a flat surface, and that you are travelling between the two points in a straight line. Go up a hill and the rate at which you cover the horizontal distance decreases. If you go round a corner, because you are taking a longer route that the straight line, the GPS sees this as a slower speed.