How accurate are Speed Indicator Devices

[Shef1985]

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

 

Ahh, got it. Because the Sat Nav works by determining the travel time between point A and Point B, if you go round a sharp bend very fast the Sat Nav cannot account for you turning at the time, it will simply assume you'd gone straight from A to B and therefore underestimate the speed necessary to have covered that distance.

 

Its marginal though.

 

---------- Post added 18-06-2016 at 19:12 ----------

 

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.

 

Was writing as you were

[dillm]

Hi thanks for the input

My query was how accurate the Speed indicator Devices the ones that either flash ≤30 and give a smiley face or if you are over give a sad face

[Shef1985]

Hi thanks for the input

My query was how accurate the Speed indicator Devices the ones that either flash ≤30 and give a smiley face or if you are over give a sad face

 

Oh yeah forgot about that...they seem to match the car speedo more than the Sat Nav.

 

Just make them happy

[Eater Sundae]

R

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

 

When I get a "new" car I calibrate the speedo by using the sat nav on a straight, flatish section of motorway at around 70 mph. This avoids the problems associated with bends and hills. I also run at a constant speed to avoid any time lag problems. I do the same on a quiet section of 30 mph road, to claibrate for a lower speed.

 

Once I've done that, I know how inaccurate the speedo is, so I can adjust for its inaccuracy in my head and be confident of the speed I am doing at any time.

 

I find that the roadside speed indicators show faster speeds than reality, but only by a couple of mph.