Speed detection displays

[jj4091]

Noticed this a few times and always been puzzled about how these things measure your speed. If I'm driving behind a high vehicle ( wagon or even van) as the vehicle in front of me passes the display and then I pass in my little Suzuki the reading always increases by 2 or 3 mph even though we are both driving at the same speed. It doesn't seem to happen in a line of cars as far as I can tell.
It's usually a 30 mph sign. Could this be correct or is it my imagination.
Just happened again on my way home which has prompted the post

 

[Wayners]

I'm going to guess. I don't actually know for fact. Maybe you can look up online and give an opinion.
I think the further you are away then the better the line of sight for the radar.
As the lorry passed it could be showing slower than the actual speed, then when you be become visible the speed display jumps up because the radar is bouncing better and more accurate
There is a optimal distance for accelerate radar reading and it's not close or by the side of the radar from what I understand

 

[jj4091]

" Maybe you can look up online and give an opinionn"

No info that might explain this online that I have been able to find.

 

[foxhole]

I think I read somewhere reflection from vehicles in close proximity can cause errors , a vehicle towing at low speed was recorded as breaking speed limit by more than the speed the vehicle could possibly achieve but was proved to be an error.

 

[jj4091]

I think I read somewhere reflection from vehicles in close proximity can cause errors , a vehicle towing at low speed was recorded as breaking speed limit by more than the speed the vehicle could possibly achieve but was proved to be an error.

Thanks that would seem to fit what I have noticed although I am not close enough to be mistaken for a trailer and not sure if both devices
( camera and warning sign ) operate in the same manner.

 

[robinbanks]

Warnings signs operate on vehicles travelling towards; Gatsos etc can operate towards but last I knew were only type approved for away. I expect the premise will be similar, that reflected electromagnetic waves will have a different frequency when returned than when emitted and this is used to calc the speed

Perhaps, as suggested, try increasing the spacing between you and the vehicle in front and see if it alters the behaviour. Also if you have a favourite local sign, travel past it on cruise control set to a consistent setting and see if you get a variation in the sign. I can't imagine the tech they use will be designed to be anywhere near as accurate as a gatso

Anecdotally I have always found them to be broadly accurate, and not exhibit the behaviour you describe

 

[JBR]

I think that perhaps, just to be on the safe side, I'd drive at a notch or two below the stated speed limit.
I suppose that you might arrive a minute or less behind your expected time.

 

[jj4091]

Who mentioned any figures

 

[Avocet]

Noticed this a few times and always been puzzled about how these things measure your speed. If I'm driving behind a high vehicle ( wagon or even van) as the vehicle in front of me passes the display and then I pass in my little Suzuki the reading always increases by 2 or 3 mph even though we are both driving at the same speed. It doesn't seem to happen in a line of cars as far as I can tell.
It's usually a 30 mph sign. Could this be correct or is it my imagination.
Just happened again on my way home which has prompted the post

I hadn't noticed that, to be honest. Radar (and sound and optical-based) systems tend to suffer from "cosine error".

But this should result in the equipment reading LESS than your speed, not more. I'm just wondering whether these devices have a bult-in algorithm to compensate for cosine error, and if so, whether it's a tad "optimistic" on the devices you've seen? I can't think of how the presence of a large vehicle ahead of you should make a difference, but obviously, there will be a point where it stops measuring the vehicle ahead's speed and starts measuring your speed instead. When this happens, it's going to have to have some software that handles the changeover and maybe in that split second, it over-estimates your speed? Dunno, just guessing.