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