Roadside speed indicator

Recently, as I approached a speed indicator on the freeway with my car's speedo on 100 km/h exactly it indicated I was doing 94 km/h! Later, when passing road works with a temporary speed limit of 60 km/ h a trailer-mounted indicator showed I was doing 64 km/h, but when returning through the same roadworks later I was shown as doing 56 km/ h. Same experienced driver, same car, two years old, original size tyres in good condition and recommended pressure plus two psi. I know that the ADR only requires an accuracy of 10 per cent, but is there any accuracy requirement for these speed indicators?