the threshold (i think) is in that there is some tolerance for ther gun error.
I.e. if your ticket says 34 you were probably doing 38 on the clock.
I was doing 34 (on my speedo) when i saw the camera van, i was 100% convinced i would get a ticket! nothing came, it must have read 30 on the camera! or pretty damn near
dfarrall is right, there is no THRESHOLD, only GUIDANCE.
People also need to distinguish between the speed that the police gun shows (basically your accurate speed) and the speed your in-car speedo shows.
If the limit is 30, then the police can charge you if their
speed gun shows you as doing 31 mph. However, they have GUIDLINES which usually state to only charge when their speed gun shows you as roughly +10% (ie >33). This is partly to account for any error in the speed gun, where if you're only just marginally over the limit, then they have to prove that their gun is sufficiently accurate to measure within 1mph accurately.
However, your in-car speedo is always set to over read. That is that when the police gun shows you as doing 31mph (your actual accurate speed), then your in-car speedo will have shown you as doing more (eg 33mph).
So to get done doing 34mph in a 30mph zone, it is likely his speedo read closer to 35-37mph at the time, which isn't exactly 'harsh'.