At a fixed camera, one moment counts: the instant you pass it. In an average-speed zone, everything in between counts. That sounds like a detail, and it is exactly where most drivers come unstuck.
What actually happens
A camera at the start of the zone reads your plate and records the time. Another one does the same at the end. The system knows the distance between those two points precisely, divides it by the time you took, and treats the result as your speed.
So nobody is measuring how fast you were going under the camera. They are measuring how fast you went on average across the whole stretch — sometimes a few hundred metres, sometimes ten kilometres.
Why braking at the end does not work
Driving 130 through a 90 zone and slowing only at the exit camera still leaves an average well over the limit. Easing off for the last stretch barely moves that average, especially on a long zone: you would have to make up the deficit over the remaining distance, and there is none left.
The only way through an average-speed zone is to keep to the limit for its whole length. That is precisely what it is for.
The technical correction still applies
The statutory measurement correction is applied to the calculated average as well. In Belgium it is 6 km/h for readings up to 100 km/h and 6% above that. A measured average of 100 in a 90 zone is therefore treated as 94.
It is not a margin to spend deliberately. It is a correction for measurement uncertainty, and it exists so that nobody is fined for the equipment's imprecision.
Where they are
In Belgium, average-speed zones are concentrated on motorways, on regional roads through village centres, and in tunnels. They are not always signposted, and the start and end points are rarely where you would expect them.
TrapSpotter shows where each zone starts and ends, the limit that applies inside it, and — while you are in it — your own average so far. That last number is the only one that matters at that moment.
See the zones on your route