from what point can you accelerate up to 80km/h?

You can accelerate up to 80 km/h only after you have passed the speed limit sign that indicates the new speed. Accelerating before reaching or just seeing the sign is not permitted. The correct point to begin accelerating to 80 km/h is after passing the speed limit sign or exactly at the sign itself, so the vehicle is traveling at the new speed limit when it passes the sign line.