I am looking at that again. Ignoring the speed readout on the camera (given that its GPS powered which can have anomalies), the truck may not have sped up. Keep your eyes on either the bridge or the passing lamp posts and it looks like the HGV maintains their speed. Even the dashed white lines on the slip road to appear to move any faster.
It may look like it speeds up due to the Focus hammering the brakes (see the brake lights come on).
It may look like it speeds up due to the Focus hammering the brakes (see the brake lights come on).