An improved version of the slowing tunnel can be made by placing trees adjacent to one another in the tunnel. This prevents the vaulting behaviour, and maintains the slow pawn speed for the full duration of the tunnel. Placing trees adjacent to one another can be done with the Nature Supremacy or Tree Connection memes.
I can't find a way to format an edit to include this information that wouldn't bloat the 'slowing tunnel' section, and would welcome anyone else's thoughts on how to format such an edit.