The next best thing to holo-signs: A visual barrier, but won't actually harm you if you hit it.
Brilliant, but unfortunately increases stopping distance.
The water is right at the tunnel entrance, not in the area where you would be braking to avoid impact. By the time you've reached the water you will have collided with the top of the tunnel, which should go a long way towards helping you slow down. Literally 'idiot proof' design right there.