Did you see the truck in the video take out an enormous steel beam that was presumably attached to the structure of the tunnel in some kind of permanent way? How would your solution avoid the same fate?
I'd try a 3-stage contact-warning system:
(1) hanging bar that makes noise on contact but causes little damage. This might even go before a suitable turn-off.
(2) breakaway cord with trailing noisemakers. This would be torn from its mast by the collision, and drag noisy items behind, emphasizing the need to stop and investigate.
(3) hard bar providing some slowing/resistance (but almost certainly not full stopping power, given the position and expense). By design this would actually damage the vehicle (and even risk harming the driver), approaching the same level of damage that would occur at functional tunnel-entry. But, it'd happen earlier where easier to clear/repair.
The vehicle is able to be removed under its own power, the driver is not injured, there is no debris.
If you are in a vehicle behind a truck that has damage to its top, you very likely will have damage to your vehicle from flying debris.
After all, the water wall seems primarily visual, and the driver has already missed numerous visual warnings. The contact protectors add collision-noise and vibration, which are rare enough that even a dozing/road-hypnotized driver is likely to take note.
Comparative testing is of course justified.
Or a trapdoor (warning this is a joke disregard it if you know you don't like jokes)
Excellent plan! A trapdoor! In the harbour tunnel!
Relevant anecdote: My Dad worked for the company who built the fire control system in the Sydney Harbour Tunnel. It's got (from fuzzy memory - numbers not guaranteed accurate) 128 sectors of "deluge valves". It's got pumps capable of keeping up with only 10 of those sectors of deluge valves - more than 10 sectors of valves open means even with all pumps running at maximum capacity the tunnel will fill up with water. There's some software controlling it all which is intended to ensure the operators don't accidentally flood the tunnel. Dad knows who wrote that software - he won't drive through the tunnel.