RobWard, my guess is this:
Stopping sight distance for a passenger car is ~500 feet (150m) at 55 mph (80 km/h). A loaded truck needs more than that, so let's assume the screen is triggered when the truck is 700 feet (200m) from the tunnel portal. At 2.5 second headways, there could be three vehicles between the portal and the overheight truck. Probably ten vehicles if they drive like NY City drivers.*
So, several of those vehicles are going to be closer than stopping sight distance to the portal when the system is activated. They can go through the water curtain without damaging anything except perhaps their undershorts. A physical screen would be damaged when the car collided with it.
I was wondering how well it shows up in daylight, too, especially when the sun is low on an east-west roadway.
*I was told the NYC standard is, if you can see the back tires of the vehicle in front of you, you're not tailgating.