Freaking laser beams man. Pew pew! Before the bridge have two towers on both sides of the road with a laser that goes across set to a bit below the bridge clearance (to account for vehicle shocks/bounce and snow on the road), if the beam is crossed it then activates a huge warning to stop. I like the waterfall idea. But that requires water/pipes, which will freeze in winter. So could maybe use smoke instead? It does not need to be a screen just something that grabs your attention like some kind of high pressure smoke jets from the top of the bridge or even shooting from the sides. Not sure what is the best way to create this smoke though, and also keep it safe. Needs to be a chemical you can just store and that it can activate at a moment's notice.
Best bet might just be a huge flashing warning, if you're still ignoring it then you deserve to ruin your car. Idealy there should be some solid metal beams right before the bridge that are slightly lower, so you hit that and not the bridge, if you still end up going through. All these vehicles hitting that bridge are probably damaging it over time.
The water stop sign looks cool, but the down side is that when a driver sees this, the instinct is to slam on brakes.. on wet road...
Then the truck will slip and slide into the intersection anyways and crash into any number of things in the slippery water, maybe fishtailing around and hitting other cars. True, it would be a camera moment, but doesn't fix the problem.
Hmm, probably wouldn't work, but maybe a laser that paints a monster on the windshield with big teeth, like in the movie 'Alien'. I guess it would work if there was smoke in the area in front of the windshield. And then the message "In case you missed all of the other warning signs, your vehicle will crash into the bridge if you proceed."