It depends on what you want to fix.

The 11'8 bridge does indeed seem a lost cause as far as preventing people from ruining their overheight vehicles. But the problem for the bridge has been solved: they installed a beam in front of the bridge which takes the impact.

The same sort of thing could be done with this bridge. Install a sturdy beam ahead of the structural bits. You won't stop people from driving overheight vehicles onto the bridge, but you will stop them from destroying the bridge in the process.

It depends on if you want to allow max height vehicles to use the center lanes or not.

I guess you could have a mechanism to raise the warning beam and only activate it after direct communication with the driver.

You could have sacrificial beams in front of all the important ones throughout the bridge, if you're worried about lane changes. That may not be worthwhile though.

My (not well studied) understanding is that all of the beams are important on this style of bridge.

That's sort of the other way of looking at the problem. The bridge isn't overbuilt enough.

Right, so you'd need a lot of sacrificial beams if you wanted to defend against lane changes in the middle of the bridge. Probably easier to add structural redundancy instead.

That water curtain stop sign is brilliant.

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.

There's a whole page about the Montague St bridge in Melbourne: http://howmanydayssincemontaguestreetbridgehasbeenhit.com/