What we've got, when big trucks approach low overhangs, is a disaster just waiting to happen. See, for example, what happened here last year: a ton of trucks smashing into bridges and such things when their drivers don't read the height warning signs.
While these collisions make for great video material, they can also cause real damage and, at bottleneck points like tunnel entrances, hours of delays.
Engineers in Sydney came up with an awesome solution. If a truck going into this Sydney tunnel makes it past the warning signs, hanging weights, and flashing lights, traffic engineers deploy a laser-powered water curtain stop sign. It's a big investment, but the engineers say it's nothing compared to fixing the cost of a collapsed tunnel entrance. Plus, it looks downright futuristic.