That sounds way harder. You'd first need to lift a giant pile of metal to a cartoonishly high height, then somehow time it to drop on yourself when the cars are near.

True. So if they are smart enough to fool AI, they will just remove the mid span, and have convenient weight bearing beams nearby that they put in place when they need to cross. Or if it's two lane, only fake out one side because the AI will be too clever for its own good and stay in its own lane. Or put up a sign saying "Bridge out, take temporary bridge" (which is fake).

The point is, you just need to fool the vision enough to get it to attempt the task. Play to its gullibility and trust in the camera.

Yep. Destroying an autonomous fleet would be easy with a couple million $ and a missile... but it could also be done for nearly $0 with some intuition (and depending on how easy they are to fool)