I don't think your eyes are deceiving you, I saw the same thing. My guess is (and excuse my incorrect terminology) that the hitbox and the mesh aren't the same size, so the cars are passing after the hitbox (maybe the cab) safely, and thus through the trailer mesh/texture.
It's a cool game, but it's meant to simulate an entire city. It only makes sense that you lose accuracy in details. It's just meant to look somewhat believable, and make traffic design in the game meaningful.
There is no collision detection here, and consequently no hitboxes. It’s just a visualization of some simple game simulation data
There has to be some though right? Otherwise there wouldn't be any back-ups. I guess they could be point masses that can't occupy the same space of another point mass + some radius around it.
You just calculate the total load of the road and render enough cars appropriately with defined positions. Doing that logic through a physics system would collapse the entire simulation.