What is so special about this bridge? Why do people drive under it without checking their height? You wouldn't normally think you can just drive under any bridge, would you? It doesn't even look like a very low bridge. Why do people approach it without a care in the world at full speed? My vehicle is only 2m tall and I still do take a quick look at height signs as I approach them.
A typical driver of a car, minivan or SUV in the United States will virtually never encounter a bridge or other obstacle whose height above the road is low enough to matter to them, so most people simply never have a reason to pay attention to those height signs.