The blind spot built into a box truck
A moving truck is not one uniform shape. Its cab is relatively low, while the rectangular cargo body behind it can rise much higher. From the driver’s seat, the roof of that box is difficult or impossible to see, so a bridge can strike the cargo body even after the cab has passed beneath it.
Why the posted number matters
A clearance sign gives the height available beneath a bridge or other overhead structure. The truck’s total height must fit within that space—not just the height of the cab. If the cargo box is taller than the posted clearance, the front of the vehicle may enter the opening before the driver has a clear visual warning that the roof will hit.
That mismatch is especially easy to miss when someone is driving an unfamiliar rental truck. The vehicle may feel much like a car from the driver’s seat, but its height and dimensions are very different. A truck can also approach a low bridge along a familiar road where ordinary car traffic passes without difficulty.
A collision that can stop traffic
When the upper body hits, it may become wedged beneath the structure or suffer severe damage. The truck can block the lane while drivers and road crews determine whether it can be moved safely and whether the bridge needs inspection. The practical safeguard is to know the truck’s full height and compare it with clearance signs before entering an unfamiliar route. The cab fitting under a bridge is no guarantee that the cargo box will.
