Why Does a Truck So Often Get Stuck in Our Underpass?

It just doesn't seem reasonable that trucks should have as much trouble as they do. After all, in every case I know, the road leading to the underpass has a sign indicating the clearance. Furthermore, it is natural to assume that the driver knows the height of the truck and is capable of comparing the two numbers and making an appropriate decision. And yet, the problem kept happening. I haven't kept any records, but I think they have decreased the clearance indicated on the sign at least twice in recent years.

What's the matter, does the railroad bridge over the highway settle a little more every few years, or is the highway department just unable to hire anyone who can measure the distance between the pavement and the bottom of the bridge?

Surprisingly, I think it's neither of these: I think the trouble is that trucks have gotten longer!