This morning on I St. NW, I saw this
and after a step or two wondering who it was meant for, understood that it was for the driver, who now will never need to look at a sign for a bridge or garage, and wonder whether the truck will make it through: a glance in the left-side mirror will show that.
I remember in my college years arriving in a company van at a garage. The sign announced a maximum clearance of 6'6" or 6'8". I stood beside the van, and did my best estimate of its height, then pulled in, ducking my head at every girder I drove under. I'd have been grateful for such a hint. (Yes, it probably was in the owner's manual, and the owner's manual probably was in the glove compartment, but that wasn't the sort of thing I thought of at 20.)