They get ignored because there's a fudge factor built into them.

Some states (IL in particular) have absurd fudge factors, so you have 14ft spaces signed as 12.xx and 13'6" trucks drive under them all day every day like it's nothing which basically trains them to ignore the signs.

And that's before you consider all the drivers who can't read english at road speeds so anything that isn't the standardized yellow sign right on/beside the object is going to go unnoticed to them a large amount of the time.