Yeah it's amazing (in a bad sense) how those bridge too low warnings are ignored most of the time

Well of course when you get stuck then it's too late.

Similar to how people will read a temporary hand written sign on a door, but filter out a (newly placed) professionally made sign, because the latter looks too permanent.

If something has changed, using something out of place or temporary in appearance seems to be the most effective way of getting human attention... A bale of straw feels like it fits the bill.

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.