The sign is telling them that later down the road they will absolutely get in to problems, and get literally stuck. No one would continue with that certainty of getting stuck or in trouble. On top of that, it cant be repeat offenders either. Some one who has been there before must have gotten caught out, one way or another. Its not like drivers on small roads who see a 6ft width restriction sign, but know through local knowledge that really you can squeeze 7ft wide car through the lane.
What we have is drivers who do not know the road, and in their minds don't think the big huge warning signs are meant for them. In their heads they think it for a another driver on the road in close proximity, even if there is no sign of one.
The problem with road signs is making it known to specific individuals that the message is absolutely for them. The brilliant last resort sign absolutely makes it clear who the message is for.
So the key is to some how make driver know that the message is actually for them specifically.