Moral of the story, these ad hoc checkpoints are not safe.
In that vein, snow squalls, a vehicle breakdown or any other traffic slow up comes down to the responsibility when driving a motor vehicle to always be observant of what is ahead of you. Lives are at stake when driving any umpteen thousand lb object at any speed and a second or two of inattention can have serious implications (am teaching a kid to drive and drive this home every lesson, even more so at highway speeds)