I agree and having run into H&S as self employed it riles me that bureaucrats can feel exempt from the basic consideration of risk assessment. Surely there should be a piece of paper somewhere in that department with a corresponding date, assessing the risk of their notice.
Perhaps they consider that they have closed down the risk to person by crossing a main road, but what is the alternative.
Scenario; group of young teens, returning for evening meal, caught late and faced with turning back, retracing their steps knowing they will be late. Plenty of family homes north side, open countryside south side. What is the chance they might take the risk of a gap in the traffic against being late and perhaps owning up to have been out of bounds.
Too much churlish, poorly assumed authority within that notice.