@luckytran In my job as an engineer I am often forced to make up measurable, mandatory rules to deal with safety issues.
Use shoring when digging bellow 5 ft. Wear PPE.
An excavation 2 inches deeper than 5 feet is not unsafe. An excavation 2 inches shallower than the limit may be unsafe due to soil types. I really don't know until I see indicators.
Humans need rules if you want to ensure a level of consistency and a general level of safety.
Hands down the most successful rules I've ever developed are simple in concept. See this do this. Once you start taking the math or science into account you will loose the ability to enforce and any positive safety outcomes.
Simplicity is a necessity.
I don't make non-experts know soils to understand risk. I lower the limit by a foot universally and add engineering approval for exceptions.
Attacking the science is disingenuous. There is always room for professional arguments and uncertainty. It is attacking their authority to make decisions based on knowledge.