First Incidents per hour is not arbitrary. These numbers compare very well to daily activities such as walking, driving, bathing, eating, swimming so that non specialists have a good idea of how much risk an activity carries by comparing it to an activity they’re familiar with.

Secondly ISO 26262 produces ASILs as its output which are qualitative, but still based on probably assessments in terms of chance of incidence per hour. The reason for qualitative instead of quantitative assessments of the more general SILs (based on IEC61508, the parent of ISO 26262) is that qualitative is cheaper than quantitative and the automotive industry is full of corner cutting.

Third, aircraft use QUANTITATIVE risk assessments based on ARP476, so risk can be directly measured and mathematicaly compared to any other activity. When people say “flying is safer than driving” it’s not arbitrary, it’s based on real math. The same math the FAA is using to find safety issues in the Boeing production line.

Fourth

I’m certainly not saying that safety isn’t important or that we can’t assess it.

Is this you?

safety isn’t a thing we can measure.