What is the most important element that can be used to measure "as reasonably practicable" ?

Only practical and sufficient risk assessments can prove that the work is reasonably practicable. i.e. HAZOP studies, FMEA studies etc.

Cost. Discretion is still not reasonably practicable, discretion entails a more complex problem pr analysis required that is hidden.

The most important element that can be used as reasonably and practicable is the "balance" between the cost of the control measure and the risk involve. In some instance by conducting assesment in can be shown control measures is inappropriate or not proportion to the risk where it is not practicable and reasonably to shut down one particular area where a backhaw loader had collided with the forlift.

Sorry please are those english words, because i have never heard of them before. thanks any ways for help.

Inspections and follow-up ... 

Two more elements that can measure all of the risks in the work environment. 

 

In terms of digital or numerical measurement, there are many of them .. devices such as: 

- Hajrometer | to measure air pressure. 

- Bhartencjunh | to measure the noise.