The standard in common law (and civil law) systems is the reasonable person test - what would a reasonably prudent person in the same (external) circumstances do. (law.stackexchange.com)
The standard of care of a driver is not one of perfection, but rather whether the driver acted in a manner in which an ordinarily prudent person would act. (vealelaw.com)
Stocks were too risky for prudent people to invest in and serious money should never be invested there. (fastgraphs.com)