How is a reasonable standard of care applied in assessing negligence?

The standard of conduct that will meet or breach the duty of care is measured by its reasonableness. Reasonableness is determined by all of the circumstances of the time and place. In Ryan v. Victoria (City), 1999 CanLII 706 (SCC), [1999] 1 S.C.R. 201 [Ryan], Major J. described the formula to be used when assessing the reasonableness of a standard of care. He wrote: 28 Conduct is negligent if it creates an objectively unreasonable risk of harm. To avoid liability, a person must exercise the standard of care that would be expected of an ordinary, reasonable and prudent person in the same circumstances. The measure of what is reasonable depends on the facts of each case, including the likelihood of a known or foreseeable harm, the gravity of that harm, and the burden or cost which would be incurred to prevent the injury. In addition, one may look to external indicators of reasonable conduct, such as custom, industry practice, and statutory or regulatory standards.