In determining what a reasonable person would do, the courts will consider the practicality and cost of doing anything to reduce a known risk. The American judge Learned Hand J suggested in US v Carroll Towing (1947) that this can be reduced to a mathematical relation: if the cost of damage multiplied by the probability of damage is less than the cost of prevention, then there is no breach.
The English courts have not adopted this formal test, but apply similar principles.
Morris v West Hartlepool Navigation  AC 552, HL
A seaman fell forty feet into an unguarded hold and was badly injured. Lord Reid said it is the duty of an employer in considering whether some precaution should be taken against a foreseeable risk to weigh on the one hand the magnitude of the risk, the likelihood of an accident happening, and the possible seriousness of the consequences if it does, and on the other hand the difficulty and expense and any other disadvantage of taking the precaution.
Latimer v AEC Ltd  2 All ER 449, HL
Following an exceptionally heavy storm, water entered a factory and covered much of the floor; when it drained away (after mixing with the coolant used for the machinery) it left a thin film of an oily liquid on the floor. DD put down sawdust to soak up the liquid and to reduce the risk of slipping, but did not have enough to cover the entire floor because of the exceptional nature of the storm. A workman P slipped on an untreated part of the floor and his ankle was badly injured. The House of Lords held, reversing Pilcher J, that DD had done all that a reasonable person would do in the circumstances; they could not have eliminated the risk completely without closing the factory. DD had not failed to exercise reasonable care and so were not liable for the subsequent accident.
Knight v Home Office  3 All ER 237, Pill J
A prisoner committed suicide in his cell, and it was alleged that the prison authorities had been negligent by leaving him largely unsupervised and checking him only every fifteen minutes. The judge found as a fact that the authorities had not been negligent, but said that while general practice in the prison service was to be taken into account, the plaintiff could in principle have succeeded even if the accepted practice had been followed in every respect. It was for the court to decide what standard of care was appropriate to the particular situation, and it was not a complete defence (though it might be relevant) to say that the prison staff had done what was usual and no funds were available for additional safety measures.
Walker v Northumberland CC  1 All ER 737, Colman J
A senior social worker suffered from severe stress because of overwork. DD argued that budgetary constraints made it impossible for them to take effective action to reduce P's workload, as this would have meant appointing more staff. The judge refused to accept this argument: the mutual intentions of the parties in a contract of employment might require the employer to take no more than reasonable care for the safety of his staff, but it was inconceivable that they would require him to take only such steps as political expediency from time to time permitted.