Mansfield v Weetabix Ltd [1998] 1 WLR 1263

Facts

• Mr. Tarleton, a lorry driver employed by Weetabix, was unknowingly suffering from a malignant insulinoma causing hypoglycemia, which impaired his brain function and ability to drive.
• While driving, he entered a hypoglycemic state that diminished his capacity, leading to a crash into the claimant's property.
• Mr. Tarleton was not fully unconscious but was unaware of his impairment at the time of the accident.
• Medical evidence confirmed that had he realized his condition, he would have stopped driving to avoid causing harm.
• The legal question arose whether the standard of care for a driver should account for an unknown medical condition.

Issues

1. Whether the standard of care in negligence should be modified for a defendant whose capacity is unknowingly impaired by a medical condition.
2. Whether Mr. Tarleton could be held liable for breach of duty given his lack of awareness of his impairment at the time of the accident.
3. Whether strict liability should apply in cases of harm caused by individuals suffering from undiscovered medical impairments.

Decision

• The Court of Appeal determined that the standard of care required was that of a reasonably competent driver who is unaware of any impairment affecting his ability to drive.
• The court concluded that Mr. Tarleton had not breached his duty of care, as he could not have reasonably known about his condition.
• The appeal was allowed, and Mr. Tarleton (Weetabix) was not held liable for the damage caused.
• The court rejected the imposition of strict liability in these circumstances.

Legal Principles

• The standard of care in negligence is generally objective, but can be modified if the defendant suffers from an unknown medical impairment.
• Individuals should not be required to guard against risks arising from conditions they could not reasonably have known about.
• The ruling distinguished between negligence and strict liability, clarifying that liability in tort should reflect the defendant’s knowledge and circumstances.
• The judgment does not excuse all impairments, only those of which the defendant was genuinely unaware.
• The case contrasts with later authority, particularly Dunnage v Randall, which rejected consideration of certain types of impairment.

Conclusion

The Court of Appeal in Mansfield v Weetabix Ltd established that a defendant is to be judged against the standard of a reasonably competent person unaware of their impairment, thereby introducing a necessary modification to the objective standard of care in negligence where undiagnosed medical conditions are involved and rejecting strict liability in such cases.