## Facts

- William Henry Piner was involved in two separate rear-end collisions on the same day, several hours apart, while stopped to allow pedestrians to cross.
- In the first collision, Billy Donald Jones rear-ended Piner’s truck; Piner reported neck, upper-back, left-arm, and head pain shortly afterward.
- In the second collision, Cynthia Gale Richardson rear-ended Piner’s truck; Piner experienced similar pain symptoms after the second impact.
- Piner’s physician later concluded Piner suffered multiple injuries from the collisions, but medical evidence could not attribute specific portions of the total injury to either collision.
- The parties agreed both collisions contributed to Piner’s total physical injuries and that the harm could not be apportioned between defendants by causal contribution.
- Piner sued the drivers from both collisions and sought partial summary judgment asserting the injuries were indivisible and should be treated as a single injury for liability purposes.

## Issues

1. Whether Arizona’s comparative fault and UCATA amendments abolishing most joint and several liability eliminated the common-law indivisible-injury (single-injury) rule for multiple tortfeasors.
2. In successive-collision cases producing an indivisible injury, whether the plaintiff must apportion damages between defendants, or whether defendants bear the risk and burden of non-apportionability.

## Decision

- The Arizona Supreme Court accepted special action jurisdiction after the trial court denied partial summary judgment and the court of appeals declined review.
- The court held UCATA did not abrogate Arizona’s indivisible-injury (single-injury) rule.
- The court concluded that when independent negligent acts—simultaneous or successive—cause an indivisible injury, the plaintiff is relieved of apportioning damages by causal contribution.
- The court ruled that temporal separation between events does not change the rule; the controlling fact is that each defendant’s conduct was a cause of the same indivisible harm.
- The court directed the superior court to proceed consistent with these holdings, vacating the denial of partial summary judgment to the extent inconsistent with the indivisible-injury rule.

## Legal Principles

- If multiple defendants’ negligence is a cause of a single, indivisible injury, each defendant is liable for the entire injury unless a reasonable basis for apportionment is shown.
- UCATA’s comparative-fault scheme governs allocation of fault and contribution among defendants but does not require a plaintiff to apportion indivisible damages by causal contribution.
- The plaintiff must prove negligence, causation (that each defendant’s negligence was a cause of the injury), and the extent of total damages; the plaintiff need not prove what fraction of the indivisible harm each defendant caused.
- When apportionment of an indivisible injury is not possible on the evidence, the risk of that uncertainty falls on the tortfeasors rather than the injured plaintiff.
- The indivisible-injury rule applies to successive accidents as well as to multiple-causation events occurring close in time.

## Conclusion

Arizona’s UCATA and comparative fault statutes did not eliminate the indivisible-injury doctrine: when successive tortfeasors each contribute to an injury that cannot be divided by causal contribution, the plaintiff may recover for the entire harm without apportioning damages between defendants, and fault allocation operates among defendants rather than as a bar to recovery.


Piner v. Superior Court, 192 Ariz. 182, 962 P.2d 909 (Ariz. 1998)