The general rule in the United States is that a defendant is liable for carelessly causing harm to foreseeable plaintiffs. The negligence standard tells us that if the defendant acted carefully enough, he should not be held liable.
Defendants who engage in abnormally dangerous activity are held strictly liable. They are liable no matter how cautiously they acted. The idea behind strict liability is to encourage people to not engage in an activity that is very dangerous, or to engage in the activity somewhere else so no one gets hurt. Strict liability should encourage people to find alternative, safer ways to do things.
Traditionally, U.S. courts apply multiple factors to determine whether an activity is abnormally dangerous and subject to the strict liability standard. These factors are:
(a) existence of a high degree of risk of some harm;
(b) likelihood that the harm that results from the activity will be great;
(c) inability to eliminate the risk by the exercise of reasonable care;
(d) extent to which the activity is uncommon;
(e) inappropriateness of the activity to the place where it takes place; and
(f) the extent to which the value of the activity to the community is outweighed by its danger.
The most important modern case on this issue is Indiana Harbor Belt Railroad Co. v. American Cyanamid Co., 916 F.2d 1174 (7th Cir. 1990). In that case a famous and influential judge (Judge Posner) reasoned that shipping a dangerous chemical in a major city was not abnormally dangerous because reasonable care could have prevented the accident (factor (c)). In addition, he concluded that it would not be practical to try and reroute the way the chemicals were shipped (factor (e)).
Some legal scholars want to condense the factors to the following:
An activity is abnormally dangerous and strict liability should apply if (a) the activity creates a significant risk of physical harm even if people engage in reasonable care; and (2) the activity is uncommon.