What is the expiation for accidental killing in traffic accidents that result in the death of a large number of people, when it is impossible to fast for each soul due to the extended period?

Obligations do not cease except when one is unable to fulfill them. Whoever accidentally kills a group of people is required to offer expiation for each deceased person. The expiation is to free a slave; if one cannot find one, then to fast for two consecutive months. If one is unable to fast (due to the hardship of the long duration), then one may feed the poor for each soul, acting upon the principle "What is feasible does not fall away due to what is difficult" and His — the Exalted — saying: (So fear Allah as much as you are able).