Is it obligatory to fast two months or four months as expiation for those who died in a car accident after the blood money (diyah) has been paid?

Killing two people by mistake necessitates two expiations and paying blood money to their families. The expiation is freeing a believing slave. If one cannot find [a slave to free], then one must fast for two consecutive months for each person killed, making a total of four consecutive months of fasting.