What is the expiation for someone who caused the death of a person in a traffic accident, given that their fault was 25%?

You and your companion owe expiation, which is the freeing of a slave. If you are unable to do that, then each of you must fast for two consecutive months. This is if you were complicit in the error. However, if you are certain that the error was not yours, that the police report is incorrect, and that the error was your companion's, then nothing is incumbent upon you.