All perfect praise be to Allah The Lord of The Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
Causing someone`s death as a consequence of a car accident could hold the doer accountable, unless it was a mere mistake, since Allah, The Almighty, pardons the one who does wrong by mistake. Besides, the Prophet (PBUH) said (What means):" Verily Allah has pardoned [or been lenient with] for me my ummah: their mistakes, their forgetfulness, and that which they have been forced to do under duress. [Related by Ibn Majah].
Therefore, one who had mistakenly caused someone`s death through a car accident is neither obliged to pay Diya nor expiation, unless the cause of the accident was negligence on the part of the driver and this can only be determined by the traffic sketch issued from the departments in charge.
In conclusion, since the aforementioned sketch did not hold you accountable in the court, then you are free from the liability and neither obliged to pay Diya nor expiation. And Allah knows best.