The majority said that even though mailboxes are on the public "right of way" (aka public easement), it is not on a part that is normally traveled on by vehicles, therefore there is no duty of care toward vehicles and their occupants.

In a previous case, it was decided that mailbox owners had no duty of care to cyclists, pedestrians, etc, as long as the mailboxes are legally placed.

To recap;

In Ohio, as long as you legally place your mailbox, you have no further duty of care.

Other states may have different laws and/or rulings.