designated driver
A person who stays sober during a social gathering and is responsible for safely driving others from one location to another. Since Kara never drinks alcohol, she always offers to be the designated driver for her friends.
designate (someone or something) as (something)
To label or name someone or something as something; to assign someone or something a specific role or status. The boss designated me as the point person for this project, so you guys have to listen to me. No, you can't sit here—mom designated this as the kids' table.
designate someone or something as something
to choose or name someone or something as something. Alice will designate Andrew as our representative. She designated herself as the head of the committee.