In what frame of reference would you be at rest while riding in a car?

In the frame of reference of anybody in the car.

We would be in the frame of reference to other people inside the car.

Explanation:

In Physics the frame of reference is used for the observation and mathematical description of physical phenomena that occur around us and they formulate the physical laws with respect to others. In the given case all the people inside the car are the frame of reference to one another but if someone is outside of the car then his frame of reference will be changing as the car moves with respect to him.