Your family decides to take a long trip for the summer. You will be riding in the car while it moves at a fairly constant rate (about the same speed). The longer you are in the car, the further you travel on your trip. What is the independent variable

in this scenario?

The independent variable is time, as it is the variable that is being intentionally changed (i.e. the longer the family is in the car, the more time passes and the further they travel on the trip).