There are 4 people who work and only 2 vehicles. 2 people work 6am to 6:30pm, 1 works from 8am to 3:30pm, then 1 who works from 9pm to 7am. What is the most logical carpool route as they all live together?

He who drives makes the arrangements..

You left out the important detail of how the owners of the cars work, and if they all worked at the same place on different shifts or not. The devil would be in the details, so if you have them be great to have them here for consideration. Until then I would agree with the previous poster that the vehicle owners make the plan, and no owners go along with it or get their own plan.