You are missing several parts of the equation, hoangb. First is the time required to rent the car, gas up the car, and return the car; second is the very considerable hassle factor of having to park the car at the various places where the OP wants to stop. For someone unfamiliar with DC driving, it's not all that simple to find one's way around, so that would factor in. Then there is the considerable cost of parking at any hotel in DC, if the OP is staying there. Uber presents a much better alternative.