2.5 hours is not enough time for that ground transfer if you are renting a car. Additional steps involve IAD shuttle to the rental center and substantial (although closed moving walkway) from DCA rental center to checkpoint.
If you are not familiar with the area, best bet at that time is Uber or other car service where the drivers know the back streets which go around traffic jams. Those routes won't show on a GPS map and can be worse than a major road traffic jam if you play it wrong.