Hello! I found the parking at the DoubleTree Hotel to be very easy and accommodating. However, when I arrived at IAD and was ready to leave at 6:30 am, it took 45 minutes for the shuttle to arrive. I called and they told me when the next shuttle would arrive which was accurate. While I was waiting, I saw 3-4 shuttles for Marriott and will park their in the future. It appears that their shuttles come more frequently.