Whites Ferry crosses the Potomac near Poolesville in Maryland and Leeseburg in Virginia. You can take your car across it. You pay when you get on the barge, no need to pay in advance. If you have never been on a ferry, it is a fun trip. I used to commute from Maryland to Dulles, VA and would take the trip twice a day, it was much more enjoyable than being stuck in traffic on the American Legion Bridge.