A quick update. The car rental worked quite well and I made it to IAD with plenty of time. Traffic was minimal last Saturday afternoon.

I add that I tend to strike up conversations with cabbies and two offered to take me to Dulles as a special fare when they heard of my "plight". One said 80 and the other 100. It was clear that Saturday afternoon was slow for them. My guess is that I could have bargained down a bit I had wanted to go that route. The rental totaled out to less than a 100.