@manton Washington DC: There’s lots of signs and some roads turn one-way at certain times to allow for people getting into/out of DC.
There may inevitably be Some Large Gathering happening so roads can close at random and the National Mall may be a march, protest, festival, or merely hundreds of people out enjoying a nice day. You never know!
I would also say the LA turn left rule applies to the DMV (DC and the Virginia and Maryland suburbs near it.)