I also drive around DC. I did an experiment a while ago that changed my perspective on following distance. I decided that I would leave several car lengths in front of me until I was delayed 10 cars. This doesn't count the cars that cut in and cut out less than a minute later. 10 cars means I get home ~30 seconds later.
Much to my surprise, I've never been delayed by 10 cars. Not on the beltway, not on I-95, not on I-66. I was shocked at how much stress I felt being cut off and pushed back in traffic - just to find out that the difference between diligently keeping close and being a mellow polite driver is less than 30 seconds!