Updated from this 2010 post.
As usual, bikes will be banned on Metrorail all day on the Fourth of July. As usual, it will be a totally unnecessary, and even counterproductive, precaution. Metro's policy is that
Bicycles are not permitted on Metrorail on July 4th or other special events or holidays when large crowds use the system.
Large crowds, huh?
In 2009 631,206 people used Metrorail on the Fourth of July, making it the 5th busiest Saturday in Metro history a the time. I'm not sure how many rode last year, but it was fewer than 782,000 (last year's top day). 630,000 sounds like a lot, but on an average weekday in 2008 Metrorail had 727,684 trips. That number is down to 712,843 for last year, but still shows what baseline capacity looks like.
So, the Fourth isn't actually that busy. Not busy enough to ban bikes ALL day. Of the thirty-one busiest days* in Metrorail history, not a one is a Fourth of July. Of those 31 days, bikes were only banned for #1, #8 and #29. Busiest Day #2 was on April 2, 2010 when Metro recorded 890,000+ rides. Metro banned bikes only during the morning and afternoon rush (like any other normal workday) and yet no one seemed to have a problem with it. How come we can allow bikes on for most of a nearly 900,000 rider day, but not on a 650,000 rider day?
It might be reasonable to ban bikes on for some time around the fireworks, when Metro is crazy, but why at 9 am? Considering how much financial trouble Metro is having, it doesn't really make sense to turn away paying customers.