Though it's not an exact measure of attendance, the Washington metro system reported that 193,000 metro rides had been recorded by 11 a.m. ET on Friday, versus 513,000 by that time during Obama's 2009 inauguration and 317,000 during his 2013 inauguration.
Metro Ridership: As of 11am, 193k trips taken so far today. (11am 1/20/13 = 317k, 11am 1/20/09 = 513k, 11am 1/20/05 = 197k) #wmata
— Metro (@wmata) January 20, 2017