Why don't people walking just obey traffic signals? Some signals are clearly timed around moving vehicles and not around the needs of people on foot, as this video shows:
GGWash commenter Oboe posted this video, walking northbound on 11th Street SE near Lincoln Park.
Oboe waits for a minute and 25 seconds for two consecutive crossing signals, during which time only three cars drive across the path.
This intersection gives a clear signal (ha) to everyone that vehicle flow (though not even, really) is the priority in its design. There's also a bicycle lane here, but signal timing generally accounts for two things: moving vehicles, and making each walk phase long enough for people walking at 3.5 feet per second (a Federal Highway Administration standard) to cross. It doesn't really consider how long people spend waiting at the corners for a don't walk sign.
DC retimed its signals in recent years to make the total cycle length 110 seconds instead of 100. That's moved cars in and out of downtown faster and made crossings better for slow-moving seniors and people with disabilities, but also means that the wait at each corner is about five seconds longer for most intersections.
I've encountered long (like 90 second) countdowns to cross a major arterial, like Pennsylvania Ave SE, H Street NE, or 16th Street NW, even middays on weekends or at other very low traffic volume times. The consequence is that people either cross illegally (ususally) or have to stand still for no apparent reason, as Oboe does above.