Originally Posted by
WineCountryUA
While bus is an option, it is not required. -- But it is a longer walk than most of the other airports
definitely
If IAD-PHX flight departs from higher-number end of Concourse D, it will be a long walk, too, after going through immigration and security check at Concourse C (the other end of C/D concourses).
The length of connection time between flights is most important. Both LAX and SFO offer longer connection times and shorter flight times on United.