DC actually seemed like it went the opposite way as it usually takes an hour and a half to get from VA through the beltway into DC while the map showed only an hour for as far out as Dale City. A half hour (or more) from Petworth into Downtown during rush hour is fair, since DC has probably the slowest crappiest drivers in the nation.
Updated theory: these commute times might not be based on actual driver data, it's probably a very rough estimation based on distance, travel time, speed limit, and population. It needs to be weighted based on the curve of traffic based on time and the way locals drive. Boca commuting will probably be slower than Miami commuting on average, unless you're on the highway in which case you're totally screwed in Miami. Not to mention anywhere there's an on-ramp with lots of flow you're going to make a choke point, so times would increase a lot, unless you started after that exit.
tl;dr Traffic is hard.