None of those will exceed 20mph - public transportation and biking (presumably) follow the speed limit and the average human running speed is 5 to 8 mph. Unless it's a subway or a train.
Most likely, however, you're not going to be driving 20 miles in a city center. If you're driving more than 10 miles, you'll probably want to use a highway, and the US highway system is extensive enough that you can.
As for biking, I have less of a problem with people biking over 20mph in the city. (However, you're correct that, in practice, there's no way someone's going to average 20mph for a city bike commute. Most people aren't in good enough shape to sustain 20mph on the flat.) I regularly hit 30-35mph on downhills (say, Central Park at the north end) because I know what I and the bike can handle. Bikes are small and agile (it's easy to swerve and avoid hitting someone) and much less massive than a car.