There’s always a debate about whether the Metro or car is a better way of transit. The general consensus is that the car wins hands down when it comes to speed. The DC Metro averages about 25 miles per hour, because of station stops. On the surface it appears a car can do much better.
I came to this number by looking at the Orange Line from terminus to terminus, which is Vienna, to New Carrollton, respectively. If you use a car you’re likely to use the Beltway. If you do, it takes about 46 minutes to complete the trip according to Google Maps. It takes 57 minutes using the Metro according to Metro’s website. And then considering traffic on the beltway and especially I-66 you’re probably not going to do nearly as well. And if you drive through the city instead, you’re trip will take 1 hour and 15 minutes.
The other lines average slightly slower from terminus to terminus. The Red Line runs at about 20 miles per hour. A train vs. automobile comparison for that one would be futile, because of the lines “U” shape. The Yellow Line also runs at about 20 miles per hour. The Green Line is about 18 miles per hour. The Blue Line runs averages 22 miles per hour.