Bear in mind that average speed includes station stops. It's really hard to get a good intuition of—it's not like it will tootle along at 16mph; it will run at 30+ mph, stop, run at 30+ mph, stop, etc.

For comparison, I remember seeing (a ways back admittedly) that the average speed of the J buses on East-West Hwy are about 7-8mph. Compared to that, 16mph is a huge improvement. It seems very unlikely that between Bethesda and Silver Spring, for example, that driving would be faster, even with clear roads.