How long does it take you to ride to your destination? If you use Ride the City, you'll notice that along with "total distance" it provides you with an estimated time.
cycling_time
How is "estimated time" determined? First, Ride the City calculates the total distance. Then it assumes a slow (10mph) and faster (15mph) speed to figure out the low/high estimated riding time. Aside from a slight variation for greenway miles, that's it.

How accurate does the estimated time play out for you?