How is travel time calculated?

For my current project, I need to know how RouteXL calculates the driving time? Is the the speed assumed or is it based on the actual road speed for each road segment? 

Thanks!

Our servers use OpenStreetMap (OSM) based route planners to calculate driving times between each set of points.

OSM is a kind of Wikipedia for geographical maps. If you’re not aware of OSM, check out:
http://www.openstreetmap.org/about

If road speed is correctly tagged in the map, that will be used for the driving time. If speed limits are not available, assumptions are made, e.g. based on the location.