When Truck Safe Navigation estimates the amount of time that it will take you to reach your entered destination using ActiveTraffic, it takes a few different things into consideration. First, it considers the real-time traffic speed. If this information is unavailable, it then considers the history traffic speed. Lastly, if information for the first two factors are unavailable, the system uses the default value that is configured by a road's class.
More information on ActiveTraffic and how to enable this feature can be found here: Active Traffic Settings.