In the ArcGIS Online World Traffic Service, traffic data is calculated differently for historical versus live traffic.
The World Traffic Service is a dynamic map service with capabilities for visualizing traffic speeds relative to expected averages as well as traffic incidents that can be visualized and located. With the World Traffic Service, real-time traffic information for different regions of the world is captured every five minutes.
Historical traffic creates a traffic profile of the expected speed for Sunday through Saturday based on multiple observations over a time span. The World Traffic Service calculates the expected speed from the average of the observed time span of the last three years of the traffic profile. For example, for the fourth quarter 2016 update to the World Traffic Service, observations from the fourth quarter of 2013 through the third quarter of 2016 determine the values. Once the expected speed is calculated, it would be the same throughout the seven days of the traffic profile. Therefore, traffic speeds on Monday would be the same, regardless of the date it falls on.
With the live-traffic model, the speeds are measured using varying sources, such as GPS receivers in vehicles and speed sensors on roads. Live traffic readings from sensors are saved for 12 hours, so if the map time is set back within 12 hours, actual recorded traffic speeds are shown and are supplemented with typical averages by default. By turning off the average speeds, the recorded live traffic speeds for any time within the 12-hour window can be viewed independently. Predictive traffic conditions are shown for any time in the future.
Historical traffic can be configured without live traffic. However, live traffic must be configured along with historical traffic.