TomTom maps (and probably others, I just worked for TomTom so know how theirs work) combine both real time traffic information, and road speed estimates based on data from users uploading their driving traces (with start and end locations removed to anonymize them), so it has a table of the average driving speed for all roads, on a per-hour and weekday/weekend basis, plus any real time traffic information it has, which it uses for route calculation and time estimation.