The arrival time is calculated on the type of road and the speed limits set for those roads in the map software. What it does not know about and does not take into account are how many traffic lights there are, how long you will be stopped at lights, how much under or over the speed limit you will travel. It does make adjustments as you travel and but really just takes that time you are at a certain point into account and then puts in what I stated above for the rest of the trip. Sometimes it averages out close to the actual arrival time and sometimes it is off. It is just an estimate and nothing to take very seriously.
