"Fastest" or "Shortest" - What is the underlying logic?

[Wiley]

Is there a definitive explanation somewhere of what actually is going on as TomTom works out a "Fastest" or "Shortest" route?

Shortest route:

Does it simply draw a straight line to the destination and then find the roads along that line that will get you there? Is it applying any efficiency rules?

Fastest route:

What logical criteria is required for a route to be considered fast? My guess is that it attempts to only use primary roads or a route that avoids as many turns as possible. What else?

 

[Nextourer]

Technically:

Shortest - shortest distance (logical or not)
Fastest - shortest time (based on the speed of the streets that is pre-programmed into the map. IOW, if TeleAtlas says 1st Ave is a main street and 2nd Ave is not, then it'll go with 1st Ave even though 2nd Ave may have fewer traffic lights or if the traffic lights on 2nd Ave are pedestrian controlled.)

 

[Sextant]

Technically:

Shortest - shortest distance (logical or not)
Fastest - shortest time (based on the speed of the streets that is pre-programmed into the map. IOW, if TeleAtlas says 1st Ave is a main street and 2nd Ave is not, then it'll go with 1st Ave even though 2nd Ave may have fewer traffic lights or if the traffic lights on 2nd Ave are pedestrian controlled.)

That's the way it is supposed to work but it doesn't in practice. With "fastest" my default setting the 910 wants to take me on a slightly shorter course that's several minutes longer. Logic?

 

[Pedro2NR]

fastest route is best chosen when traveling between cities.
shortest route is great when traveling within the city.
Be aware that the shortest route will make useless turns and might even have you take alley streets.