Just a note which relates to all Sat Nav systems.
They are an AID to driving, nothing more. regardless of type, they WILL send you down unsuitable roads at times, that is why they should not be used as a complete solution, they rely on YOUR input too, and using them with a degree of sense.
Most have the option to take you on either the fastest, or shortest route. The fastest "usually" means they will take you on main roads, but even then, may take you on an unsuitable towing road.
Shortest means justthat, and road type dosent matter, to the Sat Nav, anyway.
You can browse the route, on most systems, and it is worth checking against a paper map if its to a place you are unsure of, and any parts you dont like the look of, put in an "avoid" prompt.
Whilst I find my TomTom excellent, it is no good blaming the unit for ending up on roads you would rather not be on.