Be careful the smartphone will probably download constantly data.
So no signal = no directions.
Lots of data = lots of money depending upon your plan.
I can't speak for an Apple phone, but on Android google maps will download maps in advance, which saves on mobile data.
Herewego (was Nokia's maps, then bought by a consortium of car companies) also has offline capabilities.
Waze also has an offline/ map download facility, and lets you report & see reports from fellow motorists... accidents, jams, cops etc... if you're online.
If there's no signal, you'd lose live updates - but then the chances are that where there's no signal you're remote enough that there's no traffic!
Will
