Which on road sat nav?

[Merts]

I reckon they would all have fine print on what 'lifetime' actually means.

I'm pretty sure they aren't referring to the owner's lifetime. 
It just means you can keep updating the maps indefinitely until the unit dies, which at some point it will.

[bmwfreak]

I'm pretty sure they aren't referring to the owner's lifetime. 
It just means you can keep updating the maps indefinitely until the unit dies, which at some point it will.

Garmin lifetime maps is based on the market lifetime of the unit, ie when a Garmin GPS is replaced with a new model, that is the lifetime. This means you can buy a new Garmin GPS at Johnny Appleseed, take it home and register it, pay your $198 lifetime maps subscription, and the next day have a new model released superceding your purchase. At that point your lifetime maps end. This happened to me with a BMW navigator III (Garmin streetpilot 2820 from memory) although it was about 6 months, not the next day.
Map updates are only as accurate as the local roads authorities are. New 2019 Garmin maps still has a 70kph radar warning at Blacksoil from when they were building the new interchange about 10 years ago.
Cheers.

[Fizzie]

Map updates are only as accurate as the local roads authorities are.

One option there is to use one of the maps that runs on OpenStreetMap - MapsMe or OSMAND.

Then you can go in & edit the map yourself to correct street names, numbers, speed limits, lanes, traffic lights etc etc etc